Four yeast spliceosomal proteins (PRP5, PRP9, PRP11, and PRP21) interact to promote U2 snRNP binding to pre-mRNA

We have analyzed the functions of several pre-mRNA processing (PRP) proteins in yeast spliceosome formation. Here, we show that PRP5 (a DEAD box helicase-like protein), PRP9, and PRP11 are each required for the U2 snRNP to bind to the pre-spliceosome during spliceosome assembly in vitro. Genetic analyses of their functions suggest that they and another protein, PRP21, act concertedly and/or interact physically with each other and with the stem-loop IIa of U2 snRNA to bind U2 snRNP to the pre-mRNA. Biochemical complementation experiments also indicate that the PRP9 and PRP11 proteins interact. The PRP9 and PRP11 proteins may be functioning similarly in yeast and mammalian cells. The requirement for ATP and the helicase-like PRP5 protein suggests that these factors might promote a conformational change (involving either the U1 or U2 snRNP) that is required for the association of U2 snRNP with the pre-mRNA.