Functional contributions of noncysteine residues within the cystine knots of human chorionic gonadotropin subunits.

Human chorionic gonadotropin (hCG) is a heterodimeric member of a family of cystine knot-containing proteins that contain the consensus sequences Cys-X(1)-Gly-X(2)-Cys and Cys-X(3)-Cys. Previously, we characterized the contributions that cystine residues of the hCG subunit cystine knots make in folding, assembly, and bioactivity. Here, we determined the contributions that noncysteine residues make in hCG folding, secretion, and assembly. When the X(1), X(2), and X(3) residues of hCG-alpha and -beta were substituted by swapping their respective cystine knot motifs, the resulting chimeras appeared to fold correctly and were efficiently secreted. However, assembly of the chimeras with their wild type partner was almost completely abrogated. No single amino acid substitution completely accounted for the assembly inhibition, although the X(2) residue made the greatest individual contribution. Analysis by tryptic mapping, high performance liquid chromatography, and SDS-polyacrylamide gel electrophoresis revealed that substitution of the central Gly in the Cys-X(1)-Gly-X(2)-Cys sequence of either the alpha- or beta-subunit cystine knot resulted in non-native disulfide bond formation and subunit misfolding. This occurred even when the most conservative change possible (Gly --> Ala) was made. From these studies we conclude that all three "X" residues within the hCG cystine knots are collectively, but not individually, required for the formation of assembly-competent hCG subunits and that the invariant Gly residue is required for efficient cystine knot formation and subunit folding.

Intracellular folding pathway of the cystine knot-containing glycoprotein hormone alpha-subunit.

Three of the five disulfide bonds in the glycoprotein hormone alpha-subunit (GPH-alpha) form a cystine knot motif that stabilizes a three-loop antiparallel structure. Previously, we described a mutant (alpha(k)) that contained only the three knot disulfide bonds and demonstrated that the cystine knot was necessary and sufficient for efficient GPH-alpha folding and secretion. In this study, we used alpha(k) as a model to study the intracellular GPH-alpha folding pathway. Cystine knot formation proceeded through a 1-disulfide intermediate that contained the 28-82 disulfide bond. Formation of disulfide bond 10-60, then disulfide bond 32-84, followed the formation of 28-82. Whether the two non-cystine knot bonds 7-31 and 59-87 could form independent of the knot was also tested. Disulfide bond 7-31 formed rapidly, whereas 59-87 did not form when all cysteine residues of the cystine knot were converted to alanine, suggesting that 7-31 forms early in the folding pathway and that 59-87 forms during or after cystine knot formation. Finally, loop 2 of GPH-alpha has been shown to be very flexible, suggesting that loop 2 does not actively drive GPH-alpha folding. To test this, we replaced residues 36-55 in the flexible loop 2 with an artificially flexible glycine chain. Consistent with our hypothesis, folding and secretion were unaffected when loop 2 was replaced with the glycine chain. Based on these findings, we describe a model for the intracellular folding pathway of GPH-alpha and discuss how these findings may provide insight into the folding mechanisms of other cystine knot-containing proteins.

Cystine knot mutations affect the folding of the glycoprotein hormone alpha-subunit. Differential secretion and assembly of partially folded intermediates.

The common glycoprotein hormone alpha-subunit (GPH-alpha) contains five intramolecular disulfide bonds, three of which form a cystine knot motif (10-60, 28-82, and 32-84). By converting each pair of cysteine residues of a given disulfide bond to alanine, we have studied the role of individual disulfide bonds in GPH-alpha folding and have related folding ability to secretion and assembly with the human chorionic gonadotropin beta-subunit (hCG-beta). Mutation of non-cystine knot disulfide bond 7-31, bond 59-87, or both (leaving only the cystine knot) resulted in an efficiently secreted folding form that was indistinguishable from wild type. Conversely, the cystine knot mutants were inefficiently secreted (<25%). Furthermore, mutation of the cystine knot disulfide bonds resulted in multiple folding intermediates containing 1, 2, or 4 disulfide bonds. High performance liquid chromatographic separation of intracellular and secreted forms of the folding intermediates demonstrated that the most folded forms were preferentially secreted and combined with hCG-beta. From these studies we conclude that: (i) the cystine knot of GPH-alpha is necessary and sufficient for folding and (ii) there is a direct correlation between the extent of GPH-alpha folding, its ability to be secreted, and its ability to heterodimerize with hCG-beta.

A naturally occurring genetic variant in the human chorionic gonadotropin-beta gene 5 is assembly inefficient.

The hCGbeta gene family is composed of six homologous genes linked in tandem repeat on chromosome 19; the order of the genes is 7, 8, 5, 1, 2, and 3. Previous studies have shown that hCGbeta gene 5 is highly expressed during the first trimester of pregnancy. The purpose of our study was to identify naturally occurring polymorphisms in hCGbeta gene 5 and determine whether these alterations affected hCG function. The data presented here show that hCGbeta gene 5 was highly conserved in the 334 asymptomatic individuals and 41 infertile patients examined for polymorphisms using PCR followed by single stranded conformational polymorphism analysis. Most of the polymorphisms detected were either silent or located in intron regions. However, one genetic variant identified in beta gene 5 exon 3 was a G to A transition that changed the naturally occurring valine residue to methionine in codon 79 (V79M) in 4.2% of the random population studied. The V79M polymorphism was always linked to a silent C to T transition in codon 82 (tyrosine). To determine whether betaV79M hCG had biological properties that differed from those of wild-type hCG, a beta-subunit containing the V79M substitution was created by site-directed mutagenesis and was coexpressed with the glycoprotein hormone alpha-subunit in Chinese hamster ovary cells and 293T cells. When we examined betaV79M hCG biosynthesis, we detected atypical betaV79M hCG folding intermediates, including a betaV79M conformational variant that resulted in a beta-subunit with impaired ability to assemble with the alpha-subunit. The inefficient assembly of betaV79M hCG appeared to be independent of beta-subunit glycosylation or of the cell type studied, but, rather, was due to the inability of the betaV79M subunit to fold correctly. The majority of the V79M beta-subunit synthesized was secreted as unassembled free beta. Although the amount of alphabeta hCG heterodimer formed and secreted by betaV79M-producing cells was less than that by wild-type beta-producing cells, the hCG that was secreted as alphabeta V79M heterodimer exhibited biological activity indistinguishable from that of wild-type hCG.

Dissociation of early folding events from assembly of the human lutropin beta-subunit.

The human LH of the anterior pituitary is a member of the glycoprotein hormone family that includes FSH, TSH, and placental CG. All are noncovalently bound heterodimers that share a common alpha-subunit and beta-subunits that confer biological specificity. LHbeta and CGbeta share more than 80% amino acid sequence identity; however, in transfected Chinese hamster ovary (CHO) cells, LHbeta assembles with the alpha-subunit more slowly than does hCGbeta, and only a fraction of the LHbeta synthesized is secreted, whereas CGbeta is secreted efficiently. To understand why the assembly and secretion of these related beta-subunits differ, we studied the folding of LHbeta in CHO cells transfected with either the LHbeta gene alone, or in cells cotransfected with the gene expressing the common alpha-subunit, and compared our findings to those previously seen for CG. We found that the rate of conversion of the earliest detectable folding intermediate of LH, pbeta1, to the second major folding form, pbeta2, did not differ significantly from the pbeta1-to-pbeta2 conversion of CGbeta, suggesting that variations between the intracellular fates of the two beta-subunits cannot be explained by differences in the rates of their early folding steps. Rather, we discovered that unlike CGbeta, where the folding to pbeta2 results in an assembly-competent product, apparently greater than 90% of the LH pbeta2 recovered from LHbeta-transfected CHO cells was assembly incompetent, accounting for inefficient LHbeta assembly with the alpha-subunit. Using the formation of disulfide (S-S) bonds as an index, we observed that, in contrast to CGbeta, all 12 LHbeta cysteine residues formed S-S linkages as soon as pbeta2 was detected. Attempts to facilitate LH assembly with protein disulfide isomerase in vitro using LH pbeta2 and excess urinary alpha-subunit as substrate were unsuccessful, although protein disulfide isomerase did facilitate CG assembly in this assay. Moreover, unlike CGbeta, LHbeta homodimers were recovered from transfected CHO cells. Taken together, these data suggest that differences seen in the rate and extent of LH assembly and secretion, as compared to those of CG, reflect conformational differences between the folding intermediates of the respective beta-subunits.

Human chorionic gonadotropin-beta gene expression in first trimester placenta.

The hCGbeta gene family contains six genes linked in tandem on chromosome 19 and labeled beta genes 7, 8, 5, 1, 2, and 3. Previous studies on a small number of placentas have indicated that beta gene 5 was the most highly expressed gene during the first trimester of pregnancy, followed by genes 3 and 8. Beta genes 7, 1, and 2 were expressed at very low levels. The purpose of this study was to determine 1) whether this pattern of expression was typical during normal pregnancy by sampling a large number of first trimester placentas, and 2) whether there was a correlation between gestational age and the pattern of hCGbeta gene expression. Total RNA from 27 first trimester placentas varying in age from 6-16 weeks was reverse transcribed into complementary DNA. The complementary DNA was amplified by PCR, and the amount of DNA representative of each hCGbeta gene was quantified by Genescan analysis. In 14 of the 27 placentas, hCGbeta gene 5 accounted for 50% or more of the total beta messenger RNA expressed. Beta gene 3 was expressed at levels ranging from 1-42% of the total, and beta gene 8 expression ranged from 12-32% of the total. Gene 7 expression was less than 3% of the total beta expression in all 27 placentas. Although there appeared to be a trend toward lower expression of beta gene 3 in placentas beyond 10 weeks gestational age, there was no correlation of the pattern of beta expression with placental age. Beta gene expression was also examined in two blighted ova, a spontaneous abortion sample, and a hydatidiform mole as well as in cultured JAR choriocarcinoma cells. With the exception of JAR cells, these abnormal tissues had low levels of gene 3 expression, but these levels were within the range of the patterns observed in normal placentas. These data suggest that it is the total amount of hCGbeta gene expression rather than the expression of individual beta genes that is important for the maintenance of normal pregnancy.

Novel covalent chaperone complexes associated with human chorionic gonadotropin beta subunit folding intermediates.

Molecular chaperones facilitate the folding of proteins in the endoplasmic reticulum (ER) of mammalian cells. The glycoprotein hormone chorionic gonadotropin beta subunit is a secretory protein whose folding in the ER has been demonstrated (Huth, J. R., Mountjoy, K., Perini, F., and Ruddon, R. W.(1992) J. Biol. Chem. 267, 8870-8879). Because folding of wild type hCG-beta subunit occurs in the ER with a t1/2 = 4-5 min, stable association of ER chaperones with hCG-beta have been difficult to detect probably because they have a short half-life. However, beta-chaperone complexes containing the ER chaperones BiP, ERp72, and ERp94 have been detected in slow folding mutants of hCG-beta subunit that lack both of the N-linked oligosaccharides (Feng, W., Matzuk, M. M., Mountjoy, K., Bedows, E., Ruddon, R. W., and Boime, I. (1995) J. Biol. Chem. 270, 11851-11859). The questions addressed here are 1) whether the detection of chaperone-containing complexes is related to the absence of carbohydrate or to the rate of hCG-beta subunit folding, 2) whether such complexes are dead-end or whether they lead to formation of a secreted, mature hCG-beta form, and 3) what the nature of the hCG-beta-chaperone binding is. The data obtained indicate that the amount of detectable hCG-beta-chaperone complexes correlates with the rate or extent of folding, that the complexes of hCG-beta with ER chaperones lead to the formation of secretable beta, and that the complexes of hCG-beta with chaperones involve the formation of intermolecular disulfide bonds.

Protein folding in the endoplasmic reticulum: lessons from the human chorionic gonadotropin beta subunit.

There have been few studies of protein folding in the endoplasmic reticulum of intact mammalian cells. In the one case where the in vivo and in vitro folding pathways of a mammalian secretory protein have been compared, the folding of the human chorionic gonadotropin beta subunit (hCG-beta), the order of formation of the detected folding intermediates is the same. The rate and efficiency with which multidomain proteins such as hCG-beta fold to native structure in intact cells is higher than in vitro, although intracellular rates of folding of the beta subunit can be approached in vitro in the presence of an optimal redox potential and protein disulfide isomerase. Understanding how proteins fold in vivo may provide a new way to diagnose and treat human illnesses that occur due to folding defects.

Phase I trial of an antisense oligonucleotide OL(1)p53 in hematologic malignancies.

PURPOSE: The phosphoprotein p53 is involved in transcriptional regulation and is detected in hematologic malignancies. In vitro incubation of acute myelogenous leukemia with OL(1)p53, a 20-mer phosphorothioate oligonucleotide complementary to p53 mRNA, results in leukemic cell death. A phase I dose-escalating trial was conducted to determine the toxicity of OL(1)p53 following systemic administration to patients with hematologic malignancies. PATIENTS AND METHODS: Sixteen patients with either refractory acute myelogenous leukemia (n = 6) or advanced myelodysplastic syndrome (n = 10) participated in the trial. Patients were given OL(1)p53 at doses of 0.05 to 0.25 mg/kg/h for 10 days by continuous intravenous infusion. RESULTS: No specific toxicity was directly related to the administration of OL(1)p53. One patient developed transient nonoliguric renal failure. One patient died of anthracycline-induced cardiac failure. Approximately 36% of the administered dose of OL(1)p53 was recovered intact in the urine. Plasma concentrations and area under the plasma concentration curves were linearly correlated with dose. Leukemic cell growth in vitro was inhibited as compared with pretreatment samples. There were no clinical complete responses. CONCLUSION: A phosphorothioate oligonucleotide, OL(1)p53, can be administered systemically without complications. This type of modified oligonucleotide can be administered without complete degradation, as it was recovered from the urine intact. This oligonucleotide may be useful in combination with currently available chemotherapy agents for the treatment of malignancies.

A linear 23-residue peptide reveals a propensity to form an unusual native-like conformation.

To gain insight into the earliest events of protein folding, a 23-residue peptide with a sequence corresponding to the 38-60 fragment of the beta-subunit of human chorionic gonadotropin (hCG beta) was studied by NMR. In aqueous solution the majority of the peptide residues adopted an extended polyproline II (PII) conformation similar to those in mature, fully folded hCG beta. The finding that the isolated protein fragment may acquire native-like structural motifs, even without alpha-helices or beta-structures, extends the possibility of using free peptides as model systems to better understand the protein folding mechanisms. It was shown that the PII-rich structural motif can be determined efficiently by NMR spectroscopy. The observation that in the absence of extensive medium- and long-range interactions the majority of amino acid residues may adopt the PII conformation suggests that the PII-rich structural motifs may play an important role in early events of protein folding.

The asparagine-linked oligosaccharides of the human chorionic gonadotropin beta subunit facilitate correct disulfide bond pairing.

The role of asparagine (N)-linked oligosaccharide chains in intracellular folding of the human chorionic gonadotropin (hCG)-beta subunit was determined by examining the kinetics of folding in Chinese hamster ovary (CHO) cells transfected with wild-type or mutant hCG-beta genes lacking one or both of the asparagine glycosylation sites. The half-time for folding of p beta 1 into p beta 2, the rate-determining step in beta folding, was 7 min for wild-type beta but 33 min for beta lacking both N-linked glycans. The p beta 1-->p beta 2 half-time was 7.5 min in CHO cells expressing the beta subunit missing the Asn13-linked glycan and 10 min for the beta subunit missing the Asn30-linked glycan. The inefficient folding of hCG-beta lacking both N-linked glycans correlated with the slow formation of the last three disulfide bonds (i.e. disulfides 23-72, 93-100, and 26-110) to form in the hCG-beta-folding pathway. Unglycosylated hCG-beta was slowly secreted from CHO cells, and beta subunit-folding intermediates retained in cells for more than 5 h were degraded into a hCG-beta core fragment-like protein. However, coexpression of the hCG-alpha gene enhanced folding and formation of disulfide bonds 23-72, 93-100, and 26-110 of hCG-beta lacking N-linked glycans. In addition, the molecular chaperones BiP, ERp72, and ERp94, but not calnexin, were found in a complex with unglycosylated, unfolded hCG-beta and may be involved in the folding of this beta form. These data indicate that N-linked oligosaccharides assist hCG-beta subunit folding by facilitating disulfide bond formation.

Asparagine-linked oligosaccharides facilitate human chorionic gonadotropin beta-subunit folding but not assembly of prefolded beta with alpha.

To determine the role of asparagine (N)-linked oligosaccharide chains in protein folding and assembly, the well established hCG-beta in vitro folding and assembly assays were used to analyze how the human CG (hCG) beta-subunit devoid of one or two N-linked glycans folds and assembles under different conditions. Two approaches were used: 1) site-specific mutagenesis of hCG-beta synthesized in Chinese hamster ovary cells transfected with beta-mutants lacking the asparagine glycosylation sites; and 2) enzymatic deglycosylation of hCG-beta synthesized in JAR cells with peptide N-glycosidase F or endoglycosidase H. In both cases, [35S]cysteine-labeled beta-subunits were used as substrates to measure the conversion of the hCG-beta folding intermediate p beta 1 into p beta 2 and assembly of p beta 2 with urinary alpha. Using the mutated substrates from Chinese hamster ovary cells, it was found that 60% of wild-type p beta 1 (two N-linked glycans), 60% of p beta 1 missing the Asn13-linked glycan, 40% of p beta 1 missing the Asn30-linked glycan, and 10% of p beta 1 missing two N-linked glycans were converted to the corresponding p beta 2, respectively. With the enzymatically deglycosylated substrate from JAR cells, 90% of p beta 1 (two N-linked glycans), 70% of p beta 1(1) (one N-linked glycan), and 10% of p beta 1(0) (without N-linked glycan) folded into p beta 2 under cysteamine and cystamine redox conditions with or without protein disulfide isomerase. These data demonstrate that at least one N-linked glycan is required for efficient folding of hCG-beta and that the Asn30-linked glycan is more important than Asn13-linked glycan for hCG-beta folding. It also was shown that the composition of N-linked glycans of hCG-p beta 1 did not change protein folding, since hCG-beta substrates with high mannose oligosacharides folded as efficiently as beta-substrates containing sialylated complex oligosaccharides. Moreover, assembly of the already folded, assembly-component folding intermediate, p beta 2, was not affected by removal of one or both of the N-linked glycans of the beta-subunit. These data thus show that N-linked glycans play their most important role in the folding component of the folding and assembly pathway for hCG-beta.

Bacterial expression and in vitro folding of the beta-subunit of human chorionic gonadotropin (hCG beta) and functional assembly of recombinant hCG beta with hCG alpha.

A bacterial expression system for the beta-subunit of hCG (hCG beta) has been developed to produce suitable amounts of this protein for structural and biological studies. To produce hCG beta in Escherichia coli, the nucleotide sequence that encodes the amino acid leader sequence was removed from the hCG beta complementary DNA, and the gene was cloned into a pET expression vector. After induction of protein synthesis in host bacteria, recombinant hCG beta (rhCG beta) accumulated in inclusion bodies in an unfolded state. The inclusion bodies were purified from induced cultures of E. coli, solubilized in urea, and fractionated by reverse phase HPLC. In this way, 6-7 mg unfolded hCG beta were recovered from 1 liter culture, rhCG beta was folded in the presence of 6.4 mM cysteamine and 3.6 mM cystamine at pH 8.7 at a final concentration of 0.02 mg/ml protein. The folded protein assembled with urinary hCG alpha and the purified rhCG beta/urinary alpha dimer bound to and activated the human LH/CG receptor permanently expressed in a cell line, indicating that it was a functional hormone. The rhCG beta/urinary alpha dimer also stimulated in vivo ovulation in rats, thus confirming the biological activity of bacterially expressed hCG beta. Because E. coli lacks the ability to glycosylate proteins, these activity results indicate that the N-linked and O-linked oligosaccharides of hCG beta are not required for protein folding, subunit assembly, or full biological activity. The success of producing hCG beta in bacteria and of folding it in vitro implies that the beta-subunits of the other members of the glycoprotein hormone family, LH, FSH, and TSH, can also be produced in this manner. This may facilitate structural studies of these hormones as well as lead to the production of recombinant hormones for biological studies and clinical use.

Redox conditions for stimulation of in vitro folding and assembly of the glycoprotein hormone chorionic gonadotropin.

The formation of native disulfide bonds during in vitro protein folding can be limiting in obtaining biologically active proteins. Thus, optimization of redox conditions can be critical in maximizing the yield of renatured, recombinant proteins. We have employed a folding model, that of the beta subunit of human chorionic gonadotropin (hCG- beta), to investigate in vitro oxidation conditions that facilitate the folding of this protein, and have compared the in vitro rates obtained with the rate of folding that has been observed in intact cells. Two steps in the folding pathway of hCG-beta were investigated: the rate-limiting events in the folding of this protein, and the assembly of hCG-beta with, hCG-alpha. The rates of these folding events were determined with and without protein disulfide isomerase (PDI) using two different types of redox reagents: cysteamine and its oxidized equivalent, cystamine, and reduced and oxidized glutathione. Rates of the rate-limiting folding events were twofold faster in cysteamine/cystamine redox buffers than in glutathione buffers in the absence of PDI. Optimal conditions for hCG-beta folding were attained in a 2 mM glutathione buffer, pH 7.4, that contained 1 mg/mL PDI and in 10 microM cysteamine/cystamine, pH 8.7, without PDI. Under these conditions, the half-time of the ratelimiting folding event was 16 to 20 min and approached the rate observed in intact cells (4 to 5 min). Moreover, folding of the beta subunit under these conditions yields a functional protein, based on its ability to assemble with the alpha subunit. The rates of assembly of hCG-beta with hCG-alpha in the cysteamine/cystamine or glutathione/PDI redox buffers were comparable (t(1/2/sb> = 9 to 12 min)). These studies show that rates of folding and assembly events that involve disulfide bond formation can be optimized by a simple buffer system composed of cysteamine and cystamine.

Misfolded human chorionic gonadotropin beta subunits are secreted from transfected Chinese hamster ovary cells.

There are six intramolecular disulfide (S-S) bonds that form during intracellular folding of the human chorionic gonadotropin (hCG)-beta subunit. Site-directed mutagenesis of every pair of Cys residues involved in the formation of each S-S bond was used to examine the roles that S-S bonds play in beta subunit folding and secretion. Tryptic maps of secreted hCG-beta showed that only one S-S bond formed in all S-S bond mutants that failed to fold from the earliest detectable beta folding intermediate, p beta 1, into a second major intermediate, p beta 2 (C34A-C88A, C38A-C57A or C9A-C90A mutants), whereas all 5 remaining S-S bonds formed in mutants when p beta 1-->p beta 2 conversion occurred (C23A-C72A, C93A-C100A, or C26A-C110A mutants). Nonreducing SDS-polyacrylamide gel electrophoresis showed that beta multimers were secreted from cells expressing S-S bond mutations where the folding of p beta 1-->p beta 2 was blocked. However, for mutations where p beta 1-->p beta 2 conversion was efficient, beta monomers rather than multimers were secreted. For all cell lines studied, secreted hCG-beta migrated as monomeric beta during reducing SDS-polyacrylamide gel electrophoresis, indicating that hCG-beta multimers formed via intermolecular cross-linking of unpaired thiols. Tryptic maps of hCG-beta isolated from mutants lacking the 34-88 bond, where > 80% turnover occurs, showed that only the 38-57 S-S bond formed. beta Subunits lacking the 9-90 linkage also have only S-S bond 38-57 formed, but < 10% turnover of C9A-C90A hCG-beta occurs. Thus, subtle conformational differences between partially folded or misfolded beta subunits may determine whether hCG-beta is degraded, or undergoes intracellular translocation and secretion.

The effect of garlic extract on human metabolism of acetaminophen.

Several studies suggest that the constituents of garlic may inhibit experimentally induced carcinogenesis. To evaluate the chemopreventive properties of garlic in humans, the effects of chronic administration of an aged garlic extract on the disposition of acetaminophen and metabolites were studied. This commonly used drug was chosen because it forms a reactive electrophilic metabolite after oxidative metabolism. Sixteen subjects ingested daily doses of garlic extract (approximately equivalent to six to seven cloves of garlic) for 3 months. Before the course of garlic, at the end of each month and 1 month after termination of garlic administration, a 1-g oral dose of acetaminophen was given to each subject. Plasma and urine were measured for acetaminophen and the glucuronide, sulfate, cysteinyl, mercapturate, and methylthio metabolites. It was found that garlic treatment had no discernible effect on oxidative metabolism but was associated with a slight increase in sulfate conjugation of drug. These findings suggest that garlic extract has limited potential as a chemopreventive agent.

Expression of laminin chains during myogenic differentiation.

Expression of two Ae-related chains of the extracellular matrix glycoprotein laminin was induced as multipotent C3H10T1/2 mouse embryo fibroblasts differentiated into myoblasts and myofibers. C3H10T1/2 fibroblasts expressed the B1e (M(r) = 215,000) and B2e (M(r) = 205,000) laminin chains based on metabolic radiolabeling, immunoprecipitation, peptide mapping, and mRNA analysis. In contrast, myoblasts derived from C3H10T1/2 fibroblasts treated with DNA demethylating agents or transfected with the cDNA encoding MyoD expressed the Ae (M(r) = 400,000) and a novel Ae-related laminin chain (designated Ac3h, M(r) = 350,000) in addition to the B1e and B2e chains. Expression of the Ae and Ac3h chains paralleled the capacity for myofiber formation in six additional C3H10T1/2 myoblast clones with varied potentials for terminal differentiation and coincided with a switch in laminin isoforms from those of M(r) = 850,000 synthesized by C3H10T1/2 fibroblasts to those of M(r) = 900,000-950,000 synthesized by C3H10T1/2 myoblasts and myofibers. Cultures of mouse C2C12, mouse BC3H1, rat L6, and primary mouse myoblasts also synthesized the Ae, Ac3h, B1e, and B2e laminin chains. The results demonstrate that expression of the Ae and Ac3h laminin chains is associated with expression of MyoD and the mammalian myogenic differentiation program.