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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.

References associated with 8.A.2 family:

Hardie, K.R., A. Seydel, I. Guilvout and A.P Pugsley (1996a). The secretin-specific, chaperone-like protein of the general secretory pathway: separation of proteolytic protection and piloting functions. Mol. Microbiol. 22: 967-976. 8971717
Hardie, K.R., S. Lory and A.P Pugsley (1996a). Insertion of an outer membrane protein in Escherichia coli requires a chaperone-like protein. EMBO J. 15: 978-988. 8605893
Shevchik, V.E. and G. Condemine (1998). Functional characterization of the Erwinia chrysanthemi OutS protein, an element of a type II secretion system. Microbiol. 144: 3219-3228. 9846757