2.A.116 The Peptidoglycolipid Addressing Protein (GAP) Family
The cell envelope of mycobacteria is a complex multilaminar structure that protects the cell from stresses encountered in the environment. The outermost layer of the mycobacterial envelope typically contains species-specific glycolipids. Depending on the mycobacterial species, the major glycolipid localized at the surface can be either a phenolglycolipid or a peptidoglycolipid (GPL). Six genes in Mycobacterium smegmatis involved in GPLs synthesis are clustered in a single genomic region of approximately 60 kb. A small integral membrane protein of 272 amino acids named GAP (GAP: GPL Addressing Protein) is specifically required for the transport of the GPLs to the cell surface (Sonden et al. 2005). This protein is predicted to contain six transmembrane segments and possesses homologues across the mycobacterial genus, thus delineating a new protein family. This GAP family represents a new paradigm for the transport of lipids across the mycobacterial envelope. The mechanism of its action is not known, but this family has been shown to be a member of the LsyE superfamily. Therefore, these proteins are probably secondary carriers.
The generalized reaction catalyzed by members of the GAP family is:
PGL (in) → PGL (outer membrane).