Physical Interaction with EDEM1 hinders proteolytic down‐regulation of human ER mannosidase I as a component of the mammalian UPR

The partitioning of N‐glycosylated proteins between folding and quality control requires the removal of α1,2‐linked mannose by endoplasmic reticulum mannosidase I (ERManI). The rate of mannose removal, and resultant substrate selection, are functions of the ERManI concentration. The human ortholog is not transcriptionally responsive to the mammalian unfolded protein response (UPR), but subject to proteolytic down‐regulation. Herein, the regulation of human ERManI by ER stress was elucidated. The elimination of a glycosylated ERAD substrate was accelerated in response to over‐expression of the inactive ER degradation‐enhancing mannosidase‐like protein 1 (EDEM1). Concomitant acceleration of N‐linked oligosaccharide trimming implied the involvement of ERManI. In support of this hypothesis, a physical interaction between EDEM1 and ERManI was detected by co‐immunoprecipitation. Furthermore, human ERManI was stabilized in response to experimental induction of the UPR in cells treated with tunicamycin or transfected with a spliced variant of XBP1. Ablated stabilization in response to RNAi‐mediated knock‐down of endogenous EDEM1 substantiated the physiological relevance of the regulatory system. A model is proposed in which physical interaction with transcriptionally‐elevated EDEM1 boosts glycoprotein degradation during ER stress by hindering the down‐regulation of ERManI (NIH R01DK064232).