Clathrin-immunoreactive sites in the Golgi apparatus are concentrated at the trans pole in polypeptide hormone-secreting cells.

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Abstract
By analogy with receptor-mediated endocytosis, clathrin-coated membrane segments at locations other than at the plasma membrane were implicated in intracellular membrane transport/sorting. The crucial role of the Golgi apparatus in these processes prompted a study of [rat] the distribution at this level of clathrin-immunoreactive sites in various polypeptide hormone-secreting [rat] cells. With a polyclonal antibody recognizing the polymerized form of clathrin (structural coats or cages), the Golgi area had the highest degree of specific labeling of various cytoplasmic regions, including the subplasmalemmal web. The Golgi labeling was not homogeneously distributed, most of the immunoreactive sites being associated with membrane segments at the trans pole. The labeled membranes comprised regions of trans cisternae with and without condensing secretory material, newly formed (maturing) secretory granules freshly released from trans cisternae, and typical coated vesicles. The cis Golgi cisternae, as well as the cytoplasmic region containing transfer (shuttle) vesicles and transitional cisternae of the rough endoplasmic reticulum, were only weakly labeled. Clathrin concentration, degree of assembly of triskelions into coats, duration of clathrin association with membranes, or presence of a coat protein different from clathrin could account for the low immunoreactivity observed at the cis pole as compared to the trans pole.