Carboxyl methylation of nonhistone chromosomal proteins.

The in vitro methylation of nonhistone chromosomal proteins (NHCP) was investigated in nuclei isolated from the brain, liver, and thymus of 6-8-day-old rats. After the nuclei were incubated in the presence of 20 micron M S-adenosyl-L-[methyl-3H] methionine (1 Ci/mmol), the NHCP were separated from histones on hydroxylapatite and fractionated further on sodium dodecyl sulfate-acrylamide slab gel electrophoresis. After the gels were dried, autoradiography was used to detect [3H] methyl groups associated with these proteins. Four NHCP from the liver and thymus were methylated, while six methylated proteins were detected from the brain. None of the methylated proteins in these tissues corresponded with those from other organs, except for the component with a molecular weight of 66 000. It was evident that the methyl groups were esterified to the free carboxyl groups of NHCP since they are heat labile, yielding [3H]-methanol. The carboxyl-methylated NHCP from these organs were tightly bound to chromatin. Nucleoplasm and loosely associated NHCP were essentially devoid of methylated proteins. The carboxyl methylation of NHCP was verified in vivo. Six-day-old rats were given L-[methyl-3H]methionine (7 mCi/mmol) by intraperitoneal injection. The rats were killed at varying time periods and the NHCP isolated from gradient purified nuclei. Chromosomal nonhistone proteins, particularly from the liver, contained significant amounts of alkaline labile [(3)H] methyl groups.