polII RNA polymerase II RNP ribonucleoprotein SMA spinal muscular atrophy SMN survival of motor neurons snRNP small nuclear RNP snoRNP small nucleolar RNP UsnRNP uridine-rich snRNP

Spinal muscular atrophy is a common, often lethal, neurodegenerative disease that results from low levels of, or loss-of-function mutations in, the SMN (survival of motor neurons) protein. SMN oligomerizes and forms a stable complex with five additional proteins: Gemins 2–6. SMN also interacts with several additional proteins referred to as ‘substrates’. Most of these substrates contain a domain enriched in arginine and glycine residues (the RG-rich domain), and are constituents of different ribonucleoprotein complexes. Recent studies revealed that the substrates can be modified by an arginine methyltransferase complex, the methylosome. This forms symmetrical dimethylarginines within the RG-rich domains of the substrates, thereby converting them to highaffinity binders of the SMN complex, and most likely providing regulation of the ribonucleoprotein assembly processes.