Endosomal signaling by protease-activated receptors.

Protease-activated receptors (PARs) are a family of G protein-coupled receptors (GPCRs) that are uniquely activated by proteolysis. There are four members of the PAR family including: PAR1, PAR2, PAR3, and PAR4. PARs are expressed primarily in the cells of the vasculature and elicit cellular responses to coagulant and anticoagulant proteases. PAR1 exemplifies the unusual proteolytic mechanism of receptor activation. Thrombin binds to and cleaves the N-terminal exodomain of PAR1, generating a new N-terminus that functions as a tethered ligand. The N-terminal tethered ligand domain of PAR1 binds intramolecularly to the receptor to trigger transmembrane signaling and cannot diffuse away. Similar to other GPCRs, activation of PARs promotes coupling to heterotrimeric G proteins at the plasma membrane. After activation, PARs are rapidly internalized to endosomes and then sorted to lysosomes and degraded. Internalization functions to uncouple PARs from heterotrimeric G proteins at the cell surface. However, recent studies indicate that activated internalized PARs signal from endosomes through the recruitment of β-arrestins and potentially other pathways. Here, we provide an overview of methods and strategies used to examine endosomal signaling by PARs.