O-6-METHYLGUANINE-DNA METHYLTRANSFERASE OF HUMAN LYMPHOID-CELLS - STRUCTURAL AND KINETIC-PROPERTIES AND ABSENCE IN REPAIR-DEFICIENT CELLS

Abstract

Human lymphoid [GM1953, GM0621, GM0892A, GM2246 and Raji] contain a DNA repair enzyme which removes the mutagenic alkylation lesion O6-methylguanine from DNA. The enzyme transfers the methyl group to a protein cysteine residue, generating S-methylcysteine, and is inactivated as a consequence of the reaction. Apparently the methylated enzyme represents a dead-end complex. The transfer reaction is very rapid and is completed in less than 1 min at 37.degree. C, but methyl group transfer from single-stranded DNA or heavily damaged DNA is less efficient. The active methyltransferase and the methylated protein both have MW of 21,000-22,000 as determined by gel filtration. Lymphoid cell lines proficient in repair of O6-methylguanine in vivo, Mex+, contain 10,000-25,000 molecules of the methyltransferase per cell. Repair-deficient cell lines, Mex-, do not contain detectable amounts of the enzyme. The latter point was verified by applying a partial purification procedure for the enzyme to cell-free extracts from 2 Mex- cells lines.