Enables mannosyl-oligosaccharide 1,2-alpha-mannosidase activity. Involved in mannose trimming involved in glycoprotein ERAD pathway. Acts upstream of or within glycoprotein catabolic process and proteasome-mediated ubiquitin-dependent protein catabolic process. Located in endoplasmic reticulum lumen. Human ortholog(s) of this gene implicated in congenital disorder of glycosylation type II. Orthologous to human EDEM3 (ER degradation enhancing alpha-mannosidase like protein 3). [provided by Alliance of Genome Resources, Apr 2022]