GDP-fucose protein O-fucosyltransferase 1 (POFUT1) is an enzyme responsible for adding fucose sugars in O linkage to serine or threonine residues between the second and third conserved cysteines in EGF-like repeats on the Notch protein. The protein is an inverting glycosyltransferase, which means that the enzyme uses GDP-β-L-fucose as a donor substrate and transfers the fucose in O linkage to the protein producing fucose-α-O-serine/threonine.
When the gene for POFUT1 is knocked out, or the expression is decreased to very low levels, all Notch signaling is destroyed, which means that fucose on Notch is essential for Notch function. Why this is the case is not yet well understood.
Almost all glycosyltransferases reside in the Golgi apparatus. However, POFUT1 as well as the related enzyme POFUT2 have recently been shown to reside in the endoplasmic reticulum.