Structural comparison of the transport units of type V secretion systems

Pathogenic gram-negative bacteria have evolved several secretion mechanisms to translocate adhesins, enzymes, toxins, and other virulence factors across the inner and outer membranes. Currently, eight different secretion systems, type I-type VIII (T1SS-T8SS) plus the chaperone-usher (CU) pathway, have been identified, which act in one-step or two-step mechanisms to traverse both membrane barriers. The type V secretion system (T5SS) is dependent first on the Sec translocon within the inner membrane. The periplasmic intermediates are then secreted through aqueous pores formed by beta-barrels in the outer membrane. Until now, transport across the outer membrane has not been understood on a molecular level. With respect to special characteristics revealed by crystal structure analysis, bioinformatic and biochemical data, five subgroups of T5SS were defined. Here, we compare the transport moieties of members of four subgroups based on X-ray crystal structures. For the fifth subgroup, which was identified only recently, no structures have thus far been reported. We also discuss different models for the translocation process across the outer membrane with respect to recent findings.