Functionally diversified BiP orthologs control body growth, reproduction, stress resistance, aging, and ER-Phagy in Caenorhabditis elegans.

Cellular systems that govern protein folding rely on a delicate balance of functional redundancy and diversification to maintain protein homeostasis (proteostasis). Here, we use Caenorhabditis elegans to demonstrate how both overlapping and divergent activities of two homologous endoplasmic reticulum (ER)-resident HSP70 family chaperones, HSP-3 and HSP-4, orchestrate ER proteostasis and contribute to organismal physiology. We identify tissue-, age-, and stress-specific protein expression patterns and find both redundant and distinct functions for HSP-3 and HSP-4 in ER stress resistance, reproduction, and body size regulation. We show that only HSP-3 overexpression is sufficient to improve longevity and that loss of HSP-3 or HSP-4 during distinct stages of the worm cycle or specific tissues have opposing effects on worm lifespan. Furthermore, we find that loss of HSP-4, but not HSP-3, improves tolerance to protein aggregation induced-stress by activating ER-Phagy through the engagement of IRE-1 and the putative ER-Phagy receptor, C18E9.2. Mechanistically, we show that de-repression of IRE-1 via HSP-4 dissociation allows for direct inhibition of C18E9.2- mediated ER-Phagy and demonstrate that a conserved orthologous mechanism involving the respective human orthologs, BiP, Sec-62, and IRE-1, contributes to ER proteostasis regulation in human cells. Taken as a whole, our study demonstrates that functional diversification of orthologous proteins within a single organelle is an efficient mechanism to maximize stress resilience while also defining a novel link between ER- phagy and proteostasis regulation.