Involved in endoplasmic reticulum-associated degradation (ERAD). Accelerates the glycoprotein ERAD by proteasomes, by catalyzing mannose trimming from Man8GlcNAc2 to Man7GlcNAc2 in the N-glycans (PubMed:25092655). May also participate in mannose trimming from all glycoproteins and not just misfolded ones targeted to ERAD (PubMed:34143952). May have alpha 1,2-mannosidase activity (By similarity). {ECO:0000250, ECO:0000269|PubMed:25092655, ECO:0000269|PubMed:34143952}. This is the function of EDEM3 (ER degradation enhancing alpha-mannosidase like protein 3, ENSG00000116406).