Trnasporte Del Reticulo Endoplasmatico

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Transport between ER and Golgi Judith Klumperman
In the last eighteen months, it has become clear that some classes of proteins are actively recruited into endoplasmic reticulum export carriers, whereas others are exported as bulkflow cargo. Subsequent transport to the Golgi is mediated by tubulovesicular membranes. The anterograde membrane flow is compensated for by a retrograde pathway,which, in addition to the recycling of membrane and proteins to the endoplasmic reticulum, may play a role in anterograde cargo concentration.
Addresses Department of Cell Biology, Institute of Biomembranes and Center for Biomedical Genetics, University Medical Center, Utrecht, Heidelberglaan 100, AZU Rm G02.525, 3584 CX Utrecht, The Netherlands; e-mail: J.Klumperman@lab.azu.nl Current Opinionin Cell Biology 2000, 12:445–449 0955-0674/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved.

Abbreviations GFP green fluorescent protein ERGIC ER–Golgi intermediate compartment IEM immunoelectron microscopy TCs transport complexes TEs transitional elements VSV-G G protein of vesicular stomatitis virus VTCs vesicular tubular clusters

represent the ER exit sites.Numerous smaller TEs, which show less buds, are present in the peripheral cytoplasm at some distance from the Golgi. The buds on the TEs are coated with COPII, a cytosolic complex consisting of the small GTPase Sar1p and the heterodimeric protein complexes Sec23p–Sec24p and Sec13p–Sec31p [7]. Under physiological conditions, reservoirs of nonmembrane-associated COPII are sometimes found in close vicinityto the Golgi [8••]. COPII-coated buds transform into coated vesicles that rapidly shed their coats and fuse with each other or with adjacent VTC membranes [3,4]. VTCs consist of clusters of vesicles and interconnected tubules that face the TEs [6]. Continuities between VTCs and TEs are only rarely seen, mostly under nonphysiological circumstances [4]. VTC tubules bear COPI coats at their rims.These coats consist of the small GTPase ARF and a preassembled coatomer complex of seven subunits [9]. After energy shortage, COPI coatomers aggregate in the cytosol between TEs and cis-Golgi [10]. Transport from peripheral VTCs to the Golgi is microtubule-dependent and probably followed by fusion of peripherally derived membranes with the central VTC and/or cis-Golgi [11,12]. Continuities betweenthe central VTCs and the cis-most, fenestrated Golgi cisterna have repeatedly been reported and is sometimes referred to as the cis-Golgi network [13].

Membrane proteins exit the ER by selective recruitment into COPII-coated vesicles Introduction
At the end of the 19th century, Camillio Golgi impregnated Purkinje cells with silver salts to reveal a network of interconnected structures: theinternal reticular apparatus. Adaptation of Golgi’s method for electron microscopy demonstrated that metal staining specifically occurred in the cis-most cisterna of the Golgi complex and numerous vesicular-tubular membranes adjacent to the cis-Golgi side [1,2], nowadays commonly referred to as ERGIC (ER–Golgi intermediate compartment) [3] or VTCs (vesicular tubular clusters) [4]. These earlymorphological observations indicated both the intricate relationships and the differences between VTCs and the Golgi. Since then, the role of VTCs in protein transport and the functional and structural boundaries between the ER and Golgi have been topics of intense research. This review focuses on recent advances in our understanding of how transport from the ER to VTCs is regulated and the mechanisms bywhich VTCs mediate protein and membrane transport at the ER–Golgi interface. Recent studies strongly suggest that at least some proteins are actively sorted and recruited into ER-derived transport vesicles by a direct interaction with COPII components. Examples of such proteins are the yeast ER–Golgi SNAREs Bet1p and Bos1p [14], sed5p [15] and the highly abundant VTC proteins ERGIC-53/58 [3] and...