PROTEOLYTIC CLEAVAGE OF THE BETA(1) SUBUNIT OF PLATELET ALPHA(2)BETA(1), INTEGRIN BY THE METALLOPROTEINASE JARARHAGIN COMPROMISES COLLAGEN-STIMULATED PHOSPHORYLATION OF PP72(SYK)

Early signaling events in the stimulation of platelets by collagen inc lude the tyrosine phosphorylations of FcR gamma-chain, pp72(syk) and p hospholipase C gamma 2. These events are dependent on the main platele t collagen receptor, alpha(2) beta(1) integrin (glycoprotein Ia-IIa co mplex), We recently found that jararhagin, a 52-kDa snake venom metall oproteinase, selectively inhibits collagen-induced platelet secretion and aggregation in parallel with the cleavage of the beta(1) subunit o f the alpha(2) beta(1) integrin, The present study demonstrates that j ararhagin also interferes with collagen-induced phosphorylation of the protein-tyrosine kinase pp72(syk). This effect is not observed when t he platelet aggregation response to collagen is inhibited by two venom ROD-containing disintegrins, contortrostatin and echistatin, These di sintegrins inhibit platelet aggregation through their high affinity bi nding to the platelet alpha(IIb)beta(3) integrin (glycoprotein IIb-III a complex), We also show that mild stimulation by ADP of jararhagin-tr eated platelets, but not of platelets treated with the RGD-containing disintegrins, restores the collagen-induced platelet aggregation, ADP also restored both pp72(syk) and pleckstrin phosphorylation of jararha gin-treated platelets in response to collagen, presumably via interact ion of collagen with ADP-activated alpha(IIb)beta(3) integrin, Thus, R OD-containing disintegrins do not interfere with agonist-induced pp72( syk) phosphorylation but inhibit aggregation through occupancy of the alpha(IIb)beta(3) integrin. Conversely, jararhagin affects early plate let signaling events in response to collagen through its effects on th e alpha(2) beta(1) integrin without interfering with the function of t he alpha(IIb)beta(3) integrin, Our demonstration that the degradation of the beta(1) subunit of alpha(2) beta(1) by jararhagin results in th e loss of pp72(syk) phosphorylation, suggests that this subunit is cri tically involved in collagen-induced platelet signaling.