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Identification of Two Mycobacterium smegmatis Lipoproteins Exported by a SecA2-Dependent Pathway ^,

Henry S. Gibbons,¹ Frank Wolschendorf,² Michelle Abshire,^1, Michael Niederweis,² and Miriam Braunstein^1*

Department of Microbiology and Immunology, School of Medicine, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7290,¹ Department of Microbiology, University of Alabama at Birmingham, Birmingham, Alabama 35294²

Received 31 January 2007/ Accepted 2 May 2007

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 the 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 Mycobacterium tuberculosis, as indicated by the appearance of an export intermediate when the protein was 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.

Published ahead of print on 11 May 2007.

Supplemental material for this article may be found at http://jb.asm.org/.

Present address: Department of Microbiology, University of Virginia Charlottesville, VA 22908.

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