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J Bacteriol, June 1998, p. 3062-3069, Vol. 180, No. 12
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

The Caulobacter crescentus Paracrystalline S-Layer Protein Is Secreted by an ABC Transporter (Type I) Secretion Apparatus

Peter Awram and John Smit*

Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia, Canada

Received 17 December 1997/Accepted 14 April 1998

Caulobacter crescentus is a gram-negative bacterium that produces a two-dimensional crystalline array on its surface composed of a single 98-kDa protein, RsaA. Secretion of RsaA to the cell surface relies on an uncleaved C-terminal secretion signal. In this report, we identify two genes encoding components of the RsaA secretion apparatus. These components are part of a type I secretion system involving an ABC transporter protein. These genes, lying immediately 3' of rsaA, were found by screening a Tn5 transposon library for the loss of RsaA transport and characterizing the transposon-interrupted genes. The two proteins presumably encoded by these genes were found to have significant sequence similarity to ABC transporter and membrane fusion proteins of other type I secretion systems. The greatest sequence similarity was found to the alkaline protease (AprA) transport system of Pseudomonas aeruginosa and the metalloprotease (PrtB) transport system of Erwinia chrysanthemi. The prtB and aprA genes were introduced into C. crescentus, and their products were secreted by the RsaA transport system. Further, defects in the S-layer protein transport system led to the loss of this heterologous secretion. This is the first report of an S-layer protein secreted by a type I secretion apparatus. Unlike other type I secretion systems, the RsaA transport system secretes large amounts of its substrate protein (it is estimated that RsaA accounts for 10 to 12% of the total cell protein). Such levels are expected for bacterial S-layer proteins but are higher than for any other known type I secretion system.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, University of British Columbia, 300-6174 University Blvd., Vancouver, B.C., Canada V6T 1Z3. Phone: (604) 822-4417. Fax: (604) 822-6041. E-mail: jsmit{at}unixg.ubc.ca.

J Bacteriol, June 1998, p. 3062-3069, Vol. 180, No. 12
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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