Catalytic domain of the Serine/Threonine Kinase, G protein-coupled Receptor Kinase 4
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. GRK4 has a limited tissue distribution. It is mainly found in the testis, but is also present in the cerebellum and kidney. It is expressed as multiple splice variants with different domain architectures and is post-translationally palmitoylated and localized in the membrane. GRK4 polymorphisms are associated with hypertension and salt sensitivity, as they cause hyperphosphorylation, desensitization, and internalization of the dopamine 1 (D1) receptor while increasing the expression of the angiotensin II type 1 receptor. GRK4 plays a crucial role in the D1 receptor regulation of sodium excretion and blood pressure. GRKs phosphorylate and regulate G protein-coupled receptors (GPCRs), the largest superfamily of cell surface receptors which regulate some part of nearly all physiological functions. Phosphorylated GPCRs bind to arrestins, which prevents further G protein signaling despite the presence of activating ligand. The GRK4 subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.