U snRNP assembly in yeast involves the La protein

In all eukaryotic nuclei, the La autoantigen binds nascent RNA polymerase I II transcripts, stabilizing these RNAs against exonucleases. Here we report that the La protein also functions in the assembly of certain RNA polymera se II-transcribed RNAs into RNPs, A mutation in a core protein of the splic eosomal snRNPs, Smd1p, causes yeast cells to require the La protein Lhp1p f or growth at low temperatures. Precursors to U1, U2, U4 and U5 RNAs are bou nd by Lhp1p in both wild-type and mutant cells. At the permissive temperatu re, smd1-1 cells contain higher levels of stable U1 and U5 snRNPs when Lhp1 p is present, At low temperatures, Lhp1p becomes essential for the accumula tion of U4/U6 snRNPs and for cell viability. When U4 RNA is added to extrac ts, the pre-U4 RNA, but not the mature RNA, is bound by Smd1p, These result s suggest that, by stabilizing a 3'-extended form of U4 RNA, Lhp1p facilita tes efficient Sm protein binding, thus assisting formation of the U4/U6 snR NP.