Membranes and membrane surfaces: Dynamics of cytoplasmic membranes in pancreatic acinar cells

Abstract

In pancreatic acinar cells the intracellular transport of secretory proteins occurs through the inter-connexion of distinct membrane-bounded compartments: in series, the rough surfaced endoplasmic reticulum (r.e.r.), the Golgi complex (g.c.) and the secretory (zymogen) granules (z.g.). The latter organelles are able to fuse their membrane with the plasmalemma (pm.) and discharge their content by exocytosis. In order to investigate the mechanisms by which the intracellular transport occurs we have investigated the composition as well as the rate of synthesis and turnover of the various membranes involved in the process. We found that these membranes have distinct differences in chemical composition and in the distribution of enzyme activities and that their rate of turnover is much slower than that of secretory proteins. Furthermore, SDS polyacrylamide gel electrophoresis of doubly labelled membrane proteins revealed that in each of these membranes there is a marked heterogeneity of turnover rates. These data indicate ( a ) that the membranes participating in the intracellular transport interact with one another in a non-random fashion, ( b ) that they are not synthesized concomitantly with the secretion products, ( c ) that membranes are synthesized independently from one another, and ( d ) that they are re-utilized in several secretory cycles. Consistent with these results, a model explaining the role of cellular membranes in protein secretion is described. It is proposed that the intracellular transport is effected through the specific non-random interaction (fusion-fission) and recycling of the various participating membranes. The possible relevance of these findings to other secretory systems is discussed.