Figure 7.

Scheme illustrating the functions of Aar2p during U5 snRNP assembly. In pre-U5 snRNP, Aar2p blocks RNA (e.g., U4/U6) and Brr2p binding to the Prp8p C-terminal region. Phosphorylation of Aar2p at S253 and T345 in the nucleus (by unknown kinases) leads to its detachment from Prp8p and allows Brr2p binding to form mature U5 snRNP. The same signals allow the RH domain of Prp8p to bind U4/U6 di-snRNA and inhibit their premature unwinding by Brr2p in the U4/U6-U5 tri-snRNP. The precise sequence of phosphorylation events is presently unknown. To participate in further rounds of U5 snRNP assembly, Aar2p has to be dephosphorylated by an unknown phosphatase and shuttled back to the cytoplasm. (NPC) Nuclear pore complex.