SUMMARY: Specific labelling of the plasma membrane of intact protoplasts from using lactoperoxidase-catalysed iodination, has permitted the development of a procedure for isolating relatively pure preparations of this component. The specific activity of the labelled membrane was very low, indicating that only a few proteins are exposed on the outer membrane surface. The specific activity of labelling increased almost 100-fold when both membrane surfaces were exposed to iodination. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis, in combination with autoradiography, indicated that only two glycosylated proteins are exposed on the outer membrane surface, whilst all membrane proteins can be labelled in isolated plasma membrane preparations. Extraction of mannan-protein from purified cell walls of gave material which, on sodium dodecyl sulphate-polyacrylamide gel electrophoresis, resembled the exposed plasma membrane glycoproteins.

Purified plasma membranes incorporated mannose from GDPmannose on to an endogenous protein acceptor. The addition of exogenous cell wall mannan-protein increased the degree of incorporation. Approximately 12% of the incorporated mannose was sensitive to β-elimination, but a lipid intermediate was not involved in the reaction. An Arrhenius plot of enzyme activity at different temperatures showed a discontinuity at 17 C. The mannan synthetase activity was also significantly less at 40 C than at 37 C, temperatures at which the yeast-mycelial transition has been found to occur in this organism. The possible biosynthetic relationship between the exposed glycoproteins of the plasma membrane and those of the cell wall is discussed.


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