Polyprenyl phosphates as coenzymes in protein and oligosaccharide glycosylation

Abstract

Partly saturated polyprenols of the dolichol type occur in all branches of the plant kingdom although in green plants they are quantitatively minor compared with the shorter polyprenols of the ficaprenol and betulaprenol type. Nevertheless, in all members of the plant kingdom so far studied it is the dolichyl phosphates that function in glycosyl transfer. In yeasts, dolichyl phosphate mannose is primarily an intermediate in the O-mannosylation of some membrane-bound proteins. Further mannosylation of the mannoprotein does not involve dolichyl phosphate. Yeast membrane preparations also catalyse the sequential transfer from the appropriate nucleoside diphosphate sugar of N -acetylglucosaminyl phosphate, N -acetylglucosamine, several mannose residues and several glucose residues to dolichyl monophosphate to form a dolichyl diphosphate oligosaccharide. The oligosaccharide is then transferred en bloc to protein to form an N-glycosidically linked glycoprotein. The transfer of mannose and glucose to the dolichyl diphosphate oligosaccharide is probably via dolichyl monophosphate sugar derivatives. Tunicamycin inhibits specifically the transfer of JV-acetylglucosamine phosphate which in turn blocks protein N-glycosylation. Evidence for corresponding processes in O- and N-glycosylation of proteins of hyphal fungi is also available but much less well established. The N-glycosylation of proteins by membrane preparations of green plants appears to occur by a process very similar to that found in yeast. In addition, the biosynthesis of β1-3 and β1-4 linked oligoglucans and also of an algal cellulose primer (a glucoprotein) via dolichyl phosphate glucose has been reported. Possible consequences of these phenomena are discussed.