A Predicted Amphipathic Helix Mediates Plasma Membrane Localization of GRK5

Abstract

G protein-coupled receptor kinases (GRKs) specifically phosphorylate agonist-occupied G protein-coupled receptors at the inner surface of the plasma membrane (PM), leading to receptor desensitization. GRKs utilize a variety of mechanisms to bind tightly, and sometimes reversibly, to cellular membranes. Previous studies demonstrated the presence of a membrane binding domain in the C terminus of GRK5. Here we define a mechanism by which this short C-terminal stretch of amino acids of GRK5 mediates PM localization. Secondary structure predictions suggest that a region contained within amino acids 546-565 of GRK5 forms an amphipathic helix, with the key features of the predicted helix being a hydrophobic patch of amino acids on one face of the helix, hydrophilic amino acids on the opposite face, and a number of basic amino acids surrounding the hydrophobic patch. We show that amino acids 546-565 of GRK5 are sufficient to target the cytoplasmic green fluorescent protein (GFP) to the PM, and the hydrophobic amino acids are necessary for PM targeting of GFP-546-565. Moreover, full-length GRK5-GFP is localized to the PM, but mutation of the hydrophobic patch or the surrounding basic amino acids prevents PM localization of GRK5-GFP. Last, we show that mutation of the hydrophobic residues severely diminishes phospholipid-dependent autophosphorylation of GRK5 and phosphorylation of membrane-bound rhodopsin by GRK5. The findings in this report thus suggest the presence of a membrane binding motif in GRK5 and define the importance of a group of hydrophobic amino acids within this motif in mediating its PM localization.