The trans-most cisternae of the Golgi complex: a compartment for sorting of secretory and plasma membrane proteins.

The intracellular site for the sorting of proteins destined for regulated or constitutive pathways is presently unknown for any one cell. By immunoelectron microscopy, we directly followed the routes taken by a regulated hormone, insulin, and a constitutive protein, hemagglutinin. Both proteins are present in individual Golgi stacks where they appear randomly distributed throughout the cisternae. In contrast, the two proteins do not colocalize outside the Golgi area:insulin is concentrated in dense-core secretory granules, while hemagglutinin is found predominantly in clear 100-300 nm vesicles. These vesicles do not label significantly with an endocytic tracer, indicating that they are exocytic carriers for hemagglutinin. The site at which the two proteins diverge is the clathrin-coated, trans-most cisterna of the Golgi, where the packaging of proinsulin takes place.