CHARACTERIZATION OF A NOVEL 63-KDA MEMBRANE-PROTEIN - IMPLICATIONS FOR THE ORGANIZATION OF THE ER-TO-GOLGI PATHWAY

Owing to the lack of appropriate markers the structural organization o f the ER-to-Golgi pathway and the dynamics of its membrane elements ha ve been elusive. To elucidate this organization we have taken a monocl onal antibody (mAb) approach. A mAb against a novel 63 kDa membrane pr otein (p63) was produced that identifies a large tubular network of sm ooth membranes in the cytoplasm of primate cells. The distribution of p63 overlaps with the ER-Golgi intermediate compartment, defined by a previously described 53 kDa marker protein (here termed ERGIC-53), as visualized by confocal laser scanning immunofluorescence microscopy an d immunoelectron microscopy. The p63 compartment mediates protein tran sport from the ER to Golgi apparatus, as indicated by partial colocali zation of p63 and vesicular stomatitis virus G protein in Vero cells c ultured at 15-degrees-C. Low temperatures and brefeldin A had tittle e ffect on the cellular distribution of p63, suggesting that this novel marker is a stably anchored resident protein of these pre-Golgi membra nes. p63 and ERGIC-53 were enriched to a similar degree by the same su bcellular fractionation procedure. These findings demonstrate an unant icipated complexity of the ER-Golgi interface and suggest that the ER- Golgi intermediate compartment defined by ERGIC-53 may be part of a gr eater network of smooth membranes.