This domain of unknown function is found in O-mannosyl -transferases TMTC1-4, and constitutes a loop between TM7 and TM8 located in the ER lumen that contains a small hydrophobic, but not membrane-embedded helix. This loop is critical for catalysis and binding of ligands, especially the lipid-linked sugar moiety [1]. TMTCs transfers mannosyl residues to the hydroxyl group of serine or threonine residues. The 4 members of the TMTC family are O-mannosyl-transferases acting primarily on the cadherin superfamily; each member has distinct roles in decorating the cadherin domains with O-linked mannose glycans at specific regions. These proteins also act as O-mannosyl-transferase on other proteins such as PDIA3 [2].

1: Conserved sequence motifs in human TMTC1, TMTC2, TMTC3, and TMTC4, new O-mannosyltransferases from the GT-C/PMT clan, are rationalized as ligand binding sites. Eisenhaber B, Sinha S, Jadalanki CK, Shitov VA, Tan QW, Sirota FL, Eisenhaber F; Biol Direct. 2021;16:4. PMID:33436046

2: Discovery of an O-mannosylation pathway selectively serving cadherins and protocadherins. Larsen ISB, Narimatsu Y, Joshi HJ, Siukstaite L, Harrison OJ, Brasch J, Goodman KM, Hansen L, Shapiro L, Honig B, Vakhrushev SY, Clausen H, Halim A; Proc Natl Acad Sci U S A. 2017;114:11163-11168. PMID:28973932