The Streptococcus sanguis platelet aggregation-associated protein. Identification and characterization of the minimal platelet-interactive domain.

Streptococcus sanguis expresses a cell wall-bound protein that induces the activation and aggregation of platelets. This platelet aggregation-associated protein (PAAP) contains a collagen-like, platelet-interactive domain within a 23-kDa protein fragment. To isolate the minimal platelet-interactive domain, p23 PAAP was digested with collagenase, and the digest chromatographed to isolate fractions with activity inhibitory to S. sanguis-induced platelet aggregation. The active fraction was then digested with cyanogen bromide, the product chromatographed, and a smaller inhibitory peptide isolated. Finally, this fraction was digested with endoproteinase Lys-C, and the digest fractionated. After each step, inhibitory activity resolved into single chromatographic peaks of 13 kDa (p13 PAAP), 2.7 kDa (p2.7 PAAP), and a minimal 7-mer peptide, respectively. These PAAP fragments showed similar ID50 (19-28 nM), suggesting that each contained a single copy of the platelet-interactive domain. The minimal 7-mer peptide was purified by immunoaffinity chromatography and reverse phase high pressure liquid chromatography. The primary structure was determined to be Pro-Gly-Glu-Gln-Gly-Pro-Lys. This sequence conforms to the predicted structural motif of the platelet-interactive domains of types I and III collagen. This 7-mer peptide is therefore the platelet-interactive domain of the PAAP from S. sanguis. Its structure explains the molecular basis for immunological cross-reactivity and functional similarity to the platelet-interactive domains of collagens.