Immunocytochemical localization of β-COP to the ER-Golgi boundary and the TGN

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Abstract
Recent data strongly suggest that the coatomer (COP) complex is involved in membrane transport between the ER and Golgi complex. This vesicular coat has been implicated in ER to Golgi, in intra Golgi as well as in Golgi to ER traffic. In this study we present a detailed immunocytochemical analysis of the distribution of β-COP in different tissue culture cells. Our results extend previous studies by showing, using electron microscopy, that β-COP accumulates on vesicular profiles and buds in the intermediate compartment (IC) under conditions that block ER to Golgi transport (15°C). Importantly, under these conditions β-COP co-localizes on these structures with a passenger protein, the membrane glycoprotein of vesicular stomatis virus (ts-O45-G). Furthermore, quantitative immunofluorescence microscopy of cells with ts-045-G accumulated in the ER, IC and trans-Golgi network, shifted briefly to the permissive temperature, showed that β-COP was associated with many of the putative transport intermediates containing the viral glycoprotein which is in transit between the ER/IC and the cis-Golgi. The simplest interpretation of these data is that COP-coated vesicles are involved in anterograde transport of ts-045-G from the IC to the Golgi complex. Since many putative COP vesicles lacked the G protein following release of the 15°C block this pool could be involved in retrograde transport. We also show that β-COP is present on the membranes of the transGolgi network. However, in contrast to the ER-Golgi boundary, we could find no convincing evidence that this pool of β-COP is associated with buds or trans-Golgi network-derived transport vesicles.