May play a role in male spermatogenesis. In vitro acts as inhibitor of MGAT1 activity causing cell surface proteins to carry mainly high mannose N-glycans. The function is mediated by its lumenal domain and occurs specifically in the Golgi. A catalytic glucosyltransferase activity is not detected. May be involved in regulation of Sertoli-germ cell interactions during specific stages of spermatogenesis. Isoform 2 self-associates; specifically in the endoplasmic reticulum prior to its translocation to the Golgi. Isoform 1 and isoform 2 interact with MGAT1, MGAT3 and MAN2A2; isoform 2 interacts specifically with MGAT1 in the Golgi. Isoform 1 and isoform 2 are specifically expressed in testis. Isoform 2 is expressed in spermatocytes but not in spermatids. Isoform 1 is expressed in spermatids. Expression of isoform 1 and isoform 2 is up-regulated in testis at postnatal day 17; isoform 1 maintains similar expression levels until the adult; expression of isoform 2 peaks at postnatal day 22 and is barely detected in the adult. Isoform 2 is N-glycosylated; consisting of high-mannose and/or hybrid glycans. For isoform 1 inhibition of MGAT1 activity is demonstrated in vitro upon forced membrane-association. Its rat ortholog is the most abundant protein in testis Golgi preparations. Belongs to the glycosyltransferase 54 family. GlcNAcT VI