Prenylated protein methyltransferase, an enzyme involved in the post-translational modification of many signalling proteins, has been characterized in a parasitic flagellated protozoan, Leishmania donovani. The activity of this enzyme was monitored by the methylation of an artificial substrate, an S-prenylated cysteine analogue, with S-adenosyl-l-[methyl-(3)H]methionine as methyl donor. More than 85% of the methyltransferase activity was associated with membranes. The enzyme methylates N-acetyl-S-trans, trans-farnesyl-l-cysteine and N-acetyl-S-all-trans-geranylgeranyl-l-cysteine, but N-acetyl-S-trans, trans-geranyl-l-cysteine only very weakly. In contrast with the enzyme from mammals, the leishmanial enzyme had a greater affinity for the farnesylated substrate than for the geranylgeranylated one. Activity in vitro was not modulated by cAMP, protein kinase C activator or guanosine 5'-[gamma-thio]triphosphate. An analysis of the endogenous substrates showed that the carboxymethylated proteins were also isoprenylated. The main carboxymethylated proteins have molecular masses of 95, 68, 55, 46, 34-23, 18 and less than 14 kDa. Treatment of cells with N-acetyl-S-trans,trans-farnesyl-l-cysteine decreased the carboxymethylation level, whereas treatment with guanosine 5'-[gamma-thio]triphosphate increased the carboxymethylation of various proteins, particularly those of molecular masses 30-20 kDa.
展开▼
