Enables dolichyl-phosphate beta-D-mannosyltransferase activity and enzyme regulator activity. Acts upstream of or within GPI anchor biosynthetic process and dolichol metabolic process. Located in perinuclear region of cytoplasm. Part of dolichol-phosphate-mannose synthase complex. Colocalizes with endoplasmic reticulum. Human ortholog(s) of this gene implicated in congenital disorder of glycosylation Iu. Orthologous to human DPM2 (dolichyl-phosphate mannosyltransferase subunit 2, regulatory). [provided by Alliance of Genome Resources, Apr 2022]