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Journal of Bacteriology, February 2004, p. 638-645, Vol. 186, No. 3
0021-9193/04/$08.00+0 DOI: 10.1128/JB.186.3.638-645.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
The Streptococcus gordonii Platelet Binding Protein GspB Undergoes Glycosylation Independently of Export
Barbara A. Bensing,1 Bradford W. Gibson,2 and Paul M. Sullam1*
Division
of Infectious Diseases, Veterans Affairs Medical Center, and Department
of Medicine, University of California, San
Francisco,1
Department of Pharmaceutical
Chemistry, University of California, San Francisco, and the
Buck Institute for Age Research, Novato,California2
Received 16 September 2003/
Accepted 23 October 2003
The
binding of bacteria and platelets may play a central role in the
pathogenesis of infective endocarditis. Platelet binding by
Streptococcus gordonii strain M99 is predominantly mediated by
the 286-kDa cell wall-anchored protein GspB. This unusually large
protein lacks a typical amino-terminal signal peptide and is
translocated from the cytoplasm via a dedicated transport system. A
14-kb segment just downstream of gspB encodes SecA2 and SecY2,
two components of the GspB-specific transport system. The downstream
segment also encodes several putative glycosyl transferases that may be
responsible for the posttranslational modification of GspB. In this
study, we compared the abilities of M99 and two
GspB- mutant strains to bind various
lectins. GspB was found to have affinity for lectins that bind
N-acetylglucosamine. We also examined variant forms of GspB
that lack a carboxy-terminal cell wall-anchoring domain and thus are
free of covalent linkage to cell wall peptidoglycan. Like native GspB,
these truncated proteins appear to be heavily glycosylated, as
evidenced by migration during sodium dodecyl sulfate-polyacrylamide gel
electrophoresis with an apparent molecular mass >100 kDa in
excess of the predicted mass, negligible staining with conventional
protein stains, and reactivity with hydrazide following periodate
oxidation. Furthermore, analysis of the carbohydrate associated with
the GspB variants by high-pH anion-exchange chromatography revealed the
presence of
70 to 100 monosaccharide residues per GspB
polypeptide (primarily N-acetylglucosamine and glucose).
Analysis of GspB in protoplasts of secA2 or secY2
mutant strains, which do not export GspB, indicates that GspB is
glycosylated in the cytoplasm of these strains. The combined data
suggest that the native GspB is a glycoprotein and that it may be
glycosylated prior to
export.
* Corresponding
author. Mailing address: Division of Infectious Diseases, VA Medical
Center (111W), 4150 Clement St., San Francisco, CA 94121. Phone: (415)
221-4810, ext. 2550. Fax: (415) 750-0502. E-mail:
sullam{at}itsa.ucsf.edu.
Journal of Bacteriology, February 2004, p. 638-645, Vol. 186, No. 3
0021-9193/04/$08.00+0 DOI: 10.1128/JB.186.3.638-645.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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