Exploring the chemical space of the lysine-binding pocket of the first kringle domain of hepatocyte growth factor/scatter factor (HGF/SF) yields a new class of inhibitors of HGF/SF-MET binding.

The growth/motility factor hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, the tyrosine kinase MET, constitute a signalling system essential for embryogenesis and for tissue/organ regeneration in post-natal life. HGF/SF-MET signalling, however, also plays a key role in the onset of metastasis of a large number of human tumours. Both HGF/SF and MET are high molecular weight proteins that bury an extensive interface upon complex formation and thus constitute a challenging target for the development of low molecular weight inhibitors. Here we have used surface plasmon resonance (SPR), nuclear magnetic resonance (NMR) and X-ray crystallography to screen a diverse fragment library of 1338 members as well as a range of piperazine-like compounds. Several small molecules were found to bind in the lysine-binding pocket of the kringle 1 domain of HGF/SF and its truncated splice variant NK1. We have defined the binding mode of these compounds, explored their biological activity and we show that selected fragments inhibit MET downstream signalling. Thus we demonstrate that targeting the lysine-binding pocket of NK1 is an effective strategy to generate MET receptor antagonists and we offer proof of concept that the HGF/SF-MET interface may be successfully targeted with small molecules. These studies have broad implications for the development of HGF/SF-MET therapeutics and cancer treatment.

Insights into the structure/function of hepatocyte growth factor/scatter factor from studies with individual domains.

Hepatocyte growth factor/scatter factor (HGF/SF), the ligand for the receptor tyrosine kinase encoded by the c-Met proto-oncogene, is a multidomain protein structurally related to the pro-enzyme plasminogen and with major roles in development, tissue regeneration and cancer. We have expressed the N-terminal (N) domain, the four kringle domains (K1 to K4) and the serine proteinase homology domain (SP) of HGF/SF individually in yeast or mammalian cells and studied their ability to: (i) bind the Met receptor as well as heparan sulphate and dermatan sulphate co-receptors, (ii) activate Met in target cells and, (iii) map their binding sites onto the beta-propeller domain of Met. The N, K1 and SP domains bound Met directly with comparable affinities (K(d)=2.4, 3.3 and 1.4 microM). The same domains also bound heparin with decreasing affinities (N>K1>>SP) but only the N domain bound dermatan sulphate. Three kringle domains (K1, K2 and K4) displayed agonistic activity on target cells. In contrast, the N and SP domains, although capable of Met binding, displayed no or little activity. Further, cross-linking experiments demonstrated that both the N domain and kringles 1-2 bind the beta-chain moiety (amino acid residues 308-514) of the Met beta-propeller. In summary, the K1, K2 and K4 domains of HGF/SF are sufficient for Met activation, whereas the N and SP domains are not, although the latter domains contribute additional binding sites necessary for receptor activation by full length HGF/SF. The results provide new insights into the structure/function of HGF/SF and a basis for engineering the N and K1 domains as receptor antagonists for cancer therapy.

Signalling by HGF/SF and Met: the role of heparan sulphate co-receptors.

The receptor tyrosine kinase Met and its ligand HGF/SF (hepatocyte growth factor/scatter factor) are essential in the signalling pathways required for embryogenesis and tissue regeneration. Aberrant signalling of this complex is also a feature of many tumours and appears to contribute to the growth, invasiveness and metastasis of both carcinomas and sarcomas. HGF/SF, like many other angiogenic growth factors, employs heparan sulphate as co-receptor. The role of this interaction has not been completely defined but appears to be physiologically relevant. Thus the presence of heparin increases the potency of HGF/SF in experiments with cells in culture leading to elevated downstream signalling effects and, although not vital for the Met-HGF/SF interaction, heparin or heparan sulphate is essential for the activity of certain isoforms of HGF/SF, such as NK1 and NK2. Here, we summarize the progress made in understanding the interaction between heparin and heparan sulphate and NK1, NK2 and HGF/SF and we discuss their role in HGF/SF-Met signalling.

Crystal structures of NK1-heparin complexes reveal the basis for NK1 activity and enable engineering of potent agonists of the MET receptor.

NK1 is a splice variant of the polypeptide growth factor HGF/SF, which consists of the N-terminal (N) and first kringle (K) domain and requires heparan sulfate or soluble heparin for activity. We describe two X-ray crystal structures of NK1-heparin complexes that define a heparin-binding site in the N domain, in which a major role is played by R73, with further contributions from main chain atoms of T61, K63 and G79 and the side chains of K60, T61, R76, K62 and K58. Mutagenesis experiments demonstrate that heparin binding to this site is essential for dimerization in solution and biological activity of NK1. Heparin also comes into contact with a patch of positively charged residues (K132, R134, K170 and R181) in the K domain. Mutation of these residues yields NK1 variants with increased biological activity. Thus, we uncover a complex role for heparan sulfate in which binding to the primary site in the N domain is essential for biological activity whereas binding to the K domain reduces activity. We exploit the interaction between heparin and the K domain site in order to engineer NK1 as a potent receptor agonist and suggest that dual (positive and negative) control may be a general mechanism of heparan sulfate-dependent regulation of growth factor activity.

Involvement of hepatocyte growth factor/scatter factor and met receptor signaling in hair follicle morphogenesis and cycling.

HGF/SF and its receptor (Met) are principal mediators of mesenchymal-epithelial interactions in several different systems and have recently been implicated in the control of hair follicle (HF) growth. We have studied their expression patterns during HF morphogenesis and cycling in C57BL/6 mice, whereas functional hair growth effects of HGF/SF were assessed in vivo by analysis of transgenic mice and in skin organ culture. In normal mouse skin, follicular expression of HGF/SF and Met was strikingly localized: HGF/SF was found only in the HF mesenchyme (dermal papilla fibroblasts) and Met in the neighboring hair bulb keratinocytes. Both HGF/SF and Met expression peaked during the initial phases of HF morphogenesis, the stage of active hair growth (early and mid anagen), and during the apoptosis-driven HF regression (catagen). Met+ cells in the regressing epithelial strand appeared to be protected from undergoing apoptosis. Compared to wild-type controls, transgenic mice overexpressing HGF/SF under the control of the MT-1 promoter had twice as many developing HF and displayed accelerated HF development on postnatal day 3. They also showed significant catagen retardation on P17. In organ culture and in vivo, HGF/SF i.c. resulted in a significant catagen retardation. These results demonstrate an important role of HGF/SF and Met in murine hair growth control and suggest that Met-mediated signaling might be exploited for therapeutic manipulation of human hair growth disorders.-Lindner, G., Menrad, A., Gherardi, E., Merlino, G., Welker, P., Handjiski, B., Roloff, B., Paus, R. Involvement of hepatocyte growth factor/scatter factor and Met receptor signaling in hair follicle morphogenesis and cycling.

Expression of a Cx43 deletion mutant in 3T3 A31 fibroblasts prevents PDGF-induced inhibition of cell communication and suppresses cell growth.

Communication through gap junctions was first suggested to have a role in the social control of cell growth over 30 years ago. However, despite extensive experimentation, the importance of gap junctions as a general mechanism of growth control remains to be established. A number of different studies have shown that a common early response of cells in culture to polypeptide growth factors such as PDGF is a rapid and transient inhibition of cell communication suggesting that a cell may have to lose communication with its neighbors before it can undergo cell division. Here we show that 3T3 A31 fibroblasts exposed to PDGF exhibit a 50% decrease in cell communication as measured by dye transfer in the absence of significant changes in the cellular content and distribution of Cx43. Likewise, PDGF inhibited cell communication in cells transfected either with a vector which did not contain a cDNA or with an expression vector encoding full-length Cx43 fused to a c-myc tag (Cx43-M). In contrast, 3T3 A31 fibroblasts transfected with an expression construct encoding a deletion mutant of Cx43 (Cx43-256M) consisting of amino acids 1-256 of Cx43 fused to a c-myc tag maintain high levels of gap junction activity following exposure to PDGF. These results suggest that sites which trigger loss of cell communication in response to PDGF are located within amino acids 257 to 382 of the Cx43 molecule. Cells transfected with an expression vector encoding full-length Cx43 fused to a c-myc tail exhibited a reduced basal growth rate compared to both parent cells and cells transfected with a control vector but maintained a strong mitogenic response to PDGF. In contrast, both the basal growth rate and the mitogenic response to PDGF was markedly reduced in cells which expressed Cx43-256M consistent with the hypothesis that loss of cell communication is required before a cell can respond to mitogenic stimuli.

Heparan sulfate-modified CD44 promotes hepatocyte growth factor/scatter factor-induced signal transduction through the receptor tyrosine kinase c-Met.

CD44 has been implicated in tumor progression and metastasis, but the mechanism(s) involved is as yet poorly understood. Recent studies have shown that CD44 isoforms containing the alternatively spliced exon v3 carry heparan sulfate side chains and are able to bind heparin-binding growth factors. In the present study, we have explored the possibility of a physical and functional interaction between CD44 and hepatocyte growth factor/scatter factor (HGF/SF), the ligand of the receptor tyrosine kinase c-Met. The HGF/SF-c-Met pathway mediates cell growth and motility and has been implicated in tumor invasion and metastasis. We demonstrate that a CD44v3 splice variant efficiently binds HGF/SF via its heparan sulfate side chain. To address the functional relevance of this interaction, Namalwa Burkitt's lymphoma cells were stably co-transfected with c-Met and either CD44v3 or the isoform CD44s, which lacks heparan sulfate. We show that, as compared with CD44s, CD44v3 promotes: (i) HGF/SF-induced phosphorylation of c-Met, (ii) phosphorylation of several downstream proteins, and (iii) activation of the MAP kinases ERK1 and -2. By heparitinase treatment and the use of a mutant HGF/SF with greatly decreased affinity for heparan sulfate, we show that the enhancement of c-Met signal transduction induced by CD44v3 was critically dependent on heparan sulfate moieties. Our results identify heparan sulfate-modified CD44 (CD44-HS) as a functional co-receptor for HGF/SF which promotes signaling through the receptor tyrosine kinase c-Met, presumably by concentrating and presenting HGF/SF. As both CD44-HS and c-Met are overexpressed on several types of tumors, we propose that the observed functional collaboration might be instrumental in promoting tumor growth and metastasis.

The effect of high-frequency random mutagenesis on in vitro protein evolution: a study on TEM-1 beta-lactamase.

For a number of years a major limitation in genetic analysis of protein function has been the inability to introduce multiple substitutions at distant sites that would enable the selection of clusters of mutations required for improved or novel biological functions. In order to achieve this, we have recently developed a novel mutagenesis procedure in which the triphosphate derivatives of a pyrimidine (6-(2-deoxy-beta-d-ribofuranosyl)-3, 4-dihydro-8H-pyrimido-[4,5-c][1,2]oxazin-7-one; dP) and a purine (8-oxo-2'-deoxyguanosine; 8-oxodG) nucleoside analogue are employed in DNA synthesis reactions in vitro. The procedure allows control of the mutational load and can yield frequencies of amino acid residue substitutions at least one order of magnitude greater than those previously achieved. Here we report the results of an experiment in which we have hypermutated the bacterial enzyme TEM-1 beta-lactamase and selected small pools (<1.5x10(5)) of clones for enzymatic activity against the beta-lactam antibiotic cefotaxime. The experiment resulted in the isolation of a number of TEM-1 mutants with greatly improved activity against cefotaxime. Among these, clone 3D.5 (E104K:M182T:G238S) exhibited a minimum inhibitory concentration for cefotaxime 20,000-fold higher than wild-type TEM-1 and a catalytic efficiency (kcat/Km) 2383 times higher than the wild-type enzyme. Thus, small pools of hypermutated sequences enabled the selection of one of the most active extended beta-lactamases described so far. These results argue against the accepted view that multiple rounds of low-rate mutagenesis and stepwise selection are essential for in vitro protein evolution and extend the scope of directed molecular evolution to proteins for which no genetic selection is available.

Crystal structure of the NK1 fragment of HGF/SF suggests a novel mode for growth factor dimerization and receptor binding.

Although ligand-induced receptor dimerization is a common prerequisite for receptor activation, the mode by which different growth factors bind their receptors and cause them to dimerize varies considerably. Here we report the crystal structure at 2.5 A resolution of NK1, a receptor-binding fragment and a natural splice variant of hepatocyte growth factor/scatter factor (HGF/SF). NK1 assembles as a homodimer in the asymmetric unit, revealing a novel mode of growth factor dimerization produced by close packing of the N domain of one subunit and the kringle domain of the other, thus bringing the two linkers in close proximity. The structure suggests the presence of a binding site for heparan sulfate chains and a mechanism by which the NK1 dimer may engage two receptor molecules through clusters of amino acids located on each protomer and on opposite surfaces of the homodimer. We also report that short (14-mer) heparin fragments effectively dimerize NK1 in solution, implying that heparan sulfate chains may stabilize the NK1 dimer. These results provide a basis for the agonistic activity of NK1 and have implications for the mechanism of receptor binding of HGF/SF.

Developmental roles of HGF/SF and its receptor, the c-Met tyrosine kinase.

A number of developmental processes that involve cell migration, growth or morphogenesis depend on extracellular signals. A molecule that provides such signals, known as hepatocyte growth factor/scatter factor (HGF/SF), has attracted considerable interest in recent years because of its distinct structure, mechanism of activation and important roles throughout embryogenesis. This review discusses the main features of HGF/SF and its receptor, the product of the c-met protooncogene, and their role in embryogenesis.

Insights into the structure of hepatocyte growth factor/scatter factor (HGF/SF) and implications for receptor activation.

The modular structure of HGF/SF offers a reductionist or 'divide and rule' approach to the analysis of structure and function. Domain deletion experiments have established that the N domain, kringle 1 and kringle 2 are essential for HGF/SF activity and that truncated variants containing the N domain and kringle 1 (NK1) or kringles 1 and 2 (NK2) can exhibit partial agonistic or antagonistic activity depending on target cells. Comparative modelling has been used to predict the 3D structures of the six domains of HGF/SF. More recently, NMR methods have shown that the N domain has a novel fold, the charge distribution of which suggests a heparin binding site. Crystals of NK1 indicate the relationship of this domain to the kringle 1, offering further insights into the mechanism of domain interactions and receptor activation.

Engineered mutants of HGF/SF with reduced binding to heparan sulphate proteoglycans, decreased clearance and enhanced activity in vivo.

BACKGROUND: Although a number of growth factors bind cell-surface heparan sulphate proteoglycans (HSPGs), the role of this interaction is unclear except for fibroblast growth factor which requires HSPG binding for signalling. Hepatocyte growth factor/scatter factor (HGF/SF) plays important roles in mammalian development and tissue regeneration and acts on target cells through a specific receptor tyrosine kinase encoded by the c-met proto-oncogene. This factor also binds HSPGs with high affinity, but conflicting data have been reported on the role of HSPG binding in HGF/SF signalling. RESULTS: To map the binding sites for HSPG and the Met receptor in HGF/SF, we have engineered a number of HGF/SF mutants in which several clusters of solvent-accessible residues in the hairpin structure of the amino-terminal domain or in kringle 2 have been replaced. Two of the mutants (HP1 and HP2) showed greatly decreased (more than 50-fold) affinity for heparin and HSPGs but retained full mitogenic and motogenic activities on target cells in culture. Furthermore, when compared with wild-type HGF/SF, the HP1 mutant exhibited a delayed clearance from the blood, higher tissue levels and a higher induction of DNA synthesis in normal, adult murine liver. CONCLUSIONS: These results establish the following: the binding sites in HGF/SF for Met and for HSPGs can be dissociated by protein engineering; high-affinity binding of HGF/SF to HSPGs is not essential for signalling; one role of HSPG binding in the HGF/SF system appears to be sequestration and degradation of the growth factor; and HGF/SF mutants with decreased affinity for HSPGs exhibit enhanced activity in vivo.

Dimerization of Fab fragments enables ready screening of phage antibodies that affect hepatocyte growth factor/scatter factor activity on target cells.

A number of applications of antibodies in diagnosis and therapy require multivalent reagents either because of the polymeric nature of the antigens or because biological activity depends on an effect on the formation of homodimeric species. Here, we report a procedure for mass screening of phage-derived monomeric antibody fragments that depend on valency for activity. As a model system, a set of 13 phage-derived human Fab fragments were first selected against mouse and human recombinant hepatocyte growth factor/scatter factor (HGF/SF), a high molecular weight polypeptide growth factor related to the blood protease plasminogen and involved in development and cancer. These Fab fragments were subsequently screened for an effect on HGF/SF activity either as monomeric fragments or after dimerization with a monoclonal antibody (9E10) directed against a peptide tag on the fragments. Fab were identified that either inhibited or enhanced HGF/SF activity on target cell lines, but dimerization was required for this effect. The approach proposed should facilitate mass screening of phage-derived antibody fragments that depend on multiple valency for activity.

Evolution of plasminogen-related growth factors (HGF/SF and HGF1/MSP).

HGF/SF and HGF1/MSP define a novel growth factor family whose members share the domain structure and the proteolytic process of activation of the blood proteinase precursor plasminogen. The amino acid and RNA sequences of HGF/SF and HGF1/MSP, the intron-exon organization of their genes and the predicted 3D structure of individual domains indicate that HGF/SF and HGF1/MSP evolved along with plasminogen and other members of the kringle-serine proteinase (KSP) superfamily from an ancestral gene that contained a single copy of the kringle domain, a serine proteinase domain and an activation peptide connecting the two domains. A series of intragenic duplications of the kringle domain, gene duplications, exon shuffling and deletions is responsible for the genes currently present in mammals, avians and amphibians. Plasminogen, HGF/SF and HGF1/MSP represent paradigmatic examples of the modern, multi-domain proteins typically associated with vertebrate organisms and illustrate a novel evolutionary pathway that led to the emergence of molecules with growth regulatory activity from proteolytic enzymes.

Domain structure of hepatocyte growth factor/scatter factor (HGF/SF).

The modular structure of hepatocyte growth factor/scatter factor (HGF/SF) has facilitated structure-function analysis. Domain deletion experiments have established that the N-domain, kringle 1 and kringle 2 are essential for HGF/SF activity on target cells and that, conversely, truncated variants containing the N-domain and kringle 1 (NK1) or kringles 1 and 2 (NK2) exhibit partial agonistic or antagonistic activity depending on target cells and the presence of full length HGF/SF. The 3D structures of the six domains of HGF/SF have been modelled on the structure of homologues, offering interesting insights into putative mechanisms of domain interactions, receptor binding and activation. The predictions offered by such models are currently assessed by protein engineering techniques and will ultimately be measured against experimental structures.

Patients' perception of public, voluntary and private dispensaries in rural areas of Tanzania.

Eighty percent of rural dispensaries are run by the government and 19% by voluntary organisations that charge for some services. After the re-legalisation of the private health sector in 1991, private dispensaries are also emerging in villages. Privatisation is among the health reform policies of the country. Moreover, cost-sharing will be introduced at public dispensaries soon. Perception of 320 patients in the Coast Region of Tanzania on services delivered by the three health sectors has been investigated. Results show that patients are generally satisfied with the services and they would go back to the same dispensaries for treatment. Polydrug prescription was common in all sectors, while lack of prescribed drugs was a main complaint among public dispensaries patients. Voluntary dispensaries patients were less satisfied with long waiting time and with staff that did not give them enough information about the treatment. Currently, health service in public dispensaries is free but cost-sharing will be introduced soon. Most of voluntary and private dispensaries patients stated that the fees for service were moderate. The paper discusses the need for monitoring the implementation of cost sharing in public dispensaries to ensure equity in access to services by rural patients.

An approach to random mutagenesis of DNA using mixtures of triphosphate derivatives of nucleoside analogues.

We describe a new method for random mutagenesis of DNA based on the use of a mixture of triphosphates of nucleoside analogues. The method relies on DNA amplification in vitro with Taq polymerase and in the presence of the 5'-triphosphates of 6-(2-deoxy-beta-D-ribofuranosyl)-3,4-dihydro-8H-pyrimido-[4,5-C] [1,2]oxazin-7-one(dP) and of 8-oxo-2' deoxyguanosine (8-oxodG). The newly synthesised triphosphate derivative of dP (dPTP) is an excellent substrate for Taq polymerase (Km = 22 microM versus Km = 9.5 microM for TTP); it is incorporated in place of TTP and, with a approximately fourfold lower efficiency, in place of dCTP. After 30 cycles of DNA amplification, equimolar mixtures of the four normal dNTPs and dPTP yield the following frequencies of the four transition mutations: A-->G (4.4 x 10(-2), T-->C (4.3 x 10(-2), G-->A (1.1 x 10(-2) and C-->T (1.0 x 10(-2). The triphosphate derivative of 8-oxodG (8-oxodGTP) is incorporated opposite template adenine and yields two transition mutations (A-->C and T-->G) at frequencies of 0.8 x 10(-2) and 1.2 x 10(-2) respectively. Reaction mixtures containing dPTP and 8-oxodGTP results in both dP and 8-oxodG-induced mutations and an extensive array of codon changes in the absence of insertions and deletions. The method described differs from previous mutagenesis procedures in three respects: (1) it enables very high frequencies of base substitutions (up to 1.9 x 10(-1) (2) it allows control of the mutational load via the number of DNA amplification cycles and (3) it yields both transition and transversion mutations. The procedure may find application in the generation of libraries of DNA and protein mutants from which species with improved or novel activities may be selected.

Co-expression of the HGF/SF and c-met genes during early mouse embryogenesis precedes reciprocal expression in adjacent tissues during organogenesis.

Early experiments with cells in culture and recent targeting experiments have confirmed that the mesenchyme-derived growth factor hepatocyte growth factor/scatter factor (HGF/SF) is a paracrine agent that regulates the development of several epithelial and myogenic precursor cells during organogenesis. Here, we report the expression pattern of HGF/SF and its receptor, the product of the proto-oncogene c-met, during gastrulation and early organogenesis in mouse embryo. During gastrulation, the expression of HGF/SF and c-met overlaps. Initially the two genes are expressed in the endoderm and in the mesoderm along the rostro-intermediate part of the primitive streak and, later, in the node and in the notochord. Neither HGF/SF nor c-met is expressed in the ectodermal layer throughout gastrulation. During early organogenesis, overlapping expression of HGF/SF and c-met is found in heart, condensing somites and neural crest cells. However, a second and distinct pattern of expression, characterized by the presence of the ligand in mesenchymal tissues and the receptor in the surrounding ectoderm, is seen in the bronchial arches and in the limb buds. At 13 days postcoitum (d.p.c.), only this second pattern of expression is observed in differentiated somites and several major organs (i.e., lungs, liver, and gut). The expression of the HGF/SF and c-met genes throughout embryogenesis suggests a shift from an autocrine to a paracrine signaling system. The shift takes place in early organogenesis and implies different roles of HGF/SF in development. During gastrulation, HGF/SF may affect the fate of migrating mesodermal cells and may play a role in axis determination, whereas during organogenesis, the expression patterns of HGF/SF and its receptor reflect the recently established roles in the growth of certain epithelia and the migration of specific myogenic precursor cells.

Capabilities of public, voluntary and private dispensaries in basic health service provision.

A study is reported from the coastal region of the United Republic of Tanzania on the capabilities of public, voluntary and private dispensaries in the provision of peripheral health services. The implications for the future development and coordination of the different sectors are discussed.

PIP: Tanzania's primary health care strategy emphasizes the development of dispensaries and rural health centers, but economic, managerial, and organizational obstacles have delayed implementation of this plan. An evaluation of 18 dispensaries in the districts of Kibaha, Bagamoyo, and Kisarawe demonstrates that the voluntary and private sectors have worked closely with the public sector to strengthen services in this coastal region. To compare the quality of services offered by these three sectors, capability rates (the ratio of existing to expected resources) were calculated for each. The private dispensaries had more modern facilities, a broader range of auxiliary staff, and a higher infrastructure capability (86.1%) than public and voluntary facilities (both 76.1%). Capability for dealing with outpatient problems was 87.8% in the voluntary facilities, 71.2% in private ones, and 66.6% in public dispensaries. Overall, however, the highest capability for providing preventive and curative primary health care services was found in the public dispensaries. Capability in terms of the instruments necessary for delivery was 73.8% in the public dispensaries compared to 33.3% in voluntary and 14.2% in private facilities; the private dispensaries lacked delivery rooms and mother and child health rooms. All public facilities had the equipment necessary for immunizations; only half of the voluntary and none of the private ones were so equipped. The maternal-child health capabilities for the public, voluntary, and private dispensaries were 83.3%, 56.6%, and 13.3%, respectively. Sufficient numbers of mother and child aids were present only in the public sector. All the public dispensaries and two voluntary dispensaries offered at least one type of contraception; no family planning materials were found in the private facilities.

The structural repertoire of the human V kappa domain.

In humans, the gene for the V kappa domain is produced by the recombination of one of 40 functional V kappa segments and one of five functional J kappa segments. We have analysed the sequences of these germline segments and of 736 rearranged V kappa genes to determine the repertoire of main chain conformations, or canonical structures, they encode. Over 96% of the sequences correspond to one of four canonical structures for the first antigen binding loop (L1) and one canonical structure for the second antigen binding loop (L2). Junctional diversity produces some variation in the length of the third antigen binding loop (L3) and in the identity of residues at the V kappa-J kappa join. However, this is limited and 70% of the rearranged sequences correspond to one of three known canonical structures for the L3 region. Furthermore, we show that the canonical structures selected during the primary response are conserved during affinity maturation: the key residues that determine the conformations of the antigen binding loops are unmutated or undergo conservative mutation. The implications of these results for immune recognition are discussed.