The PDZ domain protein SYNJ2BP regulates GRK-dependent Sst2A phosphorylation and downstream MAPK signaling

The somatostatin receptor 2A (SST2) is a G-protein-coupled receptor (GPCR) that is expressed in neuroendocrine tissues within the GI tract and brain, and is commonly overexpressed in many neuroendocrine tumors. Moreover, SST2 agonists are used clinically as the primary pharmacological treatment to suppress excess hormone secretion in a variety of neuroendocrine tumors. Despite its wide clinical use, mechanisms controlling the trafficking and signaling of SST2 are not fully understood. SST2 contains a C-terminal PDZ domain binding motif that has been shown to interact with three different PDZ domain containing proteins. However, the consequences of these interactions are not well understood, nor is it known whether additional PDZ domain proteins interact with SST2. Through unbiased screening we have identified ten additional PDZ domain proteins that interact with SST2. We chose one of these, SYNJ2BP, for further study. We observed that SYNJ2BP interacted with SST2 in an agonist-dependent manner, and that this required the PDZ binding site of SST2. Importantly, overexpression of SYNJ2BP enhanced ligand-stimulated receptor internalization. Mechanistically, SYNJ2BP interacted with GRK2 and promoted GRK-dependent phosphorylation of the receptor after somatostatin stimulation. Interaction with GRK2 required the C-terminus of SYNJ2BP. Binding to SYNJ2BP did not affect the ability of SST2 to suppress cAMP production, but was required for optimal agonist-stimulated ERK1/2 activation. These data indicated that SYNJ2BP is SST2 interacting protein that modulates agonist-stimulated receptor regulation and downstream signaling.