During N-linked glycosylation of proteins, oligosaccharide chains are assembled on the carrier molecule dolichyl pyrophosphate in the following order: 2 molecules of N-acetylglucosamine (GlcNAc), 9 molecules of mannose, and 3 molecules of glucose. These 14-residue oligosaccharide cores are then transferred to asparagine residues on nascent polypeptide chains in the endoplasmic reticulum (ER). As proteins progress through the Golgi apparatus, the oligosaccharide cores are modified by trimming and extension to generate a diverse array of glycosylated proteins (reviewed in 6, 7).

The first seven saccharide moieties are added to dolichyl phosphate on the cytosolic side of the ER, and the last seven are added in the lumen of the ER. Rft1p is required for translocation of the nascent lipid-linked oligosaccharide (LLO) from the cytosolic to the lumenal face of the ER membrane (2), but does not co-purify with the flippase activity (4, 3). Moreover, Rft1p depletion does not affect transbilayer movement of Man5GlcNac2-P-P-Dol in sealed microsomal fractions (5). RFT1 is essential (1, 2), but conditional mutants accumulate LLO's with five mannose residues (2). Although they share no homology, RFT1 ("Requires Fifty-Three") was originally isolated as a partial loss-of-function mutation suppressed by introduction of human p53 (OMIM) (1) but p53 does not complement deletion of RFT1.

Last updated: 2005-07-12