PROJECT SUMMARY/ABSTRACT Adhesion G protein-coupled receptors (aGPCRs) are the second largest subset of the GPCR superfamily, comprising of 33 members that can be subdivided into nine subfamilies. Majority of the receptors in this subset have varying length of the N-termini with a GPCR-autoproteolysis-inducing (GAIN) domain that mediates self- cleavage into two cleaved receptor protomers, the N-terminal fragment (NTF) and the cytoplasmic C-terminal fragment (CTF), that remain noncovalently associate with each other on the plasma membrane. Previously, we had identified that urea-dissociation of the NTF unveils a cryptic N-terminal stalk, encompassing the tethered peptide agonist region, that acts on the seven transmembrane (7TM) to activate G protein signaling. Although the biochemical dissociation of the NTF of aGPCR, provided invaluable insight into a tethered-peptide-agonist mechanism, the physiological mechanism that supports this remains unknown. We postulate that a shear-force induced mechanism would dissociate the NTF to reveal the tethered-peptide agonist to activate aGPCR. We have identified platelets that endogenously express aGPCR, GPR56, which is known to interact with collagen, to be an ideal model to investigate our proposed shear-force induced mechanism of NTF removal from GPR56. This model will also facilitate in further elucidating the role of GPR56 in platelet function, especially in hemostasis and thrombosis. In addition to the in vitro shear-force dissociation approaches, ex vivo and in vivo mouse models of hemostasis and thrombosis will be employed to aid in understanding and validating the underlying mechanism of GPR56 mode of activation. Importantly this project underlies the utility of a tethered-peptide-agonist mechanism as a tool to study aGPCR activation and identify novel receptors on an endogenous cell system. Understanding the mechanism of aGPCR, such as GPR56, activation in platelets may lead to new therapeutic drug design to treat aberrant platelet-related disorders, bleeding and thrombotic-complications.