Arginine methylation of Sam68 and SLM proteins negatively regulates their poly(U) RNA binding activity.

Sam68 (Src substrate associated during mitosis) and its homologues, SLM-1 and SLM-2 (Sam68-like mammalian proteins), are RNA binding proteins and contain the arg-gly (RG) repeats, in which arginine residues are methylated by the protein arginine methyltransferase 1 (PRMT1). However, it remains unclear whether the arginine methylation affects an RNA binding. Here, we report that methylation of Sam68 and SLM proteins markedly reduced their poly(U) binding ability in vitro. The RG repeats of Sam68 bound poly(U), but arginine methylation of the RG repeats abrogated its poly(U) binding ability in vitro. Overexpression of PRMT1 increased arginine methylation of Sam68 and SLM proteins in cells, which resulted in a decrease of their poly(U) binding ability. The results suggest that the RG repeats conserved in Sam68 and SLM proteins may function as an auxiliary RNA binding domain and arginine methylation may eliminate or reduce an RNA binding ability of the proteins.