RIO Kinase 1 (RIOK1)

RIOK1 includes two alternatively spliced transcript variants, which encode different isoforms. The function of this gene has not been determined. RioK1, a new interactor of protein arginine methyltransferase 5 (PRMT5), competes with pICln for binding and modulates PRMT5 complex composition and substrate specificity. RioK1 and pICln bind to PRMT5 in a mutually exclusive fashion. This results in a PRMT5-WD45/MEP50 core structure that either associates with pICln or RioK1 in distinct complexes. RioK1 functions in analogy to pICln as an adapter protein by recruiting the RNA-binding protein nucleolin to the PRMT5 complex for its symmetrical methylation. The exclusive interaction of PRMT5 with either pICln or RioK1 thus provides the first mechanistic insight into how a methyltransferase can distinguish between its substrate proteins.