Relationship between endoplasmic reticulum and Golgi membranes; evidence for a heterogeneous localization of cytochrome b5 in the Golgi membranes.

Cytochrome b5 has been visualized by an immunoferritin labeling in a Golgi preparation. When the Golgi apparatus was first disrupted before labeling, 60% of the profiles were clearly labeled with ferritin. This proportion of labeled vesicles was too high to be explained by contamination from other membranes. When the Golgi structure was maintained during the experimental process, obviously some small vesicles located at the periphery of the Golgi complex were labeled and at least one saccule per Golgi apparatus was labeled but only at the edges. These conclusions were sustained by a quantitative analysis which showed a low but constant labeling of Golgi saccules and tubules and a very high labeling of some small profiles. These results confirm the presence of cytochrome b5 in the Golgi apparatus but stress its heterogeneous distribution; they also sustain the idea of a possible membrane specific shuttle between the ER and the Golgi apparatus by a recycling process suggested by the presence of highly labeled small vesicles found in close vicinity with the cisternae.