Expression of the Saccharomyces cerevisiae glycoprotein invertase in mouse fibroblasts: glycosylation, secretion, and enzymatic activity.

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Abstract
Oligosaccharide processing is controlled by host- and protein-dependent factors. To increase our understanding of the relative contribution of those factors we studied the glycosylation of yeast invertase expressed in a heterologous system. Invertase synthesized in psi-2 cells (an NIH 3T3-derived packaging line) is secreted efficiently, enzymatically active, and heavily glycosylated. It was estimated that the protein contains 8 or 9 carbohydrate chains. Two classes can be observed, of an approximate size of 100-110 kDa and 115-130 kDa, respectively. The size differences are due to differences in glycosylation. The smaller class contains two high-mannose carbohydrate chains; the remainder is of the complex type, sialylated and most likely tri- or tetraantennary. This profile parallels the situation observed with invertase glycosylation in yeast, where 2 of 9 or 10 chains remain unprocessed. The larger size class of invertase expressed in mouse fibroblasts has a different profile, since it contains probably only complex-type glycans. There are no apparent differences, however, in the size of the protein backbone between the two size classes. When invertase is synthesized in the presence of the mannosidase inhibitor 1-deoxymannojirimycin, processing is blocked completely, since all glycans are susceptible to endo-beta-N-acetylglucosaminidase H. The glucosidase inhibitor 1-deoxynojirimycin does not inhibit processing completely. In both cases secretion of the protein is not affected. The glycosylation inhibitor tunicamycin prevents secretion of invertase completely when cells are cultured at 37 degrees C. At 26 degrees C, however, nonglycosylated invertase can be detected in the medium. These data suggest that glycosylation of invertase seems to be essential for the early steps of the secretory pathway but is less critical for later events.