Unconventional splicing of HAC1/ERN4 mRNA required for the unfolded protein response. Sequence-specific and non-sequential cleavage of the splice sites.

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) activates an intracellular signaling pathway from the ER to the nucleus, termed the unfolded protein response. We and others recently identified transcription factor Hac1p/Ern4p responsible for the response in Saccharomyces cerevisiae and found that Hac1p expression is controlled by the regulated splicing of HAC1 mRNA. Walter and co-workers (Sidrauski, C., Cox, J. S., and Walter, P. (1996) Cell, 87, 405-413) further showed that the splicing requires tRNA ligase but not spliceosome. In this report, we carried out mutational analysis of HAC1 mRNA and revealed several unique features of the splicing. First, a mutation or deletion of the branchpoint-like sequence present in HAC1 intron did not affect the splicing. Second, cleavage of the splice sites was sequence-specific and thus completely blocked by some point mutations introduced at the 5' or 3' splice site. Third, cleavage of the 5' and 3' splice sites could occur independently as judged by the nature of splicing intermediates accumulated. Fourth, swapping the nucleotide sequences of the 5' and 3' splice sites inhibited the ligation but not the cleavage step. We conclude that signaling from the ER activates putative endonucleases that can carry out sequence-specific cleavage of the splice sites in a random order.