We have purified the yeast U5 and U6 pre-mRNA splicing small
nuclear ribonucleoproteins (snRNPs) by affinity chromatography
and analyzed the associated polypeptides by mass spectrometry.
The yeast U5 snRNP is composed of the two variants of U5 snRNA,
six U5-specific proteins and the 7 proteins of the canonical
Sm core. The U6 snRNP is composed of the U6 snRNA, Prp24, and
the 7 Sm-Like (LSM) proteins. Surprisingly, the yeast DEAD-box
helicase-like protein Prp28 is stably associated with the U5
snRNP, yet is absent from the purified U4/U6[bull ]U5 snRNP.
A novel yeast U5 and four novel yeast U4/U6[bull ]U5 snRNP
polypeptides were characterized by genetic and biochemical means
to demonstrate their involvement in the pre-mRNA splicing reaction.
We also show that, unlike the human tri-snRNP, the yeast tri-snRNP
dissociated upon addition of ATP or dATP.