Crithidia luciliae: Effect of purine starvation on S-adenosyl-L-methionine uptake and protein methylation

The utilization of S-adenosyl-L-[methyl-H-3]methionine ([H-3-methyl]AdoMet) by Crithidia luciliae was assessed under nutrient-replete and purine-starvation conditions. Uptake experiments with intact cells demonstrated that the radiolabel from this molecule was accumulated by purine-starved organisms at a rate similar to 10-fold greater than that observed in those cultivated in nutrient-replete medium. Purine-starved cells also incorporated the radiolabel into trichloroacetic acid insoluble material at an similar to 10-fold faster rate than nutrient-replete cells. No differences, however, were observed in the intracellular levels of AdoMet and its metabolites between organisms cultivated under the two conditions. Results of comparative labeling studies with [H-3-methyl]AdoMet, S-adenosyl-L-[carboxyl-C-14]methionine, L-[methyl-H-3] methionine and L-[S-35]methionine in the presence and absence of cycloheximide demonstrated that the incorporation of label from [H-3-methyl]AdoMet was due to transmethylation and was independent of protein synthesis. Further, similar to 15 methylated protein bands were identified by SDS-PAGE analysis. Lysates from both purine-starved and nutrient-replete organisms demonstrated similar levels of activity of three protein methyltransferases (PMI, II, III). The differences observed in [H-3-methyl]AdoMet utilization between purine-starved and nutrient-replete C. luciliae may reflect the enhanced purine transport capacity which results from purine starvation. (C) 1995 Academic Press, Inc.