A MODELING FRAMEWORK FOR THE STUDY OF PROTEIN GLYCOSYLATION

The key step in the asparagine-linked glycosylation of secretory prote ins is the transfer of oligosaccharide from a dolichol precursor to th e polypeptide at an Asp-X-Ser/Thr (NXS/T) consensus sequence. It is of ten the case, both in cultured cells and in vivo, that this reaction d oes not occur for every molecule of a given protein. Thus, the cell ma y create two protein populations, one bearing and one lacking oligosac charide, for each potential glycosylation site. We present a structure d kinetic modeling framework of the initial glycosylation event based on a balance of available glycosylation sites through the region of en doplasmic reticulum lumen proximal to the membrane. Oligosaccharyltran sferase, a multimeric protein complex, catalyzes the sugar transfer. T his enzyme is integral to the endoplasmic reticulum membrane, and it i s thought to act cotranslationally. The nascent polypeptide may also f old in such a way as to prevent glycosylation from occurring. The net result is a potentially complex spatial and temporal relationship amon g translation, glycosylation, and other cotranslational events. Model results predict how fractional glycosylation site occupancy may depend on protein synthesis rate, oligosaccharyldolichol availability, and m RNA elongation rate. Although we are currently unable to quantitativel y compare predicted to experimentally obtained fractional site occupan cy, we are able to determine qualitative trends which may be confirmed experimentally. (C) 1996 John Wiley & Sons, Inc.