GspB is a 3072-residue cell-wall-anchored glycoprotein from Streptococcus gordonii
that mediates the binding of this bacterium to salivary glycoproteins and human platelets (Bensing & Sullam, 2002
). The former interaction is important for colonization of the oral cavity, while the latter is a key event in the pathogenesis of infective endocarditis (Xiong et al.
). This unusual adhesin is a member of an expanding family of serine-rich glycoproteins that are found in most pathogenic streptococci and staphylococci (Takahashi et al.
; Siboo et al.
; Rose et al.
; Seifert et al.
) and are believed to mediate pathogen host attachment and promote bacterial biofilm formation (Sanchez et al.
; Wu et al.
). GspB contains an atypical 90-amino-acid N-terminal signal peptide, a serine-rich region, a basic region, a second serine-rich region and a C-terminal cell-wall-anchoring domain (Fig. 1). The basic region (GspBBR
) mediates the binding of GspB to sialylated carbohydrate moieties on platelet glycoprotein Ibα (Takamatsu et al.
), salivary mucin MG2 and salivary agglutinin (Takamatsu et al.
) through its high-affinity interaction with NeuAcα(2–3)Galβ(1–3)GalNAc (sialyl-T antigen). Of note, the binding properties of GspB homologs vary considerably, with some homologs (e.g.
Hsa of S. gordonii
strain Challis and SrpA of S. sanguinis
) binding to a broader or different range of carbohydrate motifs, while others (such as PsrP of S. pneumoniae
) have no apparent lectin-like activity (Shivshankar et al.
; Takamatsu et al.
; Yajima et al.
Figure 1 Overall architecture of GspB. GspB is comprised of a signal peptide (SP), a short serine-rich region (SRR1), a unique basic region (BR) that is responsible for carbohydrate binding, a second, longer, serine-rich region (SRR2) and a cell-wall-anchoring (more ...)
These studies will ultimately identify the molecular details of carbohydrate selectivity by GspB and related serine-rich repeat adhesins. This is a critical starting point for understanding how carbohydrate binding by GspB and related lectins affects pathogen infectivity. Furthermore, a crystal structure may allow the design of small-molecular inhibitors to disrupt carbohydrate binding, which offers a new route for the design of therapeutics. Here, we report the expression, purification and crystallization of the carbohydrate-binding domain GspBBR from S. gordonii strain M99.