Regulation of Phospholipid Methylation by Reversible Phosphorylation

Abstract

At physiological doses, glucagon produces a fast and transient activation of phospholipid methyltransferase activity in isolated rat hepatocytes (Castaño et al., 1980). Similarly, glucagon produces a transient accumulation of radioactivity into phospholipids in rat hepatocytes prelabeled with (methyl–3H)-methionine (Schuller et al., 1985). Exogenous cyclic AMP (Castaño et al., 1980) or chlorophenyl thio-cyclic AMP (Pritchard et al., 1981), added to isolated rat hepatocytes, mimic the effect of glucagon on phospholipid methyltransferase. These results suggested that the activation of phospholipid methyltransferase by glucagon is a cyclic AMP-dependent process. Further evidence about this hypothesis was obtained with isolated rat liver microsomes. Cyclic AMP, in the presence of ATP, produces an activation of phospholipid methyltransferase in isolated rat liver microsomes (Mato et al., 1982). Phospholipid methyltransferase has now been purified some 300-fold from rat liver (Pajares et al., 1983). The purified enzyme is activated by incubation with ATP and the catalytic subunit of cyclic AMP dependent protein kinase (cAMP-PK) (Varela et al., 1984). Under these conditions, only one protein of Mr about 50K and pI 4.75 is phosphorylated at serine residues (Varela et al., 1984). The time course of phosphorylation of this protein correlates well with the time course of activation of the enzyme (Varela et al., 1984). These results indicate that this 50K protein modulates phospholipid methyltransferase activity. The present paper shows new data on the regulation of the phosphorylation of the 50K protein of phospholipid methyltransferase both, using a purified preparation of the enzyme and with intact rat hepatocytes.