SUMMARY: Intact cells of were able to endocytose FITC-dextrans of 70 kDa, but not of 150 kDa, whereas spheroplasts took up both components. The rate of uptake of 70 kDa dextrans by spheroplasts was about three times higher than that by intact cells. Pretreatment of intact cells with dithiothreitol (DTT) or EDTA increased the rate of uptake of 70 kDa dextrans considerably, but 150 kDa dextrans were still excluded. Release of periplasmic invertase activity into the medium by glucose-derepressed cells was negligible in control cell suspensions, but was strongly stimulated in the presence of DTT. The released invertase had an apparent molecular mass of 320 kDa, indicating that the dimeric form was released. In the presence of EDTA only a slight increase in the release of invertase was observed. Pretreatment with DTT was accompanied by an increased loss of cell wall proteins. This suggests that the loss of mannoproteins, in combination with a more general opening up of the wall by reducing disulphide bridges, increases cell wall porosity. It is argued from the Stokes radius of 70 kDa dextran (5·8 nm) that yeast cell walls are, in principle, permeable to globular proteins with a molecular mass up to 400 kDa.


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