CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector Magazine R583 Complex carbohydrate between related signals specifying Primer modifications in eukaryotes are either forward or retrograde transport compartmentalised into the ER and through the Golgi apparatus. A series Golgi apparatus. Proteins can be of recent studies indicate that COP-I The Golgi apparatus glycosylated in two different ways, in conjunction with the small Ben Short and N-linked glycosylation on GTPase ARF1 and its specific asparagine residues, and O-linked GTPase-activating protein (ARF- Francis A. Barr glycosylation on serine or threonine GAP) operate a kinetic proofreading residues. N-linked glycosylation is mechanism for signal decoding. How At some point in their lives all cells initiated in the ER, with the transfer this would result in the formation of need to grow and divide. In of a branched sugar structure from two classes of COP-I vesicle going in eukaryotic cells these processes the isoprenoid lipid dolichol to different directions is unclear. require the delivery of new proteins specific asparagine residues, while Finally, at the trans-face of the and lipids from their site of synthesis O-linked glycosylation is initiated in Golgi, is the trans-Golgi network in the endoplasmic reticulum (ER) the Golgi. The exact series of sugar (TGN) where proteins are sorted for to the cell surface. The Golgi modifications in both cases depends delivery to their final subcellular apparatus, first visualised under the not only on the species, but the destination. Proteins destined for the light microscope in 1898 by Camillo tissue, cell type and even growth endocytic pathway, such as lysosomal Golgi using a staining method he state of the cell under investigation. enzymes, are bound by a receptor in had developed to study neurons, lies Complex carbohydrate processing the TGN which recognises a specific on the route between the ER and continues in the Golgi apparatus, sugar modification on these proteins, other destinations within the cell. where sugar transferases and sugar- mannose-6-phosphate, and are The electron microscopic (EM) trimming enzymes add or remove a segregated into forming vesicles studies of Felix and Dalton in the variety of sugars at various covered with the clathrin coat 1950s revealed that the Golgi stereospecific positions. These protein. Other transport events at the apparatus is comprised of a series of enzymes are localised to discrete TGN are less well understood, the overlapping cisternal or plate-like, cisternae within the Golgi exact nature of the transport membrane structures and associated apparatus, in an order that intermediates formed and the coat 60 to 70 nm diameter vesicles. corresponds to their sequence of proteins, if any, that cover them Palade and co-workers then went on action. The Golgi apparatus could remaining mysterious. to demonstrate from their studies of therefore be viewed as an assembly pancreatic cells that secreted line for the production of correctly Transport mechanisms proteins pass through the Golgi glycosylated proteins. Despite many years of intensive apparatus on their way to the cell The first, or cis-Golgi cisterna is a research there is no definitive answer surface or other destinations within major site for the sorting of secretory to how secretory cargo is transported the cell such as the lysosomes. This and ER-derived proteins. Many ER through the Golgi apparatus, and it is layered cisternal structure is often proteins contain the carboxy- therefore still an area of some called a stack, and has a defined terminal sequence KDEL, which is controversy. Two mechanisms for polarity, cis- being the face accepting recognised by an integral membrane cargo transport have been proposed, material from the ER, and trans- receptor in the Golgi and results in referred to as the cisternal maturation being the face where material exits the packaging of these molecules in and vesicle transport models for secretion. vesicles for transport back to the ER. (Figure 1). Since the groundbreaking Other signals are found in the work of Palade and colleagues in the The function of the Golgi apparatus cytoplasmic domains of 1960s, the vesicle transport model has The Golgi apparatus has two key transmembrane proteins. The di- held centre stage, although the functions, and could be seen as a lysine motif is one such signal found cisternal maturation model actually combined assembly line and logistics at the carboxyl terminus of pre-dates it by a decade. The current centre. It houses the enzymes membrane proteins. It binds directly versions of both models envisage responsible for the synthesis of to the proteinaceous vesicle coat secretory cargo exiting the complex carbohydrate structures termed COP-I, causing proteins that endoplasmic reticulum in some form found on many proteins and lipids, bear di-lysine to become of vesicular carrier, but even at this while both the cis- and trans-faces of concentrated in vesicles destined for early point the two models then the Golgi apparatus are important the ER. The exact mechanism by diverge. The vesicle transport model sites for the sorting of proteins and which COP-I recognises and decodes proposes that the Golgi cisternae are lipids for delivery to specific sorting signals of proteins is highly stable pre-existing structures through subcellular destinations. complex, as it appears to discriminate which the cargo molecules pass. R584 Current Biology Vol 10 No 16 Figure 1 (a) Vesicle transport (b) Cisternal maturation Plasma membrane trans-Golgi network trans medial COP-I COP-I cis cis-Golgi network COP-II COP-II Endoplasmic reticulum Current Biology Transport through the Golgi apparatus. (a) Vesicle transport envisages ER-derived vesicles, and then matures by exchanging enzymes and stable cisternae between which cargo molecules are transported in other components of the Golgi, but not secretory cargo, both with small vesicular carriers in a vectorial fashion. (b) Cisternal maturation other cisternae and with the ER. The Golgi apparatus cisternae are proposes that the first Golgi cisterna is created de novo from colour coded to indicate their unique but related composition. Vesicles coming from the studies of Palade and co-workers who (NSF) and its cofactors in order to endoplasmic reticulum recognise and provided both morphological and recognise and fuse with their target fuse with the first, or cis-Golgi, biochemical evidence for the membrane. Small coiled-coil cisterna and deliver their content. vectorial transport of secretory transmembrane proteins, the This content can then undergo one of proteins through the Golgi apparatus SNAREs, originally identified as a number of fates: Golgi enzymes are in vesicular carriers. Latterly, the receptors for NSF, mediate the fusion retained if the cisterna represents work of Rothman and colleagues has of the lipid bilayers of the vesicle and their final destination, while secretory reconstituted this process in vitro, and target membranes. cargo molecules get packaged into allowed the identification of both The cisternal maturation vesicles destined for the next cisterna membrane and cytosolic proteins hypothesis proposes that ER-derived in the Golgi stack. Escaped ER required for vesicle formation and vesicles contain all the necessary proteins and structural components fusion. This work culminated in the components to build a new Golgi cis- such as the SNAREs and putative isolation of Golgi transport vesicles cisterna. This new cisterna then cargo receptors involved in ER-to- and the characterisation of COP-I, matures by exchanging proteins and Golgi traffic are packaged into which is comprised of seven lipids by the process of vesicle vesicles destined for the ER. At the polypeptides and the small GTPase transport with the ER and other later trans-most face of the Golgi, the ARF1. Subsequently it has been Golgi cisterna. Cargo molecules are trans-Golgi network, proteins and demonstrated that these vesicles not transported in vesicles in this lipids are packed into a variety of appear to function as highly selective model, only molecules such as the classes of vesicle destined for bi-directional carriers of material enzymes that process the cargo, different destinations within the cell, through the Golgi apparatus, and and structural components of the such as the endosomal system and from the Golgi apparatus to the ER. Golgi. When a cisterna reaches the plasma membrane. Much of the They require the Golgi tethering its final maturation state at the evidence supporting the vesicle factor p115, plus the N-ethyl trans face of the Golgi it is then transport model comes from the early maleimide sensitive fusion factor broken down, and the Golgi Magazine R585 Figure 2 elements rather than vesicles. In this the ER during the mitotic model, the COP-I vesicle is proposed disassembly process, and that it is the to function as a retrograde carrier, ER which is dispersed between the recycling enzymes and other Golgi two daughter cells along components either back to the ER or microtubules. The Golgi apparatus is Cell to a previous Golgi cisterna. then proposed to rebuild at sites division A final twist that can be applied where secretory vesicles form from Golgi to both models is that transient the ER, the so-called ER exit sites. disassembly tubular connections, rather than There is compelling evidence for and vesicles, might mediate transport both views, suggesting that aspects of partitioning