Modulation by the ATP/GTP ratio of the phosphorylation level of P-glycoprotein and of various plasma membrane proteins of KB-V1 multidrug resistant cells.

The level of protein phosphorylation is known to affect the properties of various membrane proteins. We have previously shown that GTP is capable of greatly enhancing the phosphorylation by [gamma-32P]ATP of P-glycoprotein (Pgp) from KB-V1 cells (3). Investigating the possibility of a general modulation of [gamma-32P]ATP plasma membrane protein phosphorylation, we found that phosphorylation of other membrane proteins are also modulated by various combinations of [ATP + GTP]. The ATP/GTP ratio giving the highest phosphorylation level depended on the protein studied. Modulation of the [gamma-32P]ATP-mediated phosphorylation of numerous membrane proteins requires hydrolysis of both ATP and GTP. ADP and GDP also increased [gamma-32P]ATP-driven phosphorylation but to a lesser extent than GTP. This plasma membrane endogenous phosphorylation activity was neither inhibited by specific inhibitors of protein kinase C, nor by inhibitors of cAMP- or cGMP-dependent protein kinases or of casein kinase II, respectively. Mastoparan, a G-protein regulator, increased the phosphorylation of some proteins that were already enhanced by the presence of [ATP + GTP] mixtures, especially proteins migrating in gels at the same position as P-glycoprotein.