JB Accepts, published online ahead of print on 11 May 2007

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J. Bacteriol. doi:10.1128/JB.00163-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Identification of two Mycobacterium smegmatis lipoproteins exported by a SecA2-dependent pathway

Henry S. Gibbons, Frank Wolschendorf, Michelle Abshire, Michael Niederweis, and Miriam Braunstein^*

Department of Microbiology and Immunology, School of Medicine, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7290, Department of Microbiology, University of Alabama at Birmingham, Birmingham AL 35294

^* To whom correspondence should be addressed. Email: Miriam_Braunstein{at}med.unc.edu.

The SecA2 protein is part of a specialized protein export system of mycobacteria. We set out to identify proteins exported to the bacterial cell envelope by the mycobacterial SecA2 system. By comparing the protein profiles of cell wall and membrane fractions from wild-type and secA2 mutant Mycobacterium smegmatis, we identified Msmeg1712 and Msmeg1704 proteins as SecA2-dependent cell envelope proteins. These are the first endogenous M. smegmatis proteins identified as dependent on SecA2 for export. Both proteins are homologous to periplasmic sugar-binding proteins of other bacteria and both contain functional amino-terminal signal sequences with lipobox motifs. These two proteins appeared to be genuine lipoproteins as shown by Triton X-114 fractionation and sensitivity to globomycin, an inhibitor of lipoprotein signal peptidase. The role of SecA2 in the export of these proteins was specific; not all mycobacterial lipoproteins required SecA2 for efficient localization or processing. Finally, Msmeg1704 was recognized by the SecA2 pathway of M. tuberculosis, as indicated by the appearance of an export intermediate when expressed in a secA2 mutant of M. tuberculosis. Taken together, these results indicate that a select subset of envelope proteins containing amino-terminal signal sequences can be substrates of the mycobacterial SecA2 pathway and that some determinants for SecA2-dependent export are conserved between M. smegmatis and M. tuberculosis.

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