CD36 peptides enhance or inhibit CD36-thrombospondin binding. A two-step process of ligand-receptor interaction.
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CD36 (glycoprotein IV or IIIB) is an integral plasma membrane protein of wide cellular distribution and functions as a receptor site for thrombospondin (TSP), an adhesive protein important in cell-cell and cell-matrix interactions. OKM5, a monoclonal anti-CD36 antibody, has been reported to block CD36 cell adhesive functions suggesting that the OKM5 epitope on CD36 is functionally important. A panel of 10 synthetic CD36 peptides was made. One peptide, P139-155, specifically inhibited the immunoadsorption of CD36 by OKM5, and P139-155 was directly immunoadsorbed by OKM5, indicating that CD36 sequence 139-155 represents part of the OKM5 epitope. TSP bound to immobilized P139-155 in a dose-dependent and saturable manner. Surprisingly, P139-155 significantly augmented, instead of inhibited, binding of CD36 to TSP. This peptide did not induce platelet aggregation but augmented ADP- and collagen-induced aggregation in platelet-rich plasma. Another CD36 peptide, P93-110, which had no effect on OKM5 immunoadsorption, blocked binding of CD36 to immobilized TSP and partially inhibited collagen-induced platelet aggregation. P93-110 by itself did not bind to TSP; however, in the presence of P139-155, there was a marked enhancement of P93-110 binding to TSP, with a stoichiometry consistent with the trimeric nature of TSP. The data suggest that CD36-TSP interaction is a two-step process; the sequence 139-155 region of CD36 binds first to TSP, triggering a change in TSP to reveal a second site, which binds the 93-110 region of CD36 with high affinity. CD36 peptides can be used as stimulators or inhibitors in cellular adhesive events involving TSP-CD36 interaction. Conformational changes leading to the exposure or activation of high affinity binding sites may occur in both the receptor and the ligand upon cell-cell and cell-matrix adhesion.
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