ALTERED EXPRESSION OF THE PLATELET AGGREGATION-ASSOCIATED PROTEIN FROM STREPTOCOCCUS-SANGUIS AFTER GROWTH IN THE PRESENCE OF COLLAGEN

Certain strains of Streptococcus sanguis adhere selectively to human p latelets (Adh(+)) and, in plasma, induce them to aggregate into in vit ro thrombi (Agg(+)). The induction of aggregation is mediated by the p latelet aggregation-associated protein (PAAP) expressed on the cell su rface of the streptococcus. In endocarditis, expression of PAAP may be regulated by association with host proteins on damaged heart valves. To begin to test this hypothesis, three strains of S. sanguis were eac h cultured in the presence or absence of collagens (types I to X), lam inin, or PAAP-derived peptide preparations. After harvesting and washi ng, the platelet-interactive phenotype of strains 133-79 (Adh(+) Agg()), L74 (Adh(+) Agg(-)), and 10556 (Adh(-) Agg(-)) was unchanged. The cells from each culture were then digested mildly with trypsin to isol ate PAAP. PAAP isolated from strain 133-79 (Adh(+) Agg(+)) grown in th e absence of added collagen, other proteins, or peptides inhibited pla telet aggregation in response to untreated cells of S. sanguis. Platel et aggregation was induced immediately however, by PAAP from strain 13 3-79 isolated after growth in the presence of 300 nM type I collagen, while lower concentrations yielded protein fragments that potentiated the response to intact cells. Aggregation-inducing PAAP could be remov ed by anti-PAAP (PGEQGPK) immunoaffinity chromatography, but only inhi bitory activity could be recovered. The agonist effect of PAAP was not associated with collagen itself, since the PAAP preparations did not contain detectable amounts of hydroxyproline. PAAP antigens isolated f rom cells grown in the presence and absence of collagen had similar ap parent molecular weights, as estimated by sodium dodecyl sulfate-polya crylamide gel electrophoresis and Western immunoblotting. When electro phoresis was performed under nondenaturing conditions, however, PAAP i solated from cells grown in type I collagen migrated more slowly. Stra in L74 grown with type I collagen yielded tryptic fragments of protein s that inhibited aggregation significantly better than control peptide s (no collagen in the medium). Strain 10556 was apparently unaffected by growth in type I collagen. The effect of type I collagen was somewh at unique. Growth in the presence of collagen types II to VI (300 nM) yielded protein fragments that potentiated without inducing platelet a ggregation, while other collagens, laminin, and PAAP-derived peptides did not affect platelet aggregation. These results suggest that growth in the presence of type I collagen and, perhaps, collagens II to VI a lters the expression and conformation of PAAP in certain strains of S. sanguis.