alpha-Actinin is a potent regulator of G protein-coupled receptor kinase activity and substrate specificity in vitro

G protein-coupled receptor kinases (GRKs) phosphorylate G protein-coupled r eceptors, thereby terminating receptor signaling. Herein me report that alp ha-actinin potently inhibits all GRK family members. In addition, calcium-b ound calmodulin and phosphatidylinositol 4,5-bisphosphate (PIP2), two regul ators of GRK activity, coordinate with alpha-actinin to modulate substrate specificity of the GRKs. In the presence of calmodulin and alpha-actinin, G RK5 phosphorylates soluble, but not membrane-incorporated substrates. In co ntrast, in the presence of PIP2 and alpha-actinin, GRK5 phosphorylates memb rane-incorporated, but not soluble substrates. Thus, modulation of alpha-ac tinin-mediated inhibition of GRKs by PIP2 and calmodulin has profound effec ts on both GRK activity and substrate specificity. (C) 2000 Federation of E uropean Biochemical Societies.