IAI Accepts, published online ahead of print on 13 October 2008

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Infect. Immun. doi:10.1128/IAI.00534-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

A conserved C-terminal thirteen amino acid motif of Gap1 is required for the Gap1 function and necessary for biogenesis of a serine-rich glycoprotein of Streptococcus parasanguinis

Meixian Zhou, Zhixiang Peng, Paula Fives-Taylor, and Hui Wu^*

Departments of Pediatric Dentistry and Microbiology, University of Alabama at Birmingham Schools of Medicine and Dentistry, Birmingham, AL 35244; Department of Microbiology and Molecular Genetics, College of Medicine and College of Life Science and Agriculture, Burlington, VT 05405

^* To whom correspondence should be addressed. Email: hwu{at}uab.edu.

Adhesion of Streptococcus parasanguinis to saliva-coated hydroxyapatite (SHA), an in vitro tooth model, is mediated by long peritrichous fimbriae. Fap1, a fimbriae associated serine-rich glycoprotein is required for fimbrial assembly. Biogenesis of Fap1 is controlled by an eleven gene cluster that contains gly, nss, galT1-2, secY2, gap1-3, secA2 and gtf1-2. We had previously isolated a collection of nine nonadherent mutants using random chemical mutagenesis approaches. These mutants fail to adhere to the in vitro tooth model and to form fimbriae. In this report, we further characterized these randomly selected nonadherent mutants and classified them into three distinct groups. Two groups of genes were previously implicated in the Fap1 biogenesis. One group has a mutation in a glycosyltransferase gene gtf1 that is essential for the first step of Fap1 glycosylation, whereas the other group has defects in the fap1 structural gene. The third group mutant produces an incompletely glycosylated Fap1 and exhibits a mutant phenotype similar to a glycosylation-associated protein 1 (Gap1) mutant. Analysis of this new mutant revealed that a conserved C-terminal 13 amino acid motif was missing in Gap1. Site-directed mutagenesis of a highly conserved amino acid tryptophan within this motif recapitulated the deletion phenotype, demonstrating the importance of the Gap1 C-terminal motif for the Fap1 biogenesis. Furthermore the C-terminal mutation does not affect Gap1-Gap3 protein-protein interaction which has been shown to mediate Fap1 glycosylation, suggesting the C-terminal motif has a distinct function related to the Fap1 biogenesis.

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