Inhibition of α-aminoisobutyric acid transport in membrane vesicles from mouse fibroblasts after phosphorylation by cyclic AMP-dependent protein kinase

Cyclic AMP-dependent protein kinases from several mammalian sources inhibit Na⁺-dependent α-aminoisobutyric acid transport by membrane vesicles isolated from 3T3 cells. Evidence is provided that phosphorylation of membrane proteins by the enzyme is responsible for the inhibition. Lysis of the vesicles, or a reduction in the intravesicular volume is not the cause of reduced transport.The cyclic AMP-dependent protein kinase and its catalytic subunit phosphorylate a number of membrane proteins. Most of these proteins are phosphorylated, but to a lesser extent in the absence of protein kinase or cyclic AMP. The phosphorylated proteins remain associated with the membranes during hypotonic lysis treatments, which would be expected to release intra-vesicular contents and loosely associated membrane proteins. ³²P-labeled bands detected on sodium dodecyl sulfate polyacrylamide gels after phosphorylation of membranes by the catalytic subunit of the cyclic AMP-dependent kinase are eliminated by treatment with either pronase or 1 N NaOH, but not by ribonuclease nor by phospholipase C. The stability of the incorporated radioactivity to hot acid and hydroxylamine relative to hot base suggests that most of the ³²P from γ-³²P]ATP is incorporated into protein phosphomonoester linkages.