Functional monoclonal antibody acts as a biased agonist by inducing internalization of metabotropic glutamate receptor 7.

BACKGROUND AND PURPOSE: The mGlu(7) receptors are strategically located at the site of vesicle fusion where they modulate the release of the main excitatory and inhibitory neurotransmitters. Consequently, they are implicated in the underlying pathophysiology of CNS diseases such as epilepsy and stress-related psychiatric disorders. Here, we characterized a selective, potent and functional anti-mGlu(7) monoclonal antibody, MAB1/28, that triggers receptor internalization. EXPERIMENTAL APPROACH: MAB1/28's activity was investigated using Western blot and direct immunofluorescence on live cells, in vitro pharmacology by functional cAMP and [(35) S]-GTPγ binding assays, the kinetics of IgG-induced internalization by image analysis, and the activation of the ERK1/2 by elisa. KEY RESULTS: mGlu(7) /mGlu(6) chimeric studies located the MAB1/28 binding site at the extracellular amino-terminus of mGlu(7) . MAB1/28 potently antagonized both orthosteric and allosteric agonist-induced inhibition of cAMP accumulation. The potency of the antagonistic actions was similar to the potency in triggering receptor internalization. The internalization mechanism occurred via a pertussis toxin-insensitive pathway and did not require Gα(i) protein activation. MAB1/28 activated ERK1/2 with potency similar to that for receptor internalization. The requirement of a bivalent receptor binding mode for receptor internalizations suggests that MAB1/28 modulates mGlu(7) dimers. CONCLUSIONS AND IMPLICATIONS: We obtained evidence for an allosteric-biased agonist activity triggered by MAB1/28, which activates a novel IgG-mediated GPCR internalization pathway that is not utilized by small molecule, orthosteric or allosteric agonists. Thus, MAB1/28 provides an invaluable biological tool for probing mGlu(7) function and selective activation of its intracellular trafficking.

Antiproliferative activity of the human IFN-alpha-inducible protein IFI44.

The interferon-alpha (IFN-alpha)-inducible protein IFI44 is associated with hepatitis C virus (HCV) infection, and its function is unknown. We show here in two human melanoma cell lines (ME15 and D10) that transcription starts 4 h after induction, and peak protein levels are reached 24 h after stimulation. We show by immunofluorescence, viral overexpression, and cellular fractionation that IFI44 is a cytoplasmic protein. Overexpression of IFI44 cDNA induces an antiproliferative state in vitro, even in cells that are not responsive to IFN-alpha. IFI44 contains a perfect GTP binding site but has no homology to known GTPases or G proteins. Based on these results, we propose a model in which IFI44 binds intracellular GTP, and this depletion abolishes extracellular signal-regulated kinase (ERK) signaling and results finally in cell cycle arrest.

High sensitivity analysis of amyloid-beta peptide composition in amyloid deposits from human and PS2APP mouse brain.

Cortical amyloid-beta (Abeta) deposition is considered essential in Alzheimer's disease (AD) and is also detectable in nondemented individuals with pathologic aging (PA). The present work presents a detailed analysis of the Abeta composition in various plaque types from human AD and PA cases, compared with plaque Abeta isolated from PS2APP mice. To determine minute amounts of Abeta from 30 to 50 laser-dissected amyloid deposits, we used a highly sensitive mass spectrometry procedure after restriction protease lysyl endopeptidase (Lys-C) digestion. This approach allowed the analysis of the amino-terminus and, including a novel ionization modifier, for the first time the carboxy-terminus of Abeta at a detection limit of approximately 200 fmol. In addition, full length Abeta 40/42 and pyroglutamate 3-42 were analyzed using a highly sensitive urea-based Western blot procedure. Generally, Abeta fragments were less accessible in human deposits, indicative of more posttranslational modifications. Thioflavine S positive cored plaques in AD were found to contain predominantly Abeta 42, whereas thioflavine S positive compact plaques and vascular amyloid consist mostly of Abeta 40. Diffuse plaques from AD and PA, as well as from PS2APP mice are composed predominantly of Abeta 1-42. Despite biochemical similarities in human and PS2APP mice, immuno-electron microscopy revealed an extensive extracellular matrix associated with Abeta fibrils in AD, specifically in diffuse plaques. Amino-terminal truncations of Abeta, especially pyroglutamate 3-40/42, are more frequently found in human plaques. In cored plaques we measured an increase of N-terminal truncations of approximately 20% between Braak stages IV to VI. In contrast, diffuse plaques of AD and PA cases, show consistently only low levels of amino-terminal truncations. Our data support the concept that diffuse plaques represent initial Abeta deposits but indicate a structural difference for Abeta depositions in human AD compared with PS2APP mice already at the stage of diffuse plaque formation.

Endothelin-1 and endothelin converting enzyme-1 in human atherosclerosis--novel targets for pharmacotherapy in atherosclerosis.

The role of chronic inflammation in the pathogenesis of the acute coronary syndromes has received increasing attention since active plaques rich in macrophages (Mphi's) are more prone to rupture whereas plaques rich in myofibroblasts are considered to be stable. Functionally, active plaques show a locally enhanced vasoreactivity. Endothelin-1 (ET-1) a potent vasoconstrictor acts in a paracrine fashion to regulate vascular tone. ET-1 is also produced by inflammatory cells suggesting a role for ET-1 in inflammation. Additionally, ET-1 is a mitogen. Endothelin converting enzyme-1 (ECE-1) activates ET-1 and may thus contribute to the regulation of vascular tone and cell growth during atherosclerosis. We evaluated the presence of ECE-1 and big ET-1/ET-1 and the activity of ECE-1 in different plaque types. Together with ET-1, ECE-1 is present in endothelial cells (ECs), vascular smooth muscle cells (VSMCs) and Mphi's. ECE-1 activity and ET-1-immunoreactivity (IR) both are upregulated during the progression of atherosclerosis from a non-inflammatory to an inflammatory stage. Thus, enhanced production of active ET-1 may contribute to cell growth and regulation of vascular tone in advanced plaques and also in very early stages of atherosclerosis. Furthermore, we examined the presence of ET-1 in coronary plaque tissue obtained by directional coronary atherectomy. ET-1 IR localized to plaque components indicative of chronic inflammation. Semiquantitative analysis of ET-1 IR revealed significantly higher staining grades in active coronary lesions compared with nonactive lesions. The increased ET-1 content in active coronary lesions may be beneficial to the stabilization of the vessel wall after plaque rupture and disadvantageous because it may lead to vasospasm and to the progression of atherosclerosis.

Inhibition of proliferation by 1-8U in interferon-alpha-responsive and non-responsive cell lines.

Interferon (IFN)-inducible proteins of the 1-8 gene family mediate homotypic adhesion and transduction of antiproliferative signals. Their induction correlates with inhibition of cell growth while they are often repressed in the course of malignant transformation and tumor development. Ras-mediated transformation of mouse mast cells is associated with downregulation of 1-8U expression and interferon-alpha (IFN-alpha) treatment reverts the proliferation rate to normal levels together with induction of 1-8U. Conversely, the antiproliferative responses of IFN-alpha in sensitive human melanoma cells are accompanied by 1-8U induction. Here we provide direct evidence that recombinant expression of 1-8U in human cell lines is sufficient to block cell proliferation. Based on the abundant expression and subcellular localization to the plasma membrane and exosome-like structures, we propose a model capable of explaining the pleiotropic functions of 1-8 family proteins in tumor cells and during normal development.

Coexpression of endothelin-converting enzyme-1 and endothelin-1 in different stages of human atherosclerosis.

BACKGROUND: Endothelin-converting enzyme (ECE)-1 activates endothelin-1 (ET-1) and may thus contribute to the regulation of vascular tone and cell growth during atherosclerosis. METHODS AND RESULTS: To evaluate ECE-1 immunoreactivity concerning big ET-1/ET-1, we performed qualitative and quantitative immunohistochemistry in normal internal mammary arteries (n=10), in coronary arteries with adaptive intimal fibrosis (n=10), in aortic fatty streaks (n=10), and in distinct regions of advanced carotid plaques (n=15). Furthermore, we determined ECE-1 activity in the control specimens and in the inflammatory intimal regions of carotid plaques. Double immunolabeling showed that ECE-1 was present in endothelial cells, vascular smooth muscle cells, and macrophages. All ET-1(+) cells were simultaneously ECE-1(+). Most importantly, there were significantly more ET-1(+) cells in the intima and media when atherosclerosis was in an inflammatory stage than when it was in a noninflammatory stage. Moreover, ECE-1 activity was upregulated in the intima of carotid plaques, although immunohistochemically, there were no significant differences between the number of ECE(+) cells in the different compartments of the arterial wall. CONCLUSION: Together with ET-1, ECE-1 is abundantly present in human arteries and at different stages of atherosclerotic plaque evolution. The upregulation of the ECE-1/ET-1 system is closely linked to the presence of chronic inflammation and is present in very early stages of plaque evolution. Therefore, enhanced production of active ET-1 may substantially contribute to cell growth and the regulation of vascular tone in advanced atherosclerotic lesions and in the very early stages of plaque evolution, when a plaque is still imperceptible clinically.

Cryosubstitution of frozen biological specimens in electron microscopy: use and application as an alternative to chemical fixation.

Frozen bacterial cells were low-temperature embedded after cryosubstitution at 185K in organic solvent. Temperature elevation of cryosubstituted E. coli cells, when still in organic solvent, had no effect on the preservation of chromatin structure. The achieved stabilization was found to be established independent of the presence of the chemical fixative. Cryosubstitution without the use of a chemical fixative allows for excellent preservation of cellular ultrastructure. Beyond that, the approach is preferential for sensitive antigens in immuno-electron microscopy. We conclude that low-temperature dehydration by cryosubstitution in organic solvents is able to form stable cross-links between macromolecules by hydrophobic interactions.

Self-assembly of beta-amyloid 42 is retarded by small molecular ligands at the stage of structural intermediates.

Assemblyof the amyloid-beta peptide (Abeta) into fibrils and its deposition in distinct brain areas is considered responsible for the pathogenesis of Alzheimer's disease (AD). Thus, inhibition of fibril assembly is a potential strategy for therapeutic intervention. Electron cryomicroscopy was used to monitor the initial, native assembly structure of Abeta42. In addition to the known fibrillar intermediates, a nonfibrillar, polymeric sheet-like structure was identified. A temporary sequence of supramolecular structures was revealed with (i) polymeric Abeta42 sheets during the onset of assembly, inversely related to the appearance of (ii) fibril intermediates, which again are time-dependently replaced by (iii) mature fibrils. A cell-based primary screening assay was used to identify compounds that decrease Abeta42-induced toxicity. Hit compounds were further assayed for binding to Abeta42, radical scavenger activity, and their influence on the assembly structure of Abeta42. One compound, Ro 90-7501, was found to efficiently retard mature fibril formation, while extended polymeric Abeta42 sheets and fibrillar intermediates are accumulated. Ro 90-7501 may serve as a prototypic inhibitor for Abeta42 fibril formation and as a tool for studying the molecular mechanism of fibril assembly.

Transfer of tissue factor from leukocytes to platelets is mediated by CD15 and tissue factor.

We describe thrombogenic tissue factor (TF) on leukocyte-derived microparticles and their incorporation into spontaneous human thrombi. Polymorphonuclear leukocytes and monocytes transfer TF(+) particles to platelets, thereby making them capable of triggering and propagating thrombosis. This phenomenon calls into question the original dogma that vessel wall injury and exposure of TF within the vasculature to blood is sufficient for the occurrence of arterial thrombosis. The transfer of TF(+) leukocyte-derived particles is dependent on the interaction of CD15 and TF with platelets. Both the inhibition of TF transfer to platelets by antagonizing the interaction CD15 with P-selectin and the direct interaction of TF itself suggest a novel therapeutic approach to prevent thrombosis.

Controlling polymerization of beta-amyloid and prion-derived peptides with synthetic small molecule ligands.

The Alzheimer beta-amyloid peptide (Abeta) and a fragment of the prion protein have the capacity of forming amyloid-like fibrils when incubated under physiological conditions in vitro. Here we show that a small amyloid ligand, RO-47-1816/001, enhances this process severalfold by binding to amyloid molecules and apparently promote formation of the peptide-to-peptide bonds that join the monomers of the amyloid fibrils. This effect could be antagonized by other ligands, including analogues of RO-47-1816/001, as well as the structurally unrelated ligand Congo red. Analogues of RO-47-1816/001 with low affinity for amyloid did not display any antagonistic effect. In conclusion, these data suggest that synthetic molecules, and possibly also small natural substances present in the brain, may act in a chaperone-like fashion, promoting Abeta polymerization and growth of amyloid fibrils in vitro and possibly also in vivo. Furthermore, we demonstrate that small organic molecules can be used to inhibit the action of amyloid-enhancing compounds.

Endogenous proteins controlling amyloid beta-peptide polymerization. Possible implications for beta-amyloid formation in the central nervous system and in peripheral tissues.

We report that certain plasma proteins, at physiological concentrations, are potent inhibitors of amyloid beta-peptide (Abeta) polymerization. These proteins are also present in cerebrospinal fluid, but at low concentrations having little or no effect on Abeta. Thirteen proteins representing more than 90% of the protein content in plasma and cerebrospinal fluid were studied. Quantitatively, albumin was the most important protein, representing 60% of the total amyloid inhibitory activity, followed by alpha1-antitrypsin and immunoglobulins A and G. Albumin suppressed amyloid formation by binding to the oligomeric or polymeric Abeta, blocking a further addition of peptide. This effect was also observed when the incorporation of labeled Abeta into genuine beta-amyloid in tissue section was studied. The Abeta and the anti-diabetic drug tolbutamide apparently bind to the same site on albumin. Tolbutamide displaces Abeta from albumin, increasing its free concentration and enhancing amyloid formation. The present results suggest that several endogenous proteins are negative regulators of amyloid formation. Plasma contains at least 300 times more amyloid inhibitory activity than cerebrospinal fluid. These findings may provide one explanation as to why beta-amyloid deposits are not found in peripheral tissues but are only found in the central nervous system. Moreover, the data suggest that some drugs that display an affinity for albumin may enhance beta-amyloid formation and promote the development of Alzheimer's disease.

Blood-borne tissue factor: another view of thrombosis.

Arterial thrombosis is considered to arise from the interaction of tissue factor (TF) in the vascular wall with platelets and coagulation factors in circulating blood. According to this paradigm, coagulation is initiated after a vessel is damaged and blood is exposed to vessel-wall TF. We have examined thrombus formation on pig arterial media (which contains no stainable TF) and on collagen-coated glass slides (which are devoid of TF) exposed to flowing native human blood. In both systems the thrombi that formed during a 5-min perfusion stained intensely for TF, much of which was not associated with cells. Antibodies against TF caused approximately 70% reduction in the amount of thrombus formed on the pig arterial media and also reduced thrombi on the collagen-coated glass slides. TF deposited on the slides was active, as there was abundant fibrin in the thrombi. Factor VIIai, a potent inhibitor of TF, essentially abolished fibrin production and markedly reduced the mass of the thrombi. Immunoelectron microscopy revealed TF-positive membrane vesicles that we frequently observed in large clusters near the surface of platelets. TF, measured by factor Xa formation, was extracted from whole blood and plasma of healthy subjects. By using immunostaining, TF-containing neutrophils and monocytes were identified in peripheral blood; our data raise the possibility that leukocytes are the main source of blood TF. We suggest that blood-borne TF is inherently thrombogenic and may be involved in thrombus propagation at the site of vascular injury.

The toxicity of the Alzheimer's beta-amyloid peptide correlates with a distinct fiber morphology.

In an attempt to elucidate the relationship among aggregation properties, fiber morphology, and cellular toxicity several beta-amyloid peptides (A beta) were prepared according to a standardized procedure. Peptides either carried mutations inside the membrane anchor segment around amino acid position 35 or their carboxy terminus was shortened from 42 to 41, 40, or 39 amino acids. The time-dependent self-assembly of monomeric A beta into fibers was simultaneously monitored by electron microscopy, circular dichroism spectroscopy, analytical ultracentrifugation, and A beta-mediated cellular toxicity using the reduction of 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) to measure cell viability. The transition of A beta monomers into fibers was analyzed by more than 600 electron micrographs. Distinct morphological changes from seed-like structures to immature and mature fibers were observed. Seeds were of spherical appearance. Immature fibers were typically elongated structures with a rough surface and with varying thickness depending on the A beta sequence. Mature fibers were characterized by a periodic variation of their thickness along the fiber axis. The proportion of these different structures and the total amount of aggregated A beta was amino acid sequence-dependent. Wild-type A beta 1-42 and its oxidized derivative carrying a methionine sulfoxide residue at position 35 showed the highest rate of fiber formation and exerted toxic activity in the MTT assay at very low nanomolar concentrations. The fibers formed by these two peptides were predominantly of the mature type. In contrast, carboxyl-terminus truncated peptides A beta 1-41, A beta 1-40, and A beta 1-39 or most A beta 1-42 derivatives mutated around amino acid position 35 showed a reduced aggregation rate, the immature fibers predominated, and the toxicity was orders of magnitude lower. Thus, a correlation can be drawn among the chemical structure, aggregation properties, fiber morphology, and cellular toxicity.

Integrin alphaIIb beta3 reconstituted into lipid bilayers is nonclustered in its activated state but clusters after fibrinogen binding.

Integrin activation, ligand binding, and integrin clustering were analyzed using alphaIIb beta3 reconstituted into phospholipid vesicles and into supported planar lipid bilayers. Strong and specific binding of fibrinogen and the gamma-chain dodecapeptide of fibrinogen to alphaIIb beta3 indicated that the integrin is in an activated state after membrane reconstitution. Cryoelectron and fluorescence microscopy suggested a nonclustered state of the protein in the vesicle membrane. Supported planar lipid membranes were generated by fusion of vesicles in which approximately equal fractions of integrins were pointing inside-out and outside-in. This distribution led to an immobilization of about 40% of the integrin in supported bilayers due to attachment of the large extracellular domains to the quartz support. Fluorescence recovery after photobleaching indicated a diffusion coefficient of D = (0.70 +/- 0.06) x 10(-8) cm2/s, consistent with a nonclustered state of the mobile integrin. Upon fibrinogen binding, the integrins became immobile, and fluorescence micrographs showed a patchy distribution of fibrinogen-integrin complexes consisting of approximately 250 molecules. In addition to the expected dimer formation by bivalent fibrinogen, additionally induced fibrinogen clustering may account for the large size of the complexes. In contrast, binding of monovalent GRGDS pentapeptide or the gamma-chain dodecapeptide of fibrinogen altered neither the mobile fraction nor the association state of alphaIIb beta3. Our data indicate that integrin alphaIIbb3 is activated while monodisperse, and became clustered upon fibrinogen binding, leading to an irreversibly bound state.

Dioleoylmelittin as a novel serum-insensitive reagent for efficient transfection of mammalian cells.

Amphipathic peptides can be useful effectors to enhance gene delivery. However, peptide/DNA complexes usually require additional effectors, such as fusogenic lipids, to mediate efficient transfection. Due to weak and/or multiple interactions between the various components of the system, the transfecting complexes are often heterogeneous and unstable in biological fluids. Accordingly, a hybrid molecule resulting from the covalent coupling of an amphipathic, membrane-disturbing peptide to a lipid moiety might create a stable and efficient peptide-based gene transfer system. The present work describes such a novel hybrid molecule, dioleoylmelittin, resulting from the conjugation of dioleoylphosphatidylethanolamine-N-[3-(2-pyridyldithio)propionate] with [Cys1]melittin. Dioleoylmelittin had a lower hemolytic and membrane-disturbing activity than melittin. Size and zeta potential measurements, DNA gel electrophoresis, and electron microscopy showed that dioleoylmelittin, unlike melittin, was able to complex plasmid DNA to form spherical particles with a net positive charge and a diameter between 50 and 250 nm. These particles, prepared at an optimal 10/1 dioleoylmelittin/DNA ratio (w/w), mediated efficient transient transfection of reporter genes in cultured mammalian cells including primary cells. The luciferase activity induced by the dioleoylmelittin/DNA complex was 5-500-fold higher than that induced by a cationic lipid/DNA complex, depending on the cationic lipid and the cell-line. Surprisingly, the presence of 10-50% fetal calf serum during dioleoylmelittin-mediated transfection enhanced 1.5-3-fold gene expression. Dioleoylmelittin represents a new class of efficient peptide-based transfection reagents, especially suited for serum-sensitive cells.

Large and small splice variants of collagen XII: differential expression and ligand binding.

Collagen XII has a short collagenous tail and a very large, three-armed NC3 domains consisting primarily of fibronectin type III repeats. Differential splicing within this domain gives rise to a large (320 kD) and a small (220 kD) subunit; the large but not the small can carry glycosaminoglycan. To investigate whether collagen XII variants have distinct expression patterns and functions, we generated antibody and cDNA probes specific for the alternatively spliced domain. We report here that the large variant has a more restricted expression in embryonic tissue than the small. For example, whereas the small variant is widespread in the dermis, the large is limited to the base of feather buds. Distinct proportions of mRNA for the two variants were detected depending on the tissue. Monoclonal antibodies allowed us to separate collagen XII variants, and to show that homo- and heterotrimers exist. Collagen XII variants differ in ligand binding. Small subunits interact weakly with heparin via their COOH-terminal domain. Large subunits have additional, stronger heparin-binding site(s) in their NH2-terminal extra domain. In vivo, both large and small collagen XII are associated with interstitial collagen. Here we show biochemically and ultrastructurally that collagen XII can be incorporated into collagen I fibrils when it is present during, but not after, fibril formation. Removal of the collagenous domain of collagen XII reduces its coprecipitation with collagen I. Our results indicate that collagen XII is specifically associated with fibrillar collagen, and that the large variant has binding sites for extracellular ligands not present in the small variant.

Immunostaining of DNA in electron microscopy: an amplification and staining procedure for thin sections as alternative to gold labeling.

We describe a new electron microscopic on-section staining technique with high specificity and sensitivity for DNA-containing structures. Lowicryl HM20 sections of specimens obtained by cryofixation and freeze-substitution are incubated in a first step with a primary IgM antibody specific for double-stranded DNA. The layer of bound antibodies at the section surface is amplified in a successive step by a secondary IgG antibody. Finally, electron scattering of the antibody layer produced is enhanced by staining with a mixture of uranyl acetate and potassium permanganate. The applicability of the method is exemplified by the detection of shape and distribution of various types of bacterial and eukaryotic chromatin.

In situ localization of the 60 k protein of Helicobacter pylori, which belongs to the family of heat shock proteins, by immuno-electron microscopy.

The groEl homologue of Helicobacter pylori was isolated and characterized by means of immunoelectron microscopy, after cryosectioning. The 60 k protein was isolated from Helicobacter pylori by treatment of the cells with 2-butanol and purified by anion exchange chromatography. The native molecular weight of the 60 k protein was estimated to be 420 k by size exclusion chromatography. The purified 60 k protein showed the typical rotational symmetry of chaperonins when analyzed by electron microscopy. Ultrathin sections of Helicobacter pylori were immunostained by a polyclonal antibody directed against the hsp-65 of Mycobacterium tuberculosis. The label revealed a clustered localization of the 60 k protein on the cell surface as well as in the periplasmic space.

Concentration evaluation of chromatin in unstained resin-embedded sections by means of low-dose ratio-contrast imaging in STEM.

Quantitative STEM with the imaging mode of ratio-contrast was investigated in order to evaluate the local concentration of DNA in situ for different kinds of DNA plasms in terms of intracellular packing densities (p.d.). The ability of ratio imaging to suppress thickness variations provided the basis to use unstained sections from cryofixed and freeze-substituted material. The DNA p.d. within the nucleoid of E. coli was determined to be about 100 mg ml-1. Quantitative data concerning the p.d. of DNA in condensed eukaryotic chromatin assuming equal amounts of DNA and protein were evaluated for the first time: approximately 400 mg ml-1 chromatin which corresponds to 200 mg ml-1 DNA. The p.d. of DNA in chromosomes from the dinoflagellate Amphidinium carterae, a eukaryote devoid of histones and with only small relative amounts of histone-like protein, was also found to be of the order of 200 mg ml-1. The highest p.d. of DNA was measured for the head of the bacteriophage T4 with more than 800 mg ml-1, in fair agreement with previous calculations. The results provide further support for a condensation mode of low protein chromatins that involves a liquid-crystalline organization of the DNA filaments.

The inactive form of recA protein: the 'compact' structure.

When recA protein is enzymatically inactive in vitro, it adopts a more compact helical polymer form than that of the active protein polymerized onto DNA in the presence of ATP. Here we describe some aspects of this structure. By cryo-electron microscopy, a pitch of 76 A is found for both the self-polymer and the inactive complex with ssDNA. A smaller pitch of 64 A is observed in conventional electron micrographs. The contour length of complexes with ssDNA was used to estimate the binding stoichiometry in the compact complex, 6 +/- 1 nt/recA. In addition, the compact structure was observed in vivo in Escherichia coli: inclusion bodies produced upon induction of recA expression in an overproducing strain have a fibrous morphology with the structural parameters of the compact polymer.