8.A.2 The Secretin Auxiliary Lipoprotein (SAL) Family
The PulS protein of Klebsiella pneumoniae, the OutS protein of Erwinia chrysanthemi and a functionally uncharacterized E. coli protein (EtpO, gbY09824) are homologous lipoproteins of 125-133 amino acyl residues. The former two proteins interact with and facilitate insertion of the secretins (TC #1.B.22), PulD and OutD, respectively. These secretins are the oligomeric, pore-forming, outer membrane protein constituents of the main terminal branch of the type II general protein secretory pathway (IISP; TC #3.A.5). Secretins of the IISP family contain two functional domains: a periplasmic N-terminal domain that recognizes the substrate secretory proteins, while the C-terminal domain forms the oligomeric pore. The extreme C-terminal 60 residues of PulD and OutD interact with PulS and OutS, respectively, which stabilize and facilitate insertion of the secretin into the outer membrane. PulS is therefore probably a chaperone and a "pilotin", guiding the secretin to the outer membrane. The pilotin function is dependent on the N-terminal acylated part of the SAL family member, while the C-terminal domain of the SAL protein interacts with the secretin.