Phosphorylation of Membrane Proteins by Protein Kinase in a Smooth Muscle Plasma Membrane Fraction

Abstract

Properties of protein kinase and its phosphoprotein substrates associated with a plasma membrane (sarcolemmal) fraction isolated from rat uterine smooth muscle were investigated. The enzyme, a mixture of cAMP-dependent and -independent types, was active against exogenous substrates only in the presence of Triton X-100. In contrast, a few membrane proteins were actively phosphorylated in the absence of detergent. Protein kinase was resistant to trypsin digestion when associated with the plasma membrane but not when solubilized with Triton X-100. Membrane phosphoproteins, on the other hand, were sensitive to proteolysis in their native state. These results suggest the smooth muscle sarcolemmal protein kinase interacts extensively with the membrane lipid bilayer and its intrinsic substrates, though heterogeneous, share a common membrane orientation.