Alpha-1,2-specific exomannosidase of the ER; involved in glycan trimming of both folded and misfolded glycoproteins; complexes with Pdi1p, and recognizes misfolded, globular glycoproteins through the C-terminal domain, trimming a mannose from Man8GlcNac2 glycan to generate Man7GlcNac2, an oligosaccharide signal for ER-associated protein degradation (ERAD); modifies Pdi1p redox reactions; requires Pdi1p for stability and substrate recognition; human homolog EDEM1 can complement yeast null mutant