| Abstract: | We have previously shown that newly synthesized liver secretory proteins are exported at three distinct characteristic rates, with intracellular retention half-times of 110-120 min (e.g. transferrin), 75-80 min (e.g. ceruloplasmin), and 30-40 min (e.g. alpha 1-protease inhibitor) (J. B. Parent, H. Bauer, and K. Olden (1985) Biochim. Biophys. Acta, in press). In the present study we have determined the average time required for specific glycoproteins to move through the various compartments of the intracellular transport pathway, consisting of endoplasmic reticulum and Golgi complex. Localization in particular compartments was monitored by the use of the following complementary approaches: (i) Percoll density gradient fractionation of the subcellular organelles, (ii) sensitivity of the glycan moiety of N-linked glycosylation to endo-beta-N-acetylglucosaminidase H, and (iii) by the lectin-binding characteristics. The cell fractionation studies revealed that alpha 1-protease inhibitor, ceruloplasmin, and transferrin were transported from the rough endoplasmic reticulum with a retention half-time of 10, 30, or 45 min, respectively. Measurements of the rate at which newly synthesized glycoprotein became endo H-resistant (an event localized near the medial region of Golgi) demonstrated that it took 60-70, 30, and 18 min for 50% of transferrin, ceruloplasmin, and alpha 1-protease inhibitor, respectively, to reach the medial Golgi. Consistent with this finding, maximal binding of transferrin to wheat germ agglutinin (also a medial Golgi event) and Ricinus communis agglutinin I (a trans Golgi event) required 75 and 90 min, respectively, and maximal binding of ceruloplasmin to both lectins occurred in approximately 30 min. Maximal binding of alpha 1-protease inhibitor to wheat germ agglutinin and Ricinus communis agglutinin I required 15 and 30 min, respectively. The results presented here clearly indicate that (i) the time required for protein secretion cannot be entirely accounted for by lag in transport from the rough endoplasmic reticulum to the Golgi since the glycoproteins examined are retained in the former organelle for no more than two-fifths of the total intracellular retention half-time, and (ii) the variability in rates of protein secretion is not due solely to differences in rates of transport from the rough endoplasmic reticulum to the Golgi as variability in retention within the Golgi is also demonstrated. The results are discussed in terms of their compatibility with receptor-mediated transport of glycoproteins in both the endoplasmic reticulum and Golgi. |
