Assembly of the B subunit pentamer of Escherichia coli heat-labile enterotoxin - Kinetics and molecular basis of rate-limiting steps in vitro

Ruddock, Lloyd W. and Coen, Jeremy J. F. and Cheesman, Caroline and Freedman, Robert B. and Hirst, Timothy R. (1996) Assembly of the B subunit pentamer of Escherichia coli heat-labile enterotoxin - Kinetics and molecular basis of rate-limiting steps in vitro. Journal of Biological Chemistry, 271 (32). pp. 19118-19123. ISSN 0021-9258. (The full text of this publication is not currently available from this repository. You may be able to access a copy if URLs are provided)

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The B subunits of Escherichia coli heat-labile enterotoxin (EtxB) and cholera toxin (CtxB) assemble in vivo into exceptionally stable homopentameric complexes, which maintain their quaternary structure in a range of conditions that would normally be expected to cause protein denaturation, Recently, we showed that the simultaneous protonation of two of the COOH-terminal carboxylates in pentameric EtxB was required to cause its disassembly at pH values below 2.0 (Ruddock, L., Ruston, S. P., Kelly, S. M., Price, N. C., Freedman, R. B., and Hirst., T. R. (1995) J. Biol. Chem. 270, 29953-29958), Here, we investigate the influence of environmental parameters on the kinetics of reassembly of acid-generated EtxB monomers in vitro. Such monomers were found to undergo a further acid-mediated conformational change, with an activation energy of 76 +/- 2 J degrees mol(-1)degrees K-1, consistent with isomerization of the cis-proline residue at position 93, and which prevented subsequent EtxB reassembly. By using rapid neutralization of acid-generated monomers, a high proportion of the B-subunits adopted an assembly-competent conformation, which resulted in up to 75% of the protein reassembling into a stable pentameric complex, indistinguishable from native EtxB pentamers. The rate-limiting step in reassembly, over a concentration range of 50-200 mu g/ml, was shown 60 be due to an intramolecular event, which exhibited a pH dependence with a pK(a) of 7.0, Modification of EtxB with amine-specific probes revealed that the protonation state of the NH2-terminal alanine residue was responsible for the pH dependence of reassembly, The implications of these findings for the biogenesis of Escherichia coli enterotoxin and related enterotoxins in vivo, are considered.

Item Type: Article
Subjects: Q Science > QD Chemistry
Q Science > QP Physiology (Living systems) > QP506 Molecular biology
Divisions: Faculties > Science Technology and Medical Studies > School of Biosciences
Depositing User: F.D. Zabet
Date Deposited: 07 May 2009 15:36
Last Modified: 16 Jun 2014 14:06
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