LAUSR

Rap1 is a major convergence point of the platelet signaling pathways that result in talin1 binding to the integrin b cytoplasmic domain and consequent integrin activation, platelet aggregation, and effective hemostasis. The nature of the connection between Rap1 and talin1 in integrin activation is an important remaining gap in our understanding of this process. Previous work identified a low affinity Rap1 binding site in talin1 F0 domain that makes a small contribution to integrin activation in platelets. We recently identified an additional Rap1 binding site in talin1 F1 domain that makes a greater contribution than F0 in model systems. Here we generated mice bearing point mutations, which block Rap1 binding without affecting talin1 expression, in either talin1 F1 domain (R118E) alone, which were viable, or both F0 and F1 domains (R35E,R118E), which were embryonic lethal. Loss of the Rap1-talin1 F1 interaction in platelets markedly decreases talin1-mediated activation of platelet b1- and b3-integrins. Integrin activation and platelet aggregation in mice whose platelets express only talin1(R35E,R118E) are even more impaired, resembling the defect seen in platelets lacking both Rap1a and Rap1b. Although Rap1 is important in thrombopoiesis, platelet secretion, and surface exposure of phosphatidylserine, loss of Rap1-talin interaction in talin1(R35E,R118E) platelets had little effect on these processes. These findings show that talin1 is the principal direct effector of Rap1 GTPases that regulates platelet integrin activation in hemostasis.