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J. Bacteriol. doi:10.1128/JB.00027-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Glycine residues in the hydrophobic core of the GspB signal sequence route export towards the accessory Sec pathway

Veterans Affairs Medical Center and the University of California, San Francisco, CA

^* To whom correspondence should be addressed. Email: paul.sullam{at}ucsf.edu.

	Abstract

The Streptococcus gordonii cell-surface glycoprotein GspB mediates high affinity binding to distinct sialylated carbohydrate structures on human platelets and salivary proteins. GspB is glycosylated in the cytoplasm of S. gordonii, and is then transported to the cell surface via a dedicated transport system that includes the accessory Sec components SecA2 and SecY2. The means by which the GspB preprotein is selectively recognized by the accessory Sec system have not been characterized fully. GspB has a 90 residue amino-terminal signal sequence that displays a traditional tripartite structure, with an atypically long amino-terminal (N) region followed by hydrophobic (H) and cleavage regions. In this report, we investigate the relative importance of the N and H regions of the GspB signal peptide for trafficking of the preprotein. The results show that the extended N region does not prevent export by the canonical Sec system. Instead, three glycine residues in the H region are not only necessary for export via the accessory Sec pathway, but also interfere with export via the canonical Sec route. Replacement of the H region glycine residues with helix-promoting residues led to a decrease in the efficiency of SecA2-dependent transport of the preprotein, and a simultaneous increase in SecA2-independent translocation. Thus, the hydrophobic core of the GspB signal sequence is primarily responsible for routing towards the accessory Sec system.

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