Hemostasis, Thrombosis, and Vascular Biology

The 2 1 integrin is a major collagen receptor on platelets. Although it has been proposed that 2 1, like IIb 3, undergoes agonist-induced activation, neither the potential contributions of 2 1 receptor/ligand internalization to the increase in ligand binding nor the roles of the 2 and 1 cytoplasmic domains in activation of this integrin have been previously explored. Activation of 2 1 was assessed with fluorescein isothiocyanate–labeled soluble type I collagen binding to platelets by flow cytometry. Although collagen internalization in response to agonist activation of platelets was significant, agonist-induced collagen binding still occurred under conditions that block internalization, with minimal changes in cell surface 2 1 expression. Introduction of cell-permeable peptides containing the 2 cytoplasmic tail, and especially the membrane proximal KLGFFKR domain, induced 2 1 activation in resting platelets, whereas a cell-permeable peptide containing the 1 cytoplasmic tail was without effect. Thus, collagen binding to stimulated platelets is increased due to 2 1 activation, in addition to internalization, and the GFFKR motif appears to play an important role in the activation process. (Blood. 2003;102:1307-1315)