ASSOCIATION OF THE REGULATORY BETA-ADRENERGIC-RECEPTOR KINASE WITH RAT-LIVER MICROSOMAL-MEMBRANES

Beta-adrenergic receptor kinase (betaARK) is a regulatory enzyme involved in the modulation of beta-adrenergic and other G protein-coupled receptors. lt has been described that betaARK is a cytosolic protein that transiently translocates to the plasma membrane in order to specifically phosphorylate agonist-occupied receptors. In this report, we used betaARK-specific antibodies to demonstrate that a significant amount of this kinase is present in rat liver microsomal membranes. BeatARK seems to be peripherally associated with the cytosolic side of microsomal membranes since it can be stripped from the membranes by mild salt treatment. Cell-free association experiments indicate that the interaction of betaARK is reversible, saturable, and strongly inhibited by protease or heat treatment of the microsomes, thus suggesting that betaARK interacts with a protein component of the microsomal membrane. Gradient fractionation studies indicate that the highest betaARK-specific activity co-migrates with endoplasmic reticulum enzymatic markers. Furthermore, indirect immunofluorescence and immunogold electron microscopy experiments performed in cultured cells using affinity-purified anti-betaARK antibodies are consistent with this subcellular localization pattern. Taken together, our data suggest that several betaARK pools (i.e. microsome-bound, plasma membrane-bound, and cytosolic) may exist inside the cell. Such results are in line with recent reports showing that proteins involved in plasma membrane signal transduction, such as heterotrimeric G proteins, are also associated with membranes of different intracellular organelles.