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J. Bacteriol. doi:10.1128/JB.01347-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Outer membrane components of the Tad (tight adherence) secreton of Aggregatibacter actinomycetemcomitans

Department of Microbiology, College of Physicians and Surgeons, Columbia, University, New York, NY 10032; Molecular Laboratories, American Museum of Natural History, New York, NY, 10024

^* To whom correspondence should be addressed. Email: dhf2{at}columbia.edu.

	Abstract

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) of the Flp-pilus-biogenesis apparatus, we observed the RcpA protein in detergent-stable multimers and found that rcpA and its closely related homologs form a novel and distinct subfamily within a well-supported gene phylogeny of the entire secretin superfamily. We also found that rcpA-like genes were always linked to Aggregatibacter rcpB- or Caulobacter cpaD-like genes. Using antisera, we determined the localization and gross abundances relative to wild type of conserved (RcpA, TadC) and unique (RcpB, RcpC, TadD) Tad proteins. The three Rcp proteins (RcpA, RcpB, RcpC) and TadD, a putative lipoprotein, localized to the bacterial outer membrane. RcpA, RcpC, and TadD were also found in the inner membrane, while 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 RcpA and RcpB abundances, and RcpA did not multimerize or localize to the outer membrane without expression of TadD. These data indicate that membrane proteins TadC and TadD may influence the assembly, transport, and/or function of individual outer membrane Rcp proteins.

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• Kram, K. E., Hovel-Miner, G. A., Tomich, M., Figurski, D. H. (2008). Transcriptional Regulation of the tad Locus in Aggregatibacter actinomycetemcomitans: a Termination Cascade. J. Bacteriol. 190: 3859-3868 [Abstract] [Full Text]