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J. Bacteriol. doi:10.1128/JB.01633-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Additional in vitro and in vivo evidence for SecA functioning as dimers in the membrane: dissociation into monomers is not essential for protein translocation in Escherichia coli

Department of Biology, Georgia State University, Atlanta, GA, 30303

^* To whom correspondence should be addressed. Email: biopct{at}langate.gsu.edu.

	Abstract

SecA is an essential component in the Sec-dependent protein translocation pathway and together with ATP provides the driving force for the transport of secretory proteins across the cytoplasmic membrane of Escherichia coli. Previous studies established that SecA undergoes monomer-dimer equilibrium in solution. However, the oligomeric state of functional SecA during the protein translocation process is currently controversial. In this study, we provide additional evidence that SecA functions as a dimer in the membrane by: (1) demonstration of the capability of the presumably monomeric SecA derivative to be cross-linked as dimers in vitro and in vivo; (2) complementation of the growth of a secA(Ts) mutant with another nonfunctional SecA; or (3) in vivo complementation and in vitro function of a genetically tandem SecA dimer that does not dissociate into monomers; and (4) formation of the similar ring-like structures of the tandem SecA dimer and SecA in the presence of lipid bilayers. We conclude that SecA functions as a dimer in the membrane and dissociation into monomers is not necessary during protein translocation.