The biochemical defects of prp4-1 and prp6-1 yeast splicing mutants reveal the PRP6 protein is required for the accumulation of the (U4/U6.U5) tri-snRNP

We have raised specific antibodies against the PRP6 protein and shown that the U4, U5 and U6 snRNAs are co-precipitated with this protein. Using splicing extracts prepared from in vivo heat-inactivated cells, we have characterized the prp4-1 and prp6-1 biochemical defects. In inactivated prp4-1 cell extracts, the U6 snRNA content as well as the U6, U4/U6 snRNPs and the (U4/U6.U5) tri-snRNP particles amounts are severely reduced. In inactivated prp6-1 cell extracts, the PRP6 mutant protein is barely detectable. Glycerol gradient analyses indicate that, in these extracts, the (U4/U6.U5) tri-snRNPs are present in very low amounts, but U4/U6 snRNP particles are normally represented. The results establish that the PRP6 protein is required for the accumulation of the (U4/U6.U5) tri-snRNP. We found no evidence for the presence of the PRP6 protein in the U4/U6 particle.