9.A.47 The Tight Adherence (Pilus) Biogenesis Apparatus (TABA) Family

Prokaryotic secretion relies on proteins that are widely conserved, including NTPases and secretins, and on proteins that are system specific. The Tad secretion system in Aggregatibacter actinomycetemcomitans is dedicated to the assembly and export of Flp pili, which are needed for tight adherence. Consistent with predictions that RcpA forms the multimeric outer membrane secretion channel (secretin; TC# 1.B.22) of the Flp pilus biogenesis apparatus, Clock et al. (2007) observed that the RcpA multimers are stable in the presence of detergent and found that RcpA and its closely related homologs form a novel and distinct subfamily within the secretin superfamily. RcpA-encoding genes were linked to Aggregatibacter rcpB and rcpC. All 5 proteins were envelope-associated, and TadC localized exclusively to the inner membrane. The RcpA secretin was necessary for wild-type abundances of RcpB and RcpC, and TadC was required for normal levels of all three Rcp proteins. TadC abundance defects were observed in rcpA and rcpC mutants. TadD production was essential for wild-type RcpA and RcpB abundances, and RcpA did not multimerize or localize to the outer membrane without the expression of TadD. Thus, membrane proteins TadC and TadD may influence the assembly, transport, and/or function of individual outer membrane Rcp proteins (Clock et al., 2007). Rose et al. (2008) observed a striking correlation between the natural order of subunits in P-pili and their ability to undergo donor-strand exchange in vitro.