Nuclear Pre-mRNA Decapping and 5′ Degradation in Yeast Require the Lsm2-8p Complex

Abstract

Previous analyses have identified related cytoplasmic Lsm1-7p and nuclear Lsm2-8p complexes. Here we report that mature heat shock and MET mRNAs that are trapped in the nucleus due to a block in mRNA export were strongly stabilized in strains lacking Lsm6p or the nucleus-specific Lsm8p protein but not by the absence of the cytoplasmic Lsm1p. These nucleus-restricted mRNAs remain polyadenylated until their degradation, indicating that nuclear mRNA degradation does not involve the incremental deadenylation that is a key feature of cytoplasmic turnover. Lsm8p can be UV cross-linked to nuclear poly(A)⁺ RNA, indicating that an Lsm2-8p complex interacts directly with nucleus-restricted mRNA. Analysis of pre-mRNAs that contain intronic snoRNAs indicates that their 5′ degradation is specifically inhibited in strains lacking any of the Lsm2-8p proteins but Lsm1p. Nucleus-restricted mRNAs and pre-mRNA degradation intermediates that accumulate in lsm mutants remain 5′ capped. We conclude that the Lsm2-8p complex normally targets nuclear RNA substrates for decapping.

We thank Phil Mitchell for critical reading of the manuscript, Reinhard Lührmann for the R1131 and H20 sera, John Aris for the A66 antibodies, Bertrand Séraphin for strain BSY557 and plasmids pBS1181 and pBS1184, and Emmanuelle Fabre for providing strain FYEF95.

C.B.-A. was the recipient of a Marie Curie fellowship from the EU. This work was supported by the Wellcome Trust.