Type I Bacterial Secretion Systems
Stefan Jenewein, I. Barry Holland and Lutz Schmitt
from: Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis (Edited by: Karl Wooldridge). Caister Academic Press, U.K. (2009)
Bacteria have developed numerous systems to secrete proteins or DNA in order to modify their immediate surroundings or to obtain an advantage in a competitive and hostile environment. Since Gram-negative bacteria possess two membranes, the inner (cytoplasmic) membrane and the outer membrane, transport machines for protein secretion have the challenging task of circumventing two barriers to reach the exterior. Complex nanomachines like the Type II secretion systems, involving a two stage process with a periplasmic intermediate, have evolved for that purpose. However, a rather simple transport apparatus, the Type I secretion machinery, composed of only three proteins residing in the inner and outer membrane of Gram-negative bacteria achieve the same objective in a single step, with no periplasmic intermediate. The Type I secretion pathway although also present in Gram-positive bacteria, has been analysed in greatest detail in Gram-negative bacteria and this will be the primary focus of this chapter. Almost all Type I transport substrates are polypeptides, varying from the small Escherichia coli peptide colicin V, (10 kDa) to the impressive size of the Pseudomonas fluorescens cell adhesion protein LapA of 900 kDa. While these two examples reflect the range of the size of Type I transport substrates, the best characterized are the RTX toxins and the lipases. Note that the terms transport substrates or allocrites, the name we have coined to designate an unprocessed substrate for the ABC transporter, in contrast to the substrate ATP, which is hydrolyzed, will be used interchangeably in this chapter. Type I secretion is also apparently involved in export of non-proteinaceous substrates like cyclic β-glucans or polysaccharides, but definitive studies are lacking. Here, we shall only focus on the fundamental properties of the pathway whereby polypeptides are secreted to the extracellular medium via the Type I secretion mechanism read more ...