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

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Abstract
G protein‐coupled receptor kinases (GRKs) phosphorylate G protein‐coupled receptors, thereby terminating receptor signaling. Herein we report that α‐actinin potently inhibits all GRK family members. In addition, calcium‐bound calmodulin and phosphatidylinositol 4,5‐bisphosphate (PIP2), two regulators of GRK activity, coordinate with α‐actinin to modulate substrate specificity of the GRKs. In the presence of calmodulin and α‐actinin, GRK5 phosphorylates soluble, but not membrane‐incorporated substrates. In contrast, in the presence of PIP2 and α‐actinin, GRK5 phosphorylates membrane‐incorporated, but not soluble substrates. Thus, modulation of α‐actinin‐mediated inhibition of GRKs by PIP2 and calmodulin has profound effects on both GRK activity and substrate specificity.