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Infection and Immunity, September 2006, p. 4961-4969, Vol. 74, No. 9
0019-9567/06/$08.00+0     doi:10.1128/IAI.00103-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Role of the {alpha}-Helical Linker of the C-Terminal Translocator in the Biogenesis of the Serine Protease Subfamily of Autotransporters

Maria Kostakioti and Christos Stathopoulos*

Department of Biology and Biochemistry, University of Houston, Houston, Texas 77204

Received 19 January 2006/ Returned for modification 12 March 2006/ Accepted 13 June 2006

Autotransporters are secreted virulence factors that comprise three domains: an N-terminal signal peptide, an internal passenger domain, and a C-terminal ß-domain. The mechanism of passenger translocation across the outer membrane remains undefined, with four models having been proposed: the "hairpin," the "threading," the "multimeric," and the "Omp85 (YaeT)" models. In an attempt to understand autotransporter biogenesis, we screened the sequences of the serine protease subfamily of autotransporters (SPATEs) for conserved features indicative of a common secretion mechanism. Our analyses revealed a strictly conserved 14-amino-acid motif within the predicted {alpha}-helical linker region, upstream of the ß-domain of SPATEs. We investigated the function of this motif through a mutagenesis approach using Tsh as a model. Our studies demonstrate that mutations throughout the conserved motif do not block insertion of the ß-domain into the outer membrane. However, nonconservative mutations of four hydrophobic (V1099, L1102, G1107, and L1109) and three polar (N1100, K1104, and R1105) residues of the motif severely decrease or even abolish Tsh biogenesis. Further studies showed that these mutations interfere with passenger transport across the outer membrane. Bioinformatical analyses suggest that the critical polar and hydrophobic amino acids localize on opposite sides of the helix that runs through the ß-barrel pore. Our data indicate that the conserved motif is important for passenger secretion across the outer membrane and that mutations in certain residues severely affect the secretion process. We discuss how these results fit with the four proposed models for autotransporter secretion and potential applications in antimicrobial and vaccine development.

* Corresponding author. Mailing address: Department of Biology and Biochemistry, SRII 369, University of Houston, 4800 Calhoun St., Houston, TX 77204. Phone: (713) 743-2491. Fax: (713) 743-8351. E-mail: cstatho{at}uh.edu.

Editor: J. B. Bliska

Infection and Immunity, September 2006, p. 4961-4969, Vol. 74, No. 9
0019-9567/06/$08.00+0     doi:10.1128/IAI.00103-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.





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