Abstract
Protein traffic moving from the endoplasmic reticulum (ER) to the Golgi complex in mammalian cells passes through the tubulovesicular membrane clusters of the ER-Golgi intermediate compartment (ERGIC), the marker of which is the lectin ERGIC-53. The dynamic nature and functional role of the ERGIC have been debated for quite some time. In the most popular current view, the ERGIC clusters are mobile transport complexes that deliver secretory cargo from ER-exit sites to the Golgi. Recent live-cell imaging data revealing the formation of anterograde carriers from stationary ERGIC-53-positive membranes, however, suggest a stable compartment model in which ER-derived cargo is first shuttled from ER-exit sites to stationary ERGIC clusters in a COPII-dependent step and subsequently to the Golgi in a second vesicular transport step. This model can better accommodate previous morphological and functional data on ER-to-Golgi traffic. Such a stationary ERGIC would be a major site of anterograde and retrograde sorting that is controlled by coat proteins, Rab and Arf GTPases, as well as tethering complexes, SNAREs and cytoskeletal networks. The ERGIC also contributes to the concentration, folding, and quality control of newly synthesized proteins.
Highlights
The ER-Golgi intermediate compartment (ERGIC) is a complex membrane system between the rough endoplasmic reticulum (ER) and the Golgi that was initially defined following the identification of a 53 kDa membrane protein (ERGIC-53) that is predominantly localized to these membranes (Hauri et al, 2000; Schweizer et al, 1988)
Because it is the ERGIC-to-cis-Golgi step that requires intact microtubules, we propose that the transitory interaction between the COPII coat and p150Glued is used to maintain the kinetic stability and steady-state localization of ER-exit sites (ERES) by a COPII-mediated microtubule-trap mechanism, which in turn facilitates the association of the juxtaposed ERGIC with microtubules
Here, we argue that ERGIC clusters defined by ERGIC-53 constitute a stationary compartment closely apposed to ERES
Summary
Protein traffic moving from the endoplasmic reticulum (ER) to the Golgi complex in mammalian cells passes through the tubulovesicular membrane clusters of the ERGolgi intermediate compartment (ERGIC), the marker of which is the lectin ERGIC-53. Recent livecell imaging data revealing the formation of anterograde carriers from stationary ERGIC-53-positive membranes, suggest a stable compartment model in which ERderived cargo is first shuttled from ER-exit sites to stationary ERGIC clusters in a COPII-dependent step and subsequently to the Golgi in a second vesicular transport step. This model can better accommodate previous morphological and functional data on ER-to-Golgi traffic.
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