O-mannosyl-transferase that transfers mannosyl residues to the hydroxyl group of serine or threonine residues of proteins (PubMed:37186866). Specifically glycosylates the IPT/TIG domain of target proteins, such as MET and MST1R/RON (PubMed:37186866). TMEM260- mediated O-mannosylated residues are composed of single mannose glycans that are not elongated or modified (PubMed:37186866). . This is the function of TMEM260 (transmembrane protein 260, Ensembl gene identifier ENSG00000070269).