The serine-rich repeat (SRR) glycoproteins are a large family of adhesins found in many Gram-positive bacteria. These surface components are important virulence determinants for a broad range of human infections. GspB is an SRR adhesin of Streptococcus gordonii that mediates binding to human platelets through its interaction with the trisaccharide sialyl-T antigen (sTa) on the platelet receptor GPIb. This binding appears to be important for the pathogenesis of infective endocarditis, since mutagenesis of the GspB binding region results in decreased platelet binding in vitro, and reduced virulence in an animal model of this disease. Three properties of GspB binding may be highly important for the targeting of streptococci to the endocardium: affinity, selectivity, and fow enhancement. First, the binding of GspB to its platelet receptor is a high affinity interaction (KD 2.4 x 10-8 M). Second, GspB has a very selective binding spectrum, with sTa being its principal ligand. Third, GspB-mediated binding by bacteria to sTa is enhanced by levels of fluidic shear flow similar to those within the endovascular system. In combination, these three binding properties may target blood-borne streptococci to platelets immobilized at sites of endocardial injury (thereby initiating infection). GspB-mediated binding may also contribute to the subsequent formation of macroscopic endocardial lesions (vegetations) containing bacteria and platelets. This project seeks to define the molecular basis for GspB binding affinity, selectivity, and flow enhancement, and the relative importance of these properties for virulence.
Aim 1 will examine how the molecular architecture of the GspB binding domain confers affinity and selectivity. We will determine the structure of the GspB binding region cocrystallized with sTa and related compounds, select key domains and residues for targeted mutagenesis, and examine the impact of these mutations on binding affinity and selectivity. Two GspB homologs (Hsa and SrpA) that differ in their binding properties will also be evaluated.
Aim 2 will determine the structural features of GspB that contribute to flow-enhanced binding, and whether binding occurs via the formation of catch bonds. We will specifically examine the contribution of the serine-rich repeat domains of GspB in flow-enhanced binding.
Aim 3 will assess the impact of ligand affinity, selectivity and flow-enhanced binding on the pathogenesis of infective endocarditis. Isogenic variants of S. gordonii strain M99 that differ in their ligand binding properties will be compared for relative virulence, using a well-established co-infection model of this disease. This project will provide significant insights into the structural basis for carbohydrate binding by this novel group of bacterial adhesins, as well as the mechanisms for streptococcal binding to human platelets. In addition, these studies could provide a basis for novel therapies for infective endocarditis that target SRR glycoprotein binding.

Public Health Relevance

A key step in the development of many infections is the attachment of microbes onto host tissues. This research examines how bacteria known as streptococci attach to cardiac valves, leading to infection (endocarditis). By examining a central mechanism for producing this disease, this work will provide important mechanistic insights that may lead to novel antimicrobial therapies.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Research Project (R01)
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Special Emphasis Panel (ZRG1)
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GU, Xin-Xing
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Northern California Institute Research & Education
San Francisco
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Chen, Yu; Seepersaud, Ravin; Bensing, Barbara A et al. (2016) Mechanism of a cytosolic O-glycosyltransferase essential for the synthesis of a bacterial adhesion protein. Proc Natl Acad Sci U S A 113:E1190-9
Bensing, Barbara A; Loukachevitch, Lioudmila V; McCulloch, Kathryn M et al. (2016) Structural Basis for Sialoglycan Binding by the Streptococcus sanguinis SrpA Adhesin. J Biol Chem 291:7230-40
Loukachevitch, Lioudmila V; Bensing, Barbara A; Yu, Hai et al. (2016) Structures of the Streptococcus sanguinis SrpA Binding Region with Human Sialoglycans Suggest Features of the Physiological Ligand. Biochemistry :
Bensing, Barbara A; Khedri, Zahra; Deng, Lingquan et al. (2016) Novel aspects of sialoglycan recognition by the Siglec-like domains of streptococcal SRR glycoproteins. Glycobiology :
Yakovenko, Olga; Tchesnokova, Veronika; Sokurenko, Evgeni V et al. (2015) Inactive conformation enhances binding function in physiological conditions. Proc Natl Acad Sci U S A 112:9884-9
Deng, Lingquan; Bensing, Barbara A; Thamadilok, Supaporn et al. (2014) Oral streptococci utilize a Siglec-like domain of serine-rich repeat adhesins to preferentially target platelet sialoglycans in human blood. PLoS Pathog 10:e1004540