Kinetic studies of the intracellular transport of procollagen and fibronectin in human fibroblasts. Effects of the monovalent ionophore, monensin.

The monovalent ionophore, monensin, has been found to inhibit the secretion of both procollagen and fibronectin from human fibroblasts in cell culture. The kinetics of inhibition, as well as those for the release of inhibition, suggested that both proteins may be cotransported in the cell. In the present study, we have examined the intracellular translocation and release into the culture medium of procollagen and fibronectin, in the presence or absence of monensin. Pulse-chase studies were combined with subcellular fractionation of the fibroblasts to determine the rates of intracellular movement. We found that monensin did not significantly affect either the subcellular fractionation, or the distribution of organelle marker enzymes along the gradient, and that procollagen moved from a region of high buoyant density (primarily endoplasmic reticulum), through a mid-density region (primarily Golgi elements), to a region of low buoyant density before exiting from the cell. Monensin markedly decreased the rate of transit from one region to the other; kinetic analysis of the data showed a greater than 3-fold decrease in the rate constants for intracellular movement into the low buoyant density components and thence to the culture medium. The density gradient distribution of fibronectin was affected by monensin in similar fashion, indicating that it and procollagen probably follow the same intracellular route. Since monensin causes both proteins to accumulate in highest abundance in regions of the density gradient corresponding to Golgi and endoplasmic reticulum, the site of ionophore blockade appears to be within the Golgi apparatus. Thus, procollagen and fibronectin share a common intracellular route prior to entering the Golgi region of the cell.