Different subcellular locations of secretome components of Gram-positive bacteria.

Gram-positive bacteria contain different types of secretion systems for the transport of proteins into or across the cytoplasmic membrane. Recent studies on subcellular localization of specific components of these secretion systems and their substrates have shown that they can be present at various locations in the cell. The translocons of the general Sec secretion system in the rod-shaped bacterium Bacillus subtilis have been shown to localize in spirals along the cytoplasmic membrane, whereas the translocons in the coccoid Streptococcus pyogenes are located in a microdomain near the septum. In both bacteria the Sec translocons appear to be located near the sites of cell wall synthesis. The Tat secretion system, which is used for the transport of folded proteins, probably localizes in the cytoplasmic membrane and at the cell poles of B. subtilis. In Lactococcus lactis the ABC transporter dedicated to the transport of a small antimicrobial peptide is distributed throughout the membrane. Possible mechanisms for maintaining the localization of these secretion machineries involve their interaction with proteins of the cytoskeleton or components of the cell wall synthesis machinery, or the presence of lipid subdomains surrounding the transport systems.