Anchoring the type VI secretion system to the peptidoglycan: TssL, TagL, TagP... what else?

Abstract

The recently identified bacterial type VI secretion system (T6SS) has rapidly become one of the most interesting areas of research in microbiology. In a relatively short period of time the relationship between the T6SS and the bacteriophage T4 tail and baseplate has been established. However, a number of questions concerning the T6SS remain the focus of a large number of researchers worldwide. Key questions that need to be addressed include how this system assembles in the cell envelope and the mechanism by which it translocates effector proteins across two membranes, the identification of such effectors and their function, how this secretion system contributes to virulence, interbacterial interactions and/or adaptation to the environment, and the evolutionary relationship between T6SS machine and bacteriophage T4. Focused on how the proteins constituting the secretion system interact, we recently identified a sub-complex of the T6SS comprised of four cell envelope proteins: the inner membrane-anchored TssL, TssM and TagL proteins and the outer membrane-associated TssJ lipoprotein. We further demonstrated that the TagL subunit carries a specific domain allowing anchorage of the secretion system to the peptidoglycan (PG) layer. Herein, we discuss these results, examine whether PG-binding motifs are found within other T6SS subunits and express hypotheses regarding the role of PG-binding motifs in type VI secretion.