Ultrastructural immunocytochemistry of nascent albumin topology: proposed cytosolic folding and membrane transit of the protein.

The relationship of nascent albumin and hepatocyte organelles was studied with the immunoperoxidase reaction in rats given various drugs to alter cellular albumin content. colchicine was used to increase intracellular albumin. Cycloheximide inhibited synthesis but allowed nascent albumin to remain with its ribosome of origin. Puromycin also inhibited synthesis but released albumin from its ribosome. There was no difference in the appearance of attached ribosomes in hepatocytes from saline-injected rats and those given colchicine or cycloheximide. In these cases, membranes of the endoplasmic reticulum were consistently decorated with ribosomes positive for the presence of albumin antigenicity on their cytosolic surface. The cisternal and cytosolic compartments were negative. The situation after puromycin was different. Here the membranes appeared to be denuded of ribosomes and reaction product, indicative of albumin, was present only on the lumenal surface. To determine whether puromycin had caused the release of ribosomes, sections from puromycin-treated cells were stained nonspecifically with uranyl acetate. This showed that the normal amount of ribosomes was still bound but that they could not be seen when a probe specific only for albumin was used. It appears that nascent albumin can associate with its ribosome within the cytosol. Also, apparently after albumin passes through the membrane of the rough endoplasmic reticulum, it remains attached to its lumenal surface. A model incorporating cytosolic folding of albumin followed by its entropic membrane transit is presented.