Beta-galactosyltransferase that transfers beta-galactose to hydroxylysine residues of type I collagen (PubMed:19075007, PubMed:22216269, PubMed:27402836). By acting on collagen glycosylation, facilitates the formation of collagen triple helix (PubMed:27402836). Also involved in the biosynthesis of collagen type IV (PubMed:30412317). (5R)-5-hydroxy-L-lysyl-[collagen] + UDP-alpha-D-galactose = (5R)-5-O-(beta-D-galactosyl)-5-hydroxy-L-lysyl-[collagen] + H(+) + UDP Colocalized with PLOD3 and mannose binding lectin/MBL2. Ubiquitous with higher levels in placenta, heart, lung and spleen. N-glycosylated. The disease is caused by variants affecting the gene represented in this entry. Belongs to the glycosyltransferase 25 family. Has no glucosyltransferase activity.