Glycoprotein Synthesis in Plants: II. Structure of the Mannolipid Intermediate

This paper reports the purification and structural determination of the mannolipid shown previously (Ericson and Delmer 1977 Plant Physiol 59: 341-347) to serve as an intermediate in glycoprotein synthesis in cotyledons of Phaseolus vulgaris. The mannolipid was purified by chromatography in organic solvents on diethylaminoethyl-cellulose, followed by repeated steps of deacylation and rechromatography on Sephadex LH-20. Binding and elution behavior on diethylaminoethyl-cellulose was consistent with the presence of a monophosphate residue. Lability of the mannolipid to mild acid treatment as well as its resistance to hot phenol treatment or catalytic hydrogenation are consistent with the structure of a polyprenol having a saturated alpha-residue. After methanolysis, the chloroform-methanol-soluble portion of the mannolipid was analyzed by mass spectrometry. The fragmentation pattern obtained was nearly identical to that obtained from standard dolichol-phosphate. An intense ion at m/e 69 represented the omega-terminal isoprenoid residue, and repeating fragments separated by 68 mass units were observed up to m/e of > 1,200. All evidence supports the conclusion that the mannolipid is dolichol-monophosphate-mannose and thus provides further support for the concept that the processes involved in the glycosylation of protein in higher plants are similar to those known to occur in the animal kingdom.