A TRANS-MEMBRANE PORE CAN ACCOUNT FOR PROTEIN MOVEMENT ACROSS ZYMOGENGRANULE MEMBRANES

We have reported that the membrane of zymogen granules, secretion vesi cles from the exocrine pancreas, is permeable to its contained protein s by measuring both the loss and accumulation of protein in response t o mass action forces [1-3] However, the mechanism of transport has rem ained unknown. Here we consider evidence that this transport occurs th rough trans-membrane pores. Using freeze-fracture electron microscopic methods, Cabana et al. [4] have reported the presence of a 15 nm intr amembrane particle in zymogen granule membrane which contains a 5 nm ( +/- 0.1 nm, S.D.) diameter lucent center. In this article, we propose that this structure is a pore through which proteins can be transporte d, and test this hypothesis by comparing the predicted phenomenologica l permeability coefficient for transport by diffusion via this structu re, to that calculated from protein flux measurements on granules usin g an X-ray microscope. The predicted and experimental values were esse ntially identical and hence support the hypothesis that this structure could be a protein transporting channel.