Alpha-1,6-mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers the second mannose to the glycosylphosphatidylinositol during GPI precursor assembly. Defects in PIGV are the cause of hyperphosphatasia with mental retardation type 1 (HPMRS1). It is a syndrome characterized by elevated serum alkaline phosphatase, severe mental retardation, seizures, hypotonia, facial dysmorphism, and hypoplastic terminal phalanges. Belongs to the PIGV family. Note: This description may include information from UniProtKB.