Chaperone activity of protein O-fucosyltransferase 1 promotes Notch receptor folding

Notch proteins are receptors for a conserved signaling pathway that affects numerous cell fate decisions. We found that in, Drosophila, Protein O-fucosyltransferase 1 (OFUT1), an enzyme that glycosylates epidermal growth factor-like domains of Notch, also has a distinct Notch chaperone activity. OFUT1 is an endoplasmic reticulum protein, and its localization was essential for function in vivo. OFUT1 could bind to Notch, was required for the trafficking of wild-type Notch out of the endoplasmic reticulum, and could partially rescue defects in secretion and ligand binding associated with Notch. point mutations. This ability of OFUT1 to facilitate folding of Notch did not require its fucosyltransferase activity. Thus, a glycosyltransferase can bind its substrate in the endoplasmic reticulum to facilitate normal folding.