Golgi clusters and vesicles mediate mitotic inheritance independently of the endoplasmic reticulum

Endoplasmic reticulum positioning and partitioning in mitotic HeLa cells

Cell division serves to distribute chromosomes and organelles into two daughter cells, but the mechanism of rough endoplasmic reticulum (RER) segregation in animal cell mitosis is poorly understood. Here we study the distribution of RER in mitotic HeLa cells and its relation to the cytoskeleton. At metaphase, the RER was located in the cell cortex and was most concentrated in two locations. Close to the plasma membrane the RER was closely associated with cortical actin, and after treatment with Latrunculin A RER elements retracted to the deep cortex and became more tubular. Positioning was therefore dependent on cortical F-actin. Deeper in the cortex cisternae were wrapped tightly around the contours of the spindle body and orientated along microtubules close the spindle poles. Stereology revealed a close correlation between RER volume and cell volume in telophase daughter cells. These results suggest that the RER is positioned at the outer and inner regions of metaphase cortex by association with cytoskeleton. This arrangement combined with a disposition in concentric layers, deep to the plasma membrane, appears to distribute the RER evenly in the cortex and may help to couple quantities of RER and cell constituents.

Golgi reassembly after mitosis: the AAA family meets the ubiquitin family

The Golgi apparatus in animal cells breaks down at the onset of mitosis and is later rebuilt in the two daughter cells. Two AAA ATPases, NSF and p97/VCP, have been implicated in regulating membrane fusion steps that lead to regrowth of Golgi cisternae from mitotic fragments. NSF dissociates complexes of SNARE proteins, thereby reactivating them to mediate membrane fusion. However, NSF has a second function in regulating SNARE pairing together with the ubiquitin-like protein GATE-16. p97/VCP, on the other hand, is involved in a cycle of ubiquitination and deubiquitination of an unknown target that governs Golgi membrane dynamics. Here, these findings are reviewed and discussed in the context of the increasingly evident role of ubiquitin in membrane traffic processes.

Involvement of the actin cytoskeleton and homotypic membrane fusion in ER dynamics in Caenorhabditis elegans

The endoplasmic reticulum (ER) is the major intracellular membrane system. The ER is essential for protein and lipid biosynthesis, transport of proteins along the secretory pathway, and calcium storage. Here, we describe our investigations into the dynamics and regulation of the ER in the early Caenorhabditis elegans embryo. Using a GFP fusion to the ER-resident signal peptidase SP12, we observed the morphological transitions of the ER through fertilization and the early cell-cycles in living embryos. These transitions were tightly coordinated with the division cycle: upon onset of mitosis, the ER formed structured sheets that redispersed at the initiation of cleavage. Although microtubules were not required for the transition of the ER between these different states, the actin cytoskeleton facilitated the dispersal of the ER at the end of mitosis. The ER had an asymmetric distribution in the early embryo, which was dependent on the establishment of polarity by the PAR proteins. The small GTPase ARF-1 played an essential role in the ER dynamics, although this function appeared to be unrelated to the role of ARF-1 in vesicular traffic. In addition, the ER-resident heat shock protein BiP and a homologue of the AAA ATPase Cdc48/p97 were found to be crucial for the ER transitions. Both proteins have been implicated in homotypic ER membrane fusion. We provide evidence that homotypic membrane fusion is required to form the sheet structure in the early embryo.

Maintenance of Golgi Apparatus Structure in the Face of Continuous Protein Recycling to the Endoplasmic Reticulum: Making Ends Meet

I focus here on the Golgi apparatus and the dynamic relationship between the Golgi apparatus, the central organelle of the secretory pathway, and the endoplasmic reticulum (ER). The proteins and lipids of the Golgi apparatus originate in the ER, and cargo proteins and lipids that also originate in the ER are processed and sorted within the Golgi apparatus. The Golgi apparatus is indeed the central organelle of the secretory pathway. Surprisingly, many, if not all, of the proteins and accompanying lipids of the Golgi apparatus cycle continuously between the Golgi and the ER. Neither the Cisternal Maturation nor the Vesicular Transport/Stable Compartment model of Golgi apparatus function predicts continuous cycling of Golgi resident proteins through the ER. Evidence for this cycling comes from multiple experimental approaches, including ER-exit block-revealed accumulation of recycled Golgi resident proteins in the ER, evidence for exchange of green fluorescent protein (GFP)-tagged Golgi proteins or their analogues between Golgi and ER pools, and cisternal rab overexpression-induced redistribution of Golgi resident proteins to the ER. The implications of Golgi protein cycling for the maintenance of Golgi structure in the interphase mammalian cell are discussed. The challenge for the future is to put Golgi resident protein cycling pathway(s) to protein machinery and to characterize the cumulative, weak, dynamic interactions that hold the Golgi apparatus together. In doing so, new paradigms of organelle biogenesis will emerge.

Golgi duplication in Trypanosoma brucei

Duplication of the single Golgi apparatus in the protozoan parasite Trypanosoma brucei has been followed by tagging a putative Golgi enzyme and a matrix protein with variants of GFP. Video microscopy shows that the new Golgi appears de novo, near to the old Golgi, about two hours into the cell cycle and grows over a two-hour period until it is the same size as the old Golgi. Duplication of the endoplasmic reticulum (ER) export site follows exactly the same time course. Photobleaching experiments show that the new Golgi is not the exclusive product of the new ER export site. Rather, it is supplied, at least in part, by material directly from the old Golgi. Pharmacological experiments show that the site of the new Golgi and ER export is determined by the location of the new basal body.

Golgi inheritance: shaken but not stirred

Our view of what happens to the Golgi and ER during mitosis in mammalian cells has been shaken once more. Rather than the Golgi contents being recycled through, or mixed with the ER, two recent studies taking complementary approaches, find that the contents of these organelles remain separate throughout mitosis.

Dynamic nucleation of Golgi apparatus assembly from the endoplasmic reticulum in interphase hela cells

Models of Golgi apparatus biogenesis and maintenance are focused on two possibilities: one is self-assembly from the endoplasmic reticulum, and the other is nucleation by a stable template. Here, we asked in three different experimental situations whether assembly of the Golgi apparatus might be dynamically nucleated. During microtubule depolymerization, the integral membrane protein p27 and the peripheral Golgi protein GM130, appeared in newly formed, scattered Golgi elements before three different Golgi apparatus cisternal enzymes, whereas GRASP55, a medial peripheral Golgi protein, showed, if anything, a tendency to accumulate in scattered Golgi elements later than a cisternal enzyme. During Golgi formation after brefeldin A washout, endoplasmic reticulum exit of Golgi resident enzymes could be completely separated from that of p27 and GM130. p27 and GM130 accumulation was onto newly organized perinuclear structures, not brefeldin A remnants, and preceded that of a cisternal enzyme. Reassembly was completely sensitive to guanosine 5'-diphosphate-restricted Sar1p. When cells were microinjected with Sar1pWT DNA to reverse a guanosine 5'-diphosphate-restricted Sar1p endoplasmic reticulum-exit block phenotype, GM130 and p27 collected perinuclearly with little to no exit of a cisternal enzyme from the endoplasmic reticulum. The overall data strongly indicate that the assembly of the Golgi apparatus can be nucleated dynamically by GM130/p27 associated structures. We define dynamic nucleation as the first step in a staged organelle assembly process in which new component association forms a microscopically visible structure onto which other components add later, e.g. Golgi cisternae.

Golgi enzymes are enriched in perforated zones of golgi cisternae but are depleted in COPI vesicles

In the most widely accepted version of the cisternal maturation/progression model of intra-Golgi transport, the polarity of the Golgi complex is maintained by retrograde transport of Golgi enzymes in COPI-coated vesicles. By analyzing enzyme localization in relation to the three-dimensional ultrastructure of the Golgi complex, we now observe that Golgi enzymes are depleted in COPI-coated buds and 50- to 60-nm COPI-dependent vesicles in a variety of different cell types. Instead, we find that Golgi enzymes are concentrated in the perforated zones of cisternal rims both in vivo and in a cell-free system. This lateral segregation of Golgi enzymes is detectable in some stacks during steady-state transport, but it was significantly prominent after blocking endoplasmic reticulum-to-Golgi transport. Delivery of transport carriers to the Golgi after the release of a transport block leads to a diminution in Golgi enzyme concentrations in perforated zones of cisternae. The exclusion of Golgi enzymes from COPI vesicles and their transport-dependent accumulation in perforated zones argues against the current vesicle-mediated version of the cisternal maturation/progression model.

Golgi Inheritance Under a Block of Anterograde and Retrograde Traffic

In mitosis, the Golgi complex is inherited following its dispersion, equal partitioning and reformation in each daughter cell. The state of Golgi membranes during mitosis is controversial, and the role of Golgi-intersecting traffic in Golgi inheritance is unclear. We have used brefeldin A (BFA) to perturb Golgi-intersecting membrane traffic at different stages of the cell cycle and followed by live cell imaging the fate of Golgi membranes in those conditions. We observed that addition of the drug on cells in prometaphase prevents mitotic Golgi dispersion. Under continuous treatment, Golgi fragments persist throughout mitosis and accumulate in a Golgi-like structure at the end of mitosis. This structure localizes at microtubule minus ends and contains all classes of Golgi markers, but is not accessible to cargo from the endoplasmic reticulum or the plasma membrane because of the continuous BFA traffic block. However, it contains preaccumulated cargo, and intermixes with the reforming Golgi upon BFA washout. This structure also forms when BFA is added during metaphase, when the Golgi is not discernible by light microscopy. Together the data indicate that independent Golgi fragments that contain all classes of Golgi markers (and that can be isolated from other organelles by blocking anterograde and retrograde Golgi-intersecting traffic) persist throughout mitosis.

Golgi membranes remain segregated from the endoplasmic reticulum during mitosis in mammalian cells

What happens to organelles during mitosis, and how they are apportioned to each of the daughter cells, is not completely clear. We have devised a procedure to address whether Golgi membranes fuse with the Endoplasmic Reticulum (ER) during mitosis via the detection of interactions between ER and Golgi proteins. This procedure involves coexpressing an FKBP-tagged Golgi enzyme with an ER-retained protein fused to FRAP in COS cells. Since treatment with rapamycin induces a tight association between FKBP and FRAP, one would expect rapamycin to trap the FKBP-fused Golgi protein in the ER if it ever visits the ER during mitosis. However, after the doubly transfected cells progress through mitosis in the presence of rapamycin, we find the Golgi protein in the newly formed Golgi stacks and not in the ER. Based on these results, we conclude that Golgi membranes remain separate from the ER during mitosis in mammalian cells.

Rapid, endoplasmic reticulum-independent diffusion of the mitotic Golgi haze

Early in mitosis, the mammalian Golgi apparatus disassembles, and fluorescence microscopy reveals Golgi clusters and an extensive, nonresolvable haze that either represents scattered vesicles or a merged endoplasmic reticulum (ER)-Golgi compartment. To help decide between these alternatives, we have carried out a combined microscopic and pharmacological analysis, by using a BS-C-1 cell line stably coexpressing ER and Golgi markers. Video fluorescence microscopy showed that these two organelles were morphologically distinguishable at all stages of mitosis, and photobleaching experiments showed that diffusion of the Golgi marker was unaffected by the presence of the ER. Fragmentation of the ER by using filipin III completely blocked diffusion of the ER marker but had no effect on the Golgi marker, unless it was first relocated to the ER by using brefeldin A. The Golgi haze was also studied using BODIPY ceramide. Its diffusion was slower in mitotic Golgi than in mitotic ER, but similar to that of a Golgi enzyme marker in the mitotic Golgi haze or in Golgi vesicles generated by ilimaquinone. Together, these results support the idea that the Golgi and the ER remain separate during mitosis and strongly suggest that Golgi markers move by vesicle diffusion, as opposed to lateral diffusion in continuous membranes.

Lipid phosphate phosphatases dimerise, but this interaction is not required for in vivo activity

Background

Lipid phosphate phosphatases (LPPs) are integral membrane proteins believed to dephosphorylate bioactive lipid messengers, so modifying or attenuating their activities. Wunen, a Drosophila LPP homologue, has been shown to play a pivotal role in primordial germ cell (PGC) migration and survival during embryogenesis. It has been hypothesised that LPPs may form oligomeric complexes, and may even function as hexamers. We were interested in exploring this possibility, to confirm whether LPPs can oligomerise, and if they do, whether oligomerisation is required for either in vitro or in vivo activity.

Results

We present evidence that Wunen dimerises, that these associations require the last thirty-five C-terminal amino-acids and depend upon the presence of an intact catalytic site. Expression of a truncated, monomeric form of Wunen in Drosophila embryos results in perturbation of germ cell migration and germ cell loss, as observed for full-length Wunen. We also observed that murine LPP-1 and human LPP-3 can also form associations, but do not form interactions with Wunen or each other. Furthermore, Wunen does not form dimers with its closely related counterpart Wunen-2. Finally we discovered that addition of a trimeric myc tag to the C-terminus of Wunen does not prevent dimerisation or in vitro activity, but does prevent activity in vivo.

Conclusion

LPPs do form complexes, but these do not seem to be specifically required for activity either in vitro or in vivo. Since neither dimerisation nor the C-terminus seem to be involved in substrate recognition, they may instead confer structural or functional stability through dimerisation. The results indicate that the associations we see are highly specific and occur only between monomers of the same protein.

Dispersal of Golgi matrix proteins during mitotic Golgi disassembly

During mitosis, the mammalian Golgi disassembles into numerous vesicles and larger membrane structures referred to as clusters or remnants. Following mitosis, the vesicles and clusters reassemble to form an intact Golgi in each daughter cell. One model of Golgi biogenesis states that Golgi matrix proteins remain assembled in mitotic clusters and then serve as a template for Golgi reassembly. To test this idea, we performed a 3D-computational analysis of mitotic cells to determine the extent to which these proteins remain in mitotic clusters. As a control we used brefeldin A-induced Golgi disassembly which causes dispersal of Golgi enzymes, but leaves matrix proteins in remnant structures. Unlike brefeldin A-treated cells, in which matrix proteins were clearly sorted from non-matrix proteins, we observed extensive dispersal of matrix proteins in metaphase cells with no evidence of differential sorting of these proteins from other Golgi proteins. The extensive disassembly of matrix proteins argues against their participation in a stable template and supports a self-assembly mode of Golgi biogenesis.

Capacity of the golgi apparatus for biogenesis from the endoplasmic reticulum

It is unclear whether the mammalian Golgi apparatus can form de novo from the ER or whether it requires a preassembled Golgi matrix. As a test, we assayed Golgi reassembly after forced redistribution of Golgi matrix proteins into the ER. Two conditions were used. In one, ER redistribution was achieved using a combination of brefeldin A (BFA) to cause Golgi collapse and H89 to block ER export. Unlike brefeldin A alone, which leaves matrix proteins in relatively large remnant structures outside the ER, the addition of H89 to BFA-treated cells caused ER accumulation of all Golgi markers tested. In the other, clofibrate treatment induced ER redistribution of matrix and nonmatrix proteins. Significantly, Golgi reassembly after either treatment was robust, implying that the Golgi has the capacity to form de novo from the ER. Furthermore, matrix proteins reemerged from the ER with faster ER exit rates. This, together with the sensitivity of BFA remnants to ER export blockade, suggests that presence of matrix proteins in BFA remnants is due to cycling via the ER and preferential ER export rather than their stable assembly in a matrix outside the ER. In summary, the Golgi apparatus appears capable of efficient self-assembly.

Dynamics of the apical plasma membrane recycling system during cell division

The members of the family of Rab11 small GTPases are critical regulators of the plasma membrane vesicle recycling system. While previous studies have determined that the Golgi apparatus disperses during mitosis and reorganizes after cytokinesis, the fate of the recycling system during the cell cycle is more obscure. We have now studied in MDCK cells the fate during mitosis of an apical recycling system cargo, the polymeric IgA receptor (pIgAR), and regulators of the recycling system, Rab11a and its interacting proteins myosin Vb, Rab11-FIP1, Rab11-FIP2 and pp75/Rip11. Rab11a, pIgAR and myosin Vb containing vesicles dispersed into diffuse puncta in the cytosol during prophase and then became clustered near the spindle poles after metaphase, increasing in intensity throughout telophase. A similar pattern was observed for Rab11-FIP1 and Rab11-FIP2. However, Rab11-FIP1 lost colocalization with other recycling system markers during late prophase, relocating to the pericentriolar material. During telophase, Rab11-FIP1 returned to recycling system vesicles. Western blot analysis indicated that both Rab11a and pIgAR remained associated with membrane vesicles throughout the cell cycle. This behavior of the Rab11a-containing apical recycling endosome system during division was distinct from that of the Golgi apparatus. These results indicate that critical components of the apical recycling system remain associated on vesicles throughout the cell cycle and may provide a means for rapid re-establishment of plasma membrane components after mitosis.

Cell-cycle-specific Golgi fragmentation: how and why?

The Golgi membranes, in the form of stacks of cisternae, are contained in the pericentriolar region of mammalian cells. During mitosis, these membranes are fragmented and dispersed throughout the cell. Recent studies are revealing the significance of pericentriolar position of the Golgi apparatus and mechanism by which these membranes are fragmented during mitosis.

Cisternal maturation and vesicle transport: join the band wagon! (Review)

'No cellular organelle has been the subject of as many, as long-lasting or as diverse polemics as the Golgi apparatus'. This statement was made by Whaley almost 30 years ago in the book The Golgi Apparatus and still holds true today, perhaps more then ever. Why is this? How come something as mundane as a series of intracellular membrane bound structures continues to fascinate and captivate a large section of the cell biology community? One simple reason (putting polemics aside) is that the secretory pathway appears deceptively simple. Once probed, however, it has a persistent habit of developing into an enigma. Is one then not closer than 30 years ago? In a sense yes, in that one has more components and a better understanding of inherent membrane dynamics, but it is still not known how newly synthesized proteins and lipids make their way from the ER to the plasma membrane. Is it by vesicles, cisternal carriers or transient tubular connections? It has also been learned that newly synthesized proteins are segregated away from the resident components throughout the pathway, but not how. Do coat proteins hold the key? It is understood that the cytoskeleton is important, but not really why. It is known that each Golgi stack is a fully functional unit, but not why stacks are connected laterally into a large ribbon (the Golgi apparatus). This review focuses on how proteins make their way through the pathway, a basic question that remains to be answered.

The localization and phosphorylation of p47 are important for Golgi disassembly-assembly during the cell cycle

In mammalian cells, the Golgi apparatus is disassembled at the onset of mitosis and reassembled at the end of mitosis. This disassembly-reassembly is generally believed to be essential for the equal partitioning of Golgi into two daughter cells. For Golgi disassembly, membrane fusion, which is mediated by NSF and p97, needs to be blocked. For the NSF pathway, the tethering of p115-GM130 is disrupted by the mitotic phosphorylation of GM130, resulting in the inhibition of NSF-mediated fusion. In contrast, the p97/p47 pathway does not require p115-GM130 tethering, and its mitotic inhibitory mechanism has been unclear. Now, we have found that p47, which mainly localizes to the nucleus during interphase, is phosphorylated on Serine-140 by Cdc2 at mitosis. The phosphorylated p47 does not bind to Golgi membranes. An in vitro assay shows that this phosphorylation is required for Golgi disassembly. Microinjection of p47(S140A), which is unable to be phosphorylated, allows the cell to keep Golgi stacks during mitosis and has no effect on the equal partitioning of Golgi into two daughter cells, suggesting that Golgi fragmentation-dispersion may not be obligatory for equal partitioning even in mammalian cells.

Golgi architecture and inheritance

Golgi inheritance proceeds via sequential biogenesis and partitioning phases. Although little is known about Golgi growth and replication (biogenesis), ultrastructural and fluorescence analyses have provided a detailed, though still controversial, perspective of Golgi partitioning during mitosis in mammalian cells. Partitioning requires the fragmentation of the juxtanuclear ribbon of interconnected Golgi stacks into a multitude of tubulovesicular clusters. This process is choreographed by a cohort of mitotic kinases and an inhibition of heterotypic and homotypic Golgi membrane-fusion events. Our model posits that accurate partitioning occurs early in mitosis by the equilibration of Golgi components on either side of the metaphase plate. Disseminated Golgi components then coalesce to regenerate Golgi stacks during telophase. Semi-intact cell and cell-free assays have accurately recreated these processes and allowed their molecular dissection. This review attempts to integrate recent findings to depict a more coherent, synthetic molecular picture of mitotic Golgi fragmentation and reassembly. Of particular importance is the emerging concept of a highly regulated and dynamic Golgi structural matrix or template that interfaces with cargo receptors, Golgi enzymes, Rab-GTPases, and SNAREs to tightly couple biosynthetic transport to Golgi architecture. This structural framework may be instructive for Golgi biogenesis and may encode sufficient information to ensure accurate Golgi inheritance, thereby helping to resolve some of the current discrepancies between different workers.

Horseradish peroxidase: a valuable tool in biotechnology

Peroxidases have conquered a prominent position in biotechnology and associated research areas (enzymology, biochemistry, medicine, genetics, physiology, histo- and cytochemistry). They are one of the most extensively studied groups of enzymes and the literature is rich in research papers dating back from the 19th century. Nevertheless, peroxidases continue to be widely studied, with more than 2000 articles already published in 2002 (according to the Institute for Scientific Information). The importance of peroxidases is emphasised by their wide distribution among living organisms and by their multiple physiological roles. They have been divided into three superfamilies according to their source and mode of action: plant peroxidases, animal peroxidases and catalases. Among all peroxidases, horseradish peroxidase (HRP) has received a special attention and will be the focus of this review. A brief description of the three super-families is included in the first section of this review. In the second section, a comprehensive description of the present state of knowledge of the structure and catalytic action of HRP is presented. The physiological role of peroxidases in higher plants is described in the third section. And finally, the fourth section addresses the applications of peroxidases, especially HRP, in the environmental and health care sectors, and in the pharmaceutical, chemical and biotechnological industries.

VCIP135, a novel essential factor for p97/p47-mediated membrane fusion, is required for Golgi and ER assembly in vivo

NSF and p97 are ATPases required for the heterotypic fusion of transport vesicles with their target membranes and the homotypic fusion of organelles. NSF uses ATP hydrolysis to dissociate NSF/SNAPs/SNAREs complexes, separating the v- and t-SNAREs, which are then primed for subsequent rounds of fusion. In contrast, p97 does not dissociate the p97/p47/SNARE complex even in the presence of ATP. Now we have identified a novel essential factor for p97/p47-mediated membrane fusion, named VCIP135 (valosin-containing protein [VCP][p97]/p47 complex-interacting protein, p135), and show that it binds to the p97/p47/syntaxin5 complex and dissociates it via p97 catalyzed ATP hydrolysis. In living cells, VCIP135 and p47 are shown to function in Golgi and ER assembly.

Inheritance of the endoplasmic reticulum and Golgi apparatus

Yeast and mammalian cells use a variety of different mechanisms to ensure that the endoplasmic reticulum and Golgi apparatus are inherited by both daughter cells on cell division. In yeast, endoplasmic reticulum inheritance involves both active microtubule and passive actin-based mechanisms, while the Golgi is transported into the forming daughter cell by an active actin-based mechanism. Animal cells actively partition the endoplasmic reticulum and Golgi apparatus, but association with the mitotic spindle-rather than the actin cytoskeleton-appears to be the mechanism

Can the Golgi form de novo?

Does the Golgi apparatus proliferate by adding new material to a permanent template, or do Golgi structures form de novo by a process of self-organization? Recent work suggests that the Golgi is capable of forming de novo.

The Golgi apparatus: balancing new with old

Most models put forward to explain cellular processes do not stand the test of time. The 'lucky' few that are able to survive extensive experimental tests and peer critique may eventually become dogmas or paradigms. When this happens, the amount of experimental data required to overturn the paradigm is extensive. To some, such inertia may seem prohibitive to scientific progress but rather, in our opinion, this helps to maintain a degree of coherence. It is needed so that experiments and interpretations may be conducted within relatively safe boundaries. In the field of protein transport in the secretory pathway, we have enjoyed a relatively stable and productive period for quite some time (more than 30 years!). It is only very recently that the field has entered into a phase where all bets seem to be off. As in any paradigm shift, the accumulation of experimental observations inconsistent with the old dogma eventually reached a critical point. As we 'reluctantly' dispense with the long-standing paradigm of forward vesicular transport, we face a time that is bound to be trying as well as exciting.

The stack of the golgi apparatus

One hundred years have passed since the discovery of "the internal reticular apparatus" by Camillo GOLGI. Investigations into the structure and function of the "Golgi apparatus" have raised more and more challenging issues for cell biologists. After long debate, many new findings have accumulated in the last 10 years as a result of the availability of elegant new genetic, biochemical and morphological tools. This, in turn, has raised many new questions to be solved. In addition, numerous new findings have led to some confusion on the understanding of the Golgi apparatus. This review article deals with several modern aspects of vesicular transport versus cisternal maturation. Disruption of the stacked structure in mitotic and drug-induced conditions is also discussed to demonstrate the importance of structural integrity in the Golgi apparatus.

Inheriting a structural scaffold for Golgi biosynthesis

In animal cells, the Golgi complex undergoes reversible disassembly during mitosis. The disassembly/reassembly process has been intensively studied in order to understand the mechanisms that govern organelle assembly and inheritance during cell division. A long-standing controversy in the field has been whether formation of Golgi structure is template-mediated or self-organizes from components of the endoplasmic reticulum. A recent study1 however, has demonstrated that a subset of proteins that form a putative Golgi matrix can be inherited during cell division in the absence of membrane input from the endoplasmic reticulum. The outcome of this study suggests that a templating mechanism for the formation of Golgi structure may exist. This study has important implications for understanding mechanisms that govern Golgi biogenesis.

Golgi complex: biogenesis de novo?

Whether Golgi biogenesis occurs by self-assembly or around a pre-existing template is currently a matter of debate. Recent studies have shown that Golgi structural proteins are more dynamic than previously thought, suggesting that self-assembly of the Golgi complex may be possible.

Possible implication of Golgi-nucleating function for the centrosome

The Golgi apparatus breaks down at mitosis, resulting in the dispersal of Golgi-resident proteins. In NRK cells, however, subsets of both TGN38 and golgin-97, but not ManII and GM130, remained associated with the centrosome throughout the cell cycle. This centrosome association of TGN38 and golgin-97 was not disrupted by treatment with brefeldin A, additional inducers of retrograde trafficking and inhibitors of either kinases or protein phosphatases. Anchoring of the Golgi apparatus within the juxtanuclear region depends on microtubules; the association of TGN38 and golgin-97 subsets with the centrosome, however, was insensitive to nocodazole treatment. Drugs such as PDMP, which block Golgi dispersal both by nocodazole, despite microtubule depolymerization, and by inducers of retrograde trafficking, strengthened the microtubule-nucleating activity of the centrosome. These observations cumulatively suggest the centrosome is implicated in nucleation of the Golgi apparatus through interactions with Golgi-resident proteins, such as TGN38 and golgin-97.

Partitioning of the matrix fraction of the Golgi apparatus during mitosis in animal cells

The Golgi apparatus is partitioned during mitosis in animal cells by a process of fragmentation, dispersal, and reassembly in each daughter cell. We fractionated the Golgi apparatus in vivo using the drug brefeldin A or a dominant-negative mutant of the Sar1p protein. After these treatments, Golgi enzymes moved back to the endoplasmic reticulum, leaving behind a matrix of Golgi structural proteins. Under these conditions, cells still entered and exited mitosis normally, and their Golgi matrix partitioned in a manner very similar to that of the complete organelle. Thus, the matrix may be the partitioning unit of the Golgi apparatus and may carry the Golgi enzyme-containing membranes into the daughter cells.

Deconstructing Golgi inheritance

Eukaryotic cells use a variety of strategies to inherit the Golgi apparatus. During vertebrate mitosis, the Golgi reorganizes dramatically in a process that seems to be driven by the reversible fragmentation of existing Golgi structures and the temporary redistribution of Golgi components to the endoplasmic reticulum. Several proteins that participate in vertebrate Golgi inheritance have been identified, but their detailed functions remain unknown. A comparison between vertebrates and other eukaryotes reveals common mechanisms of Golgi inheritance. In many cell types, Golgi stacks undergo fission early in mitosis. Some cells exhibit a further Golgi breakdown that is probably due to a mitotic inhibition of membrane traffic. In all eukaryotes examined, Golgi inheritance involves either the partitioning of pre-existing Golgi elements between the daughter cells or the emergence of new Golgi structures from the endoplasmic reticulum, or some combination of these two pathways.