Abstract
A core prediction of the vesicular transport model is that COPI vesicles are responsible for trafficking anterograde cargoes forward. In this study, we test this prediction by examining the properties and requirements of inter-Golgi transport within fused cells, which requires mobile carriers in order for exchange of constituents to occur. We report that both small soluble and membrane-bound secretory cargo and exogenous Golgi resident glycosyl-transferases are exchanged between separated Golgi. Large soluble aggregates, which traverse individual stacks, do not transfer between Golgi, implying that small cargoes (which can fit in a typical transport vesicle) are transported by a different mechanism. Super-resolution microscopy reveals that the carriers of both anterograde and retrograde cargoes are the size of COPI vesicles, contain coatomer, and functionally require ARF1 and coatomer for transport. The data suggest that COPI vesicles traffic both small secretory cargo and steady-state Golgi resident enzymes among stacked cisternae that are stationary. DOI:http://dx.doi.org/10.7554/eLife.01296.001.
Highlights
The Golgi apparatus is a central feature of the secretory pathway in all eukaryotic cells
Our reasoning was that only mobile carriers, such as COPI vesicles, should be able to travel the long distance between well-separated Golgi of fused cells
Our results clearly establish that only small cargoes are exchanged, showing that inter-Golgi carriers are capable of size-filtration, and that small cargo transport must utilize a distributive mechanism different from the mechanism used by large soluble aggregates
Summary
The Golgi apparatus is a central feature of the secretory pathway in all eukaryotic cells. Proteins secreted from the cell, as well as constituents of the plasma membrane and a broad variety of membrane-enclosed compartments pass through the ER–Golgi system in the anterograde direction, diverging only as they depart the Golgi stack at its trans face ( termed the TGN). During this anterograde passage they are typically glycosylated in a step-wise fashion as they encounter the responsible enzymes (Emr et al, 2009; Klumperman, 2011). Dynamic tubular connections between cisternae have been proposed (Trucco et al, 2004)
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