A survival pathway for Caenorhabditis elegans with a blocked unfolded protein response

The unfolded protein response (UPR) counteracts stress caused by unprocessed ER client proteins. A genome-wide survey showed impaired induction of many UPR target genes in xbp-1 mutant Caenorhabditis elegans that are unable to signal in the highly conserved IRE1-dependent UPR pathway. However a family of genes, abu (activated in blocked UPR), was induced to higher levels in ER-stressed xbp-1 mutant animals than in ER-stressed wild-type animals. RNA-mediated interference (RNAi) inactivation of a representative abu family member, abu-1 (AC3.3), activated the ER stress marker hsp-4::gfp in otherwise normal animals and killed 50% of ER-stressed ire-1 and xbp-1 mutant animals. Abu-1(RNAi) also enhanced the effect of inactivation of sel-1, an ER-associated protein degradation gene. The nine abu genes encode highly related type I transmembrane proteins whose lumenal domains have sequence similarity to a mammalian cell surface scavenger receptor of endothelial cells that binds chemically modified extracellular proteins and directs their lysosomal degradation. Our findings that ABU-1 is an intracellular protein located within the endomembrane system that is induced by ER stress in xbp-1 mutant animals suggest that ABU proteins may interact with abnormal ER client proteins and this function may be particularly important in animals with an impaired UPR.