INVOLVEMENT OF PHOSPHOPROTEIN PHOSPHATASE-1 IN COLLAGEN-PLATELET INTERACTION

The binding of type I collagen to its receptor initiates platelet aggr egation, but the relationship of the receptor to other signal transduc tion components is not yet established. Correlation of platelet aggreg ation and anti-type I collagen receptor antibody immunoprecipitation o f type I collagen treated [(32)PO4]-labeled platelets showed that ther e are two phosphoproteins (M(r) 53 kDa and 21 kDa) that coprecipitated with the 65 kDa platelet type I collagen receptor. In the present inv estigation, we have identified one of the phosphoproteins. A soluble c omponent the 100,000x g supernatant fraction of 53 kDa protein is reco gnized.: by polyclonal anti-PPI antibody. The activity of the precipit ated phosphatase is inhibited by okadaic acid and inhibitor 1, suggest ing that it is protein phosphatase 1 (PP I). Phosphorylation decreases PP 1 activity as was found with [(32)PO4] phosphorylase b as the subs trate. The immunocoprecipitation of the type-1 collagen receptor and P P 1 inot the result of cross reactivity of the anti-type I collagen re ceptor antibody with the PP I protein. These results indicate that the platelet type I collagen receptor, PP 1, and unidentified 21 kDa prot ein are in close association with the platelet type I collagen recepto r upon the binding of type I collagen by the recep tor. Copyright (C) 1996 Elsevier Science Ltd