Methyl-esterified proteins in a mammalian cell line

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Abstract
Methyl esterification of carboxylic acid residues in intact mouse S49 lymphoma cells was examined, and at least 24 proteins were found to be modified. Cell fractionation revealed that a distinct set of these proteins could be found in each of the four fractions. Nuclei contained 11 methyl-esterified proteins at 12, 15.5, 18, 19, 39, 41, 45, 70, 90, 105, and 130 kilodaltons (kDa). Five proteins copurified with the plasma membrane/mitochondrial fraction at 13, 24, 25, 27, and 28 kDa. Two proteins at 32 and 56 kDa were in the microsomal fraction, and six were soluble at 16.5, 21, 24, 26, 34, and 36 kDa. Eleven of these proteins were [3H]methyl esterified when cell homogenates were incubated with S-adenosyl-L-[methyl-3H]methionine. The steady-state level of methyl group incorporation into protein in intact cells was approximately 118 pmol/mg of protein. Assuming the average protein is 40 kDa, there appears to be 1 methyl group per 210 proteins. This was compared to phosphorylation which gave approximately one phosphoryl group for every four proteins. Exogenously added L-[methyl-3H]methionine equilibrated with the cellular S-adenosylmethionine pool within 30 min which was sufficiently rapid to allow the rate of methyl group turnover to be determined. Most methyl-esterified proteins demethylated in a pulse-chase experiment with half-lives ranging from 2.6 to 9.3 h. When protein synthesis was blocked with puromycin, amino acid backbone incorporation of methionine was reduced to 2% of control. Methyl group incorporation, however, was 39% of the control. This level of methyl group incorporation could be attained whether label was added 15 or 105 min after the addition of puromycin, indicating that these proteins were being continuously methylated or demethylated. The reversible nature of this modification suggests that it may serve a regulatory function in these cells.