A Snu114 GTP Prp8 module forms a relay station for efficient splicing in yeast

The single G protein of the spliceosome, Snu114, has been proposed to facilitate splicing as a molecular motor or as a regulatory G protein. However, available structures of spliceosomal complexes show Snu114 in the same GTP bound state, and presently no Snu114 GTPase regulatory protein is known. We determined a crystal structure of Snu114 with a Snu114 binding region of the Prp8 protein, in which Snu114 again adopts the same GTP bound conformation seen in spliceosomes. Snu114 and the Snu114 Prp8 complex co purified with endogenous GTP. Snu114 exhibited weak, intrinsic GTPase activity that was abolished by the Prp8 Snu114 binding region. Exchange of GTP contacting residues in Snu114, or of Prp8 residues lining the Snu114 GTP binding pocket, led to temperature sensitive yeast growth and affected the same set of splicing events in vivo. Consistent with dynamic Snu114 mediated protein interactions during splicing, our results suggest that the Snu114 GTP Prp8 module serves as a relay station during spliceosome activation and disassembly, but that GTPase activity may be dispensable for splicin