Intracellular location of protein glycosylation in Aspergillus niger
Protein glycosylation was measured as a function of time in various subcellular organelles of Aspergillus niger. Initiation of glycosylation appeared to take place in the ribosomes, and its completion may take place in the smooth endoplasmic reticulum (SER) in which the highest mannosyl transferase activity was found. Solubilization of the SER fraction yielded two glycoproteins after SDS polyacrylamide gel electrophoresis suggesting that once the glycosylation of secretory proteins is completed, all the glycoproteins except those bound to the membrane of SER are immediately released from it. An inhibitor of cytoplasmic protein synthesis prevented glycosylation in all but the mitochondrial fraction, indicating a one to one relationship between protein synthesis and glycosylation. Since small amounts of mannosyl transferase were detected in the mitochondria, they may glycosylate some of their own proteins as well as receive glycoproteins from the SER. A considerable amount of glycoprotein was detected in the high speed supernatant (HSS) of the cell, in which a possible precursor of a secretory glycoprotein, g-glucosidase, was detected by immune precipitation. The HSS may contain several precursors of secretory glycoproteins and the plasma membrane may play a role in changing the precursor into the product.