Probable inactive glycosyltransferase required during spermatid development. May participate in protein loading into the acrosomes and accumulation of ubiquitin-proteasome systems around the head-tail coupling apparatus region. Localizes to the juxtanuclear region, possibly the late endosome. Not localized in the Golgi apparatus in round spermatids (PubMed:24398516). Expressed in testis. Mainly expressed in the round and elongated spermatids during spermiogenesis, not in the outermost cells of the seminiferous tubules, which contain spermatogonia and somatic Sertoli cells. Present in the juxtanuclear space in the round spermatids, not in the acrosomal vesicles. In the elongating spermatids, localizes strongly in the acroplaxome, the region between the developing acrosome and nucleus. During differentiation, also weakly detected in the transient manchette containing microtubules. In epididymal spermatozoa, weakly detected in the midpiece, but concentrates mainly in the neck region around the head-tail coupling apparatus (at protein level). There are two conserved domains in the glycosyltransferase region: the N-terminal domain (domain A, also called GT1 motif), which is probably involved in manganese coordination and substrate binding and the C-terminal domain (domain B, also called Gal/GalNAc-T motif), which is probably involved in catalytic reaction and UDP-Gal binding. Heterozygous mice show male infertility because of immotile sperm: glycolytic enzymes required for sperm motility are decreased, their protein loading into acrosomes disrupted, and aberrant localization of the ubiquitin-proteasome system is observed. Females show normal fertility. Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. In contrast to other members of the family, lacks the C-terminal ricin B-type lectin domain that contributes to the glycopeptide specificity, and lacks the conserved His residue in position 341. No glycosyltransferase activity has been detected in an in vitro assay (PubMed:24398516). Contaminating sequence. Potential poly-A sequence. Putative polypeptide N-acetylgalactosaminyltransferase-like protein 5