Regulation of rat liver phosphatidylethanolamine N-methyltransferase by cytosolic factors. Examination of a role for reversible protein phosphorylation

The nature of cytosolic factors which modulate the activity of rat liver phosphatidylethanolamine (PE) methyltransferase was investigated. The combined additions of cytosol, Mg∙ATP, and NaF to incubations with rat liver microsomes produced a 1.6-fold activation of the methyltransferase at pH 9.2 and a 1.3-fold stimulation at pH 7.0. Nonhydrolyzable 5′ -adenylylimidodiphosphate could not substitute for ATP, although GTP could. The activation was time dependent, stable to reisolation of the microsomes by ultracentrifugation, and partially preventable by other cytosolic components. Despite these indications that PE methyltransferase might be a substrate for cytosolic protein kinases, cAMP and Ca2+–calmodulin exerted little influence on the activation reaction. Furthermore, microsomal PE methyltransferase activity was unaffected by purified preparations of cAMP-dependent protein kinase, calmodulin–dependent protein kinase, and casein kinase II, nor was methyltransferase activity influenced by the purified catalytic subunits of protein phosphatases 1 and 2A. Cytosol also contained inhibitors of PE methyltransferase which could overcome the Mg∙ATP∙NaF-mediated activation of the enzyme, but were not affected by the thermostable phosphatase inhibitors 1 and 2. Part of this inhibitory activity (apparent molecular mass of 15 × 103 daltons) was insensitive to trypsin and chymotrypsin, stimulated by Mn2+, and partly inhibited by NaF. Therefore, regulation of methyltransferase by reversible phosphorylation, while still a tenable hypothesis, is apparently more complex than previously proposed.