Histone-lysine methyltransferase activity from sea-urchin embryo nuclei. Changes in substrate specificity upon purification.

The S-adenosylmethionine:histone-lysine methyltransferase (EC 2.1.1.43) enzyme activity, present in the chromatin of sea-urchin embryo nuclei, has been purified about 300-fold with 30% overall yield. The initial activity in the nucleus transfers methyl groups to the epsilon-amino group of lysines and acceptor proteins are chromatin-bound H3 and H4 histones. In contrast, the purified enzyme activity transfers methyl groups to the arginines and acceptor proteins are soluble H3 and H4 histones. The two changes in substrate specificity do not occur at the same time. The variation of acceptor protein from chromatin-bound to soluble histones occurs at the first step, upon nuclei sonication, when no protein fractionation has yet been performed. At that step, lysine is still the only methylated side-chain. The variation of the methylated amino acid from lysine to arginine occurs gradually with increasing enzyme purification. The enzyme activity has a molecular mass of about 200 kDa. Saturation curves for H3 and H4 histones, used as substrate either individually or in total histones, and for AdoMet show no substantial dependence on enzyme purification. Maximal activity for the enzyme, at all purification levels, occurs at about pH 8 for all substrate histones. An increase in the relative concentrations of di- and trimethyllysine derivatives is observed with the more purified enzyme preparations, while the ratio of mono- and dimethylarginine derivatives remains constant. The data are taken as evidence that the same protein molecule is responsible for the two activities.