Characterization of prmt7alpha and beta isozymes from Chinese hamster cells sensitive and resistant to topoisomerase II inhibitors

By selection of genetic suppressor elements (GSEs) conferring resistance to topoisomerase II inhibitors in Chinese hamster cells (DC-3F), we identified a gene encoding two proteins of 78 and 82 kDa which belong to the protein arginine methyltransferase (PRMT) family. Down-regulation of these enzymes (named PRMT7alpha and beta), either induced by an antisense GSE or as observed in the 9-OH-ellipticine (9-OH-E) resistant mutant DC-3F/9-OH-E, was responsible for cell resistance to various DNA damaging agents. Alternative splicing alterations in the 5'-terminal region and changes of the polyadenylation site of PRMT7 mRNAs were observed in these resistant mutant cells. PRMT7alpha and beta are isoforms of a highly conserved protein containing two copies of a module common to all PRMTs, comprising a Rossmann-fold domain and a beta-barrel domain. The C-terminal repeat appears to be degenerate and catalytically inactive. PRMT7alpha and beta form homo- and hetero-dimers but differ by their sub-cellular localization and in vitro recognize different substrates. PRMT7beta was only observed in Chinese hamster cells while mouse 10T1/2 fibroblasts only contain PRMT7alpha. Surprisingly, in human cells the anti-PRMT7 antibody essentially recognized an approximately 37 kDa peptide, which is not formed during extraction, and a faint band at 78 kDa. Analysis of in vitro and in vivo methylation patterns in cell lines under- or over-expressing PRMT7alpha and beta detected a discrete number of proteins which methylation and/or expression are under the control of these enzymes.