MEP50 (methylosome protein 50) is a component of the methylosome, a protein arginine methyltransferase complex that modifies specific arginine residues found in arginine- and glycine-rich regions of some spliceosomal Sm proteins. MEP50 is important for methylosome activity and may regulate the transfer of Sm proteins to the SMN (survival of motor neurons) complex, an early step in the assembly of U snRNPs. Both the methylosome and the SMN complex are essential for the assembly of spliceosomal snRNPs. MEP50 is a WD repeat protein that may provide an interface for multiple protein interactions between methylosome proteins. It binds to JBP1, an arginine protein methyltransferase component of the methylosome. MEP50 has been shown to interact with CTD phosphatase FCP1 (CTDP1), a protein that may link the processes of transcriptional elongation and splicing, and with SUZ12, a polycomb group protein involved in transcriptional repression. JBP1 and MEP50 have also been reported to interact with the methyl-CpG binding protein complex MBD2/NuRD.