THE USE OF COLLAGEN-BASED MODEL PEPTIDES TO INVESTIGATE PLATELET-REACTIVE SEQUENCES IN COLLAGEN

Simple collagen-like peptides comprising a repeat Gly-Pro-Hyp sequence ave highly platelet-reactive when presented to platelets in triple-he lical and polymeric form. This activity is not mediated by the platele t collagen receptor integrin alpha 2 beta 1. This may imply the existe nce of an intrinsic platelet reactivity associated with the collagen t riple helix as such or perhaps that the Gly-Pro-Hyp sequence in collag en serves as a specific cell-recognition site. In our view this basic alpha 2 beta 1-independent reactivity is modulated by the presence in collagen of sequences that may either enhance or diminish the interact ion with platelets. Inhibition studies with short linear peptides have allowed the tentative identification of sequences in collagen such as XPGEP(Q)GPX and D(N)GE(Q)X that may promote the activation of platele ts and so enhance collagen-platelet interaction. Sequences serving as integrin alpha 2 beta 1-binding sites may also promote platelet reacti vity by permitting interaction with the collagen receptor. Using tripl e-helical peptides based on the sequence of the platelet-reactive coll agen type III fragment alpha 1(III)CB4, we have been able to locate an alpha 2 beta 1-binding site in collagen type III within a 30-mer sequ ence representing residues 508-537 of the alpha 1(III) constituent alp ha-chain. Despite their alpha 2 beta 1-independent platelet reactivity , signalling by the (Gly-Pro-Hyp)(n)-based peptides shows many feature s in common with signalling by collagen fibers, including activation o f p72(SYK) and p125(FAK) the latter of which has until now been consid ered a specific consequence of ligand binding to alpha 2 beta 1. (C) 1 996 John Wiley Sons, Inc.