The Type II Secretory System (T2SS) in Gram-negative Bacteria: A Molecular Nanomachine for Secretion of Sec and Tat-Dependent Extracellular Proteins
Gérard P.F. Michel and Romé Voulhoux
from: Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis (Edited by: Karl Wooldridge). Caister Academic Press, U.K. (2009)
Gram-negative bacteria have evolved several secretory pathways to release proteins or toxic factors into their surrounding environment. Many virulence determinants, including extracellular toxins and proteases, are secreted by the type II secretion system (T2SS) which is widely conserved and common among γ-proteobacteria. Typical T2SSs are composed of 12 to 16 proteins termed Gsp (General secretion pathway) proteins. These components associate in a multiprotein complex that constitutes a large structure (the secreton) that spans the periplasm and is thought to connect inner and outer membranes. Exoproteins that use the T2SS are characterized by the presence of a leader peptide (or signal peptide) at their N terminus and are secreted in the extracellular medium by a two-step process involving a transient periplasmic intermediate. The T2SS is unique in its ability to promote secretion of large multimeric proteins that are folded in the periplasm. The system is also characterized by a species-specificity, which is mainly related to the GspC and GspD components, the gatekeepers. Although relatively little attention has been payed to the regulation of T2SSs, it was observed that expression of most of the genes encoding T2SS-dependent exoproteins is growth phase-dependent or strictly regulated by environmental signals. In Pseudomonas aeruginosa, T2SS assembly and most of the T2SS-dependent exoproteins are regulated via quorum sensing, a mechanism that senses the density of a surrounding bacterial population. Besides typical T2SSs, some secretory systems are found which contain all the T2SS components but in a different genetic organization. Some incomplete systems have also been described which contain genes homologous to T2SS but dispersed on the bacterial chromosome. Components of these systems can either associate with classical T2SS components to constitute a functional hybrid machinery or represent peculiar systems with strictly defined functions read more ...