Physical factors that affect the number and size of Golgi cisternae

Despite continual membrane reorganization in the Golgi complex, the number of cisternae in a Golgi stack is a stable parameter. The cisternal number is conserved within any given cell line and also after Golgi reassembly, e.g. following brefeldin-A-induced disruption. However, the factors that determine the cisternal number in a single Golgi stack remain to be fully determined. We propose a simple mechanical model of the Golgi stack and present a theoretical analysis of different physical factors that may affect the number of cisternae in a Golgi stack. The model takes into account the Golgi membrane bending elasticity, which is related to the membrane curvature, and the adhesion, which holds the cisternae together. The analysis shows that the equilibrium configuration of the Golgi stack can be regarded as a balance between these two effects - the adhesion, which tends to increase the number of cisternae, is opposed by the membrane resistance to bending, which does not favor highly curved cisternal rims. The adhesion strength that is needed to hold together a typical stack is calculated. In addition, the model is used to analyze changes in the cisternal numbers as a controlled traffic wave enters a Golgi stack and increases the amount of the membrane in that stack.     

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.     

Exclusion of golgi residents from transport vesicles budding from Golgi cisternae in intact cells

A central feature of cisternal progression/maturation models for anterograde transport across the Golgi stack is the requirement that the entire population of steady-state residents of this organelle be continuously transported backward to earlier cisternae to avoid loss of these residents as the membrane of the oldest (trans-most) cisterna departs the stack. For this to occur, resident proteins must be packaged into retrograde-directed transport vesicles, and to occur at the rate of anterograde transport, resident proteins must be present in vesicles at a higher concentration than in cisternal membranes. We have tested this prediction by localizing two steady-state residents of medial Golgi cisternae (mannosidase II and N-acetylglucosaminyl transferase I) at the electron microscopic level in intact cells. In both cases, these abundant cisternal constituents were strongly excluded from buds and vesicles. This result suggests that cisternal progression takes place substantially more slowly than most protein transport and therefore is unlikely to be the predominant mechanism of anterograde movement.     

The dynamics of engineered resident proteins in the mammalian Golgi complex relies on cisternal maturation

After leaving the endoplasmic reticulum, secretory proteins traverse several membranous transport compartments before reaching their destinations. How they move through the Golgi complex, a major secretory station composed of stacks of membranous cisternae, is a central yet unsettled issue in membrane biology. Two classes of mechanisms have been proposed. One is based on cargo-laden carriers hopping across stable cisternae and the other on “maturing” cisternae that carry cargo forward while progressing through the stack. A key difference between the two concerns the behavior of Golgi-resident proteins. Under stable cisternae models, Golgi residents remain in the same cisterna, whereas, according to cisternal maturation, Golgi residents recycle from distal to proximal cisternae via retrograde carriers in synchrony with cisternal progression. Here, we have engineered Golgi-resident constructs that can be polymerized at will to prevent their recycling via Golgi carriers. Maturation models predict the progress of such polymerized residents through the stack along with cargo, but stable cisternae models do not. The results support the cisternal maturation mechanism.     

Substructure of cisternal organelles of neuronal perikarya in immature rat brains revealed by quick-freeze and deep-etch techniques

Summary Membrane-bounded organelles possessing cisternae, i.e., rough endoplasmic reticulum and Golgi apparatus, in immature rat central neurons were examined by quick-freeze and deep-etch techniques to see how their intracisternal structures are organized and how ribosomes are associated with the membrane of the endoplasmic reticulum. Cisternae of endoplasmic reticulum, 60–100 nm wide, were bridged with randomly-distributed strands (trabecular strands, 12.5 nm in mean diameter). Luminal surfaces of cisternae of the endoplasmic reticulum were decorated with various-sized globular particles, some as small as intramembrane particles, and others as large as granules formed by soluble proteins seen in the cytoplasm. A closer examination revealed much thinner strands (3.3. nm in mean diameter). Such thin strands were short, usually winding toward the luminal surface, and sometimes touching the luminal surface with one end. Ribosomes appeared to be embedded into the entire thickness of cross-fractured membranes of endoplasmic reticulum, that is, their internal portions appeared to be situated at almost the same level as the cisternal luminal surface. From the internal portion of ribosomes, single thin strands occasionally protruded into the lumen, suggesting that these thin strands were newly synthesized polypeptides. A horizontal separation within ribosomes appeared to occur at the same level as the hydrophobic middle of the membrane of the endoplasmic reticulum. Interiors of the Golgi apparatus cisternae, which were much narrower than cisternae of endoplasmic reticulum, were similarly bridged with trabecular strands, but the Golgi trabecular strands were thinner and more frequent. Their cisternal lumina were also dotted with globular particles. No identifiable profiles corresponding to the thin strands in the endoplasmic reticulum were observed. Golgi cisternae showed a heterogeneous distribution of membrane granularity; the membrane in narrow cisternal space was granule-rich, while that in expanded space was granule-poor, suggesting a functional compartmentalization of the Golgi cisternae.     

Tubules of the trans Golgi apparatus visualized by immunoelectron microscopy

 Tubules constitute an integral part of the Golgi apparatus and have been shown to form a complex and dynamic network at its trans side. We have studied in detail structural features of the trans Golgi network and its relationship with the cisternal stack in thin sections of Lowicryl K4M embedded human absorptive enterocytes by immunolectron microscopy. Immunoreactive sites for α1,3 N-acetylgalactosaminyltransferase and blood group A substance were detectable troughout the cisternal stack and the entire trans Golgi network. Furthermore, the entire trans Golgi network was reactive for CMPase activity. Evidence for two kinds of tubules at the trans side of the Golgi apparatus was found: tubules that laterally connect adjacent and distant cisternal stacks, and others extending from central and lateral portions of trans cisternae to form the complex and extensive trans Golgi network. Trans cisternae showed often the peeling-off phenomenon and were continuous with the trans Golgi network. Both, trans cisternae and tubules of the trans Golgi network exhibited regionally buds and vesicles with a lace-like, non clathrin coat, previously reported by others in NRK cells, which contained glycoproteins with terminal N-acetylgalactosamine residues. These buds and vesicle are therefore involved in constitutive exocytosis. Accepted: 12 January 1998     

Electron tomography of RabA4b- and PI-4Kβ1-labeled trans Golgi network compartments in Arabidopsis

The trans Golgi network (TGN) of plant cells sorts and packages Golgi products into secretory (SV) and clathrin-coated (CCV) vesicles. We have analyzed of TGN cisternae in Arabidopsis root meristem cells by cell fractionation and electron microscopy/tomography to establish reliable criteria for identifying TGN cisternae in plant cells, and to define their functional attributes. Transformation of a trans Golgi cisterna into a Golgi-associated TGN cisterna begins with cisternal peeling, the formation of SV buds outside the plane of the cisterna and a 30-35% reduction in cisternal membrane area. Free TGN compartments are defined as cisternae that have detached from the Golgi to become independent organelles. Golgi-associated and free TGN compartments, but not trans Golgi cisternae, bind anti-RabA4b and anti-phosphatidylinositol-4 kinase (PI-4K) antibodies. RabA4b and PI-4Kβ1 localize to budding SVs in the TGN and to SVs en route to the cell surface. SV and CCV release occurs simultaneously via cisternal fragmentation, which typically yields ～30 vesicles and one to four residual cisternal fragments. Early endosomal markers, VHA-a1-green fluorescent protein (GFP) and SYP61-cyan fluorescent protein (CFP), colocalized with RabA4b in TGN cisternae, suggesting that the secretory and endocytic pathways converge at the TGN. pi4k1/pi4k2 knockout mutant plants produce SVs with highly variable sizes indicating that PI-4Kβ1/2 regulates SV size.     

Curvature of double-membrane organelles generated by changes in membrane size and composition

Transient double-membrane organelles are key players in cellular processes such as autophagy, reproduction, and viral infection. These organelles are formed by the bending and closure of flat, double-membrane sheets. Proteins are believed to be important in these morphological transitions but the underlying mechanism of curvature generation is poorly understood. Here, we describe a novel mechanism for this curvature generation which depends primarily on three membrane properties: the lateral size of the double-membrane sheets, the molecular composition of their highly curved rims, and a possible asymmetry between the two flat faces of the sheets. This mechanism is evolutionary advantageous since it does not require active processes and is readily available even when resources within the cell are restricted as during starvation, which can induce autophagy and sporulation. We identify pathways for protein-assisted regulation of curvature generation, organelle size, direction of bending, and morphology. Our theory also provides a mechanism for the stabilization of large double-membrane sheet-like structures found in the endoplasmic reticulum and in the Golgi cisternae.     

Pre- and post-Golgi vacuoles operate in the transport of Semliki Forest virus membrane glycoproteins to the cell surface

The effect of reduced temperature on synchronized transport of SFV membrane proteins from the ER via the Golgi complex to the surface of BHK-21 cells revealed two membrane compartments where transport could be arrested. At 15 degrees C the proteins could leave the ER but failed to enter the Golgi cisternae and accumulated in pre-Golgi vacuolar elements. At 20 degrees C the proteins passed through Golgi stacks but accumulated in trans-Golgi cisternae, vacuoles, and vesicular elements because of a block affecting a distal stage in transport. Both blocks were reversible, allowing study of the synchronous passage of viral membrane proteins through the Golgi complex at high resolution by immunolabeling in electron microscopy. We propose that membrane proteins enter the Golgi stack via tubular extensions of the pre-Golgi vacuolar elements which generate the Golgi cisternae. The proteins pass across the Golgi apparatus following cisternal progression and enter the post-Golgi vacuolar elements to be routed to the cell surface.     

Reassembly of Golgi stacks from mitotic Golgi fragments in a cell-free system

Rat liver Golgi stacks were incubated with mitotic cytosol for 30 min at 37 degrees C to generate mitotic Golgi fragments comprising vesicles, tubules, and cisternal remnants. These were isolated and further incubated with rat liver cytosol for 60 min. The earliest intermediate observed by electron microscopy was a single, curved cisterna with tubular networks fused to the cisternal rims. Elongation of this cisterna was accompanied by stacking and further growth at the cisternal rims. Stacks also fused laterally so that the typical end product was a highly curved stack of 2-3 cisternae mostly enclosing an electron-lucent space. Reassembly occurred in the presence of nocodazole or cytochalasin B but not at 4 degrees C or in the absence of energy supplied in the form of ATP and GTP. Pretreatment of the mitotic fragments and cytosol with N-ethylmaleimide (NEM) also prevented reassembly. GTP gamma S and A1F prevented reassembly when added during fragmentation but not when added to the reassembly mixture. In fact, GTP gamma S stimulated reassembly such that all cisternae were stacked at the end of the incubation and comprised 40% of the total membrane. In contrast, microcystin inhibited stacking so that only single cisternae accumulated. Together these results provide a detailed picture of the reassembly process and open up the study of the architecture of the Golgi apparatus to a combined morphological and biochemical analysis.     

Segregation of the Qb‐SNAREs GS27 and GS28 into Golgi Vesicles Regulates Intra‐Golgi Transport

The Golgi apparatus is the main glycosylation and sorting station along the secretory pathway. Its structure includes the Golgi vesicles, which are depleted of anterograde cargo, and also of at least some Golgi‐resident proteins. The role of Golgi vesicles remains unclear. Here, we show that Golgi vesicles are enriched in the Qb‐SNAREs GS27 (membrin) and GS28 (GOS‐28), and depleted of nucleotide sugar transporters. A block of intra‐Golgi transport leads to accumulation of Golgi vesicles and partitioning of GS27 and GS28 into these vesicles. Conversely, active intra‐Golgi transport induces fusion of these vesicles with the Golgi cisternae, delivering GS27 and GS28 to these cisternae. In an in vitro assay based on a donor compartment that lacks UDP‐galactose translocase (a sugar transporter), the segregation of Golgi vesicles from isolated Golgi membranes inhibits intra‐Golgi transport; re‐addition of isolated Golgi vesicles devoid of UDP‐galactose translocase obtained from normal cells restores intra‐Golgi transport. We conclude that this activity is due to the presence of GS27 and GS28 in the Golgi vesicles, rather than the sugar transporter. Furthermore, there is an inverse correlation between the number of Golgi vesicles and the number of inter‐cisternal connections under different experimental conditions. Finally, a rapid block of the formation of vesicles via COPI through degradation of ϵCOP accelerates the cis‐to‐trans delivery of VSVG. These data suggest that Golgi vesicles, presumably with COPI, serve to inhibit intra‐Golgi transport by the extraction of GS27 and GS28 from the Golgi cisternae, which blocks the formation of inter‐cisternal connections .     

Maturation of Golgi cisternae directly observed

Newly synthesized secretory proteins pass through the Golgi apparatus, which consists of multiple cisternae containing distinct populations of enzymes. Are the cargo proteins shuttled between cisternae in vesicles or do they remain in a cisterna while it is the Golgi enzymes that are removed and replaced? As predicted by the latter model--the cisternal maturation hypothesis--two groups have directly observed the replacement of one Golgi protein with another in individual cisternae, thus answering the question. However, its solution raises many more unknowns.     

Association of Cdc42/N-WASP/Arp2/3 signaling pathway with Golgi membranes

Recent findings indicate that Cdc42 regulates Golgi-to-ER (endoplasmic reticulum) protein transport through N-WASP and Arp2/3 (Luna et al. 2002, Mol. Biol. Cell, 13:866-879). To analyse the components of the Cdc42-governed signaling pathway in the secretory pathway, we localized Cdc42, N-WASP and Arp2/3 in the Golgi complex by cryoimmunoelectron microscopy. Cdc42 is found throughout the Golgi stack, particularly in cis/middle cisternae, whereas N-WASP and Arp3 (a component of the Arp2/3 complex) are restricted to cis cisternae. Arp3 also colocalized in peri-Golgi tubulovesicular structures with either KDEL receptor or GM130. Even though Arp3 is not found in TGN46-positive cisternal elements, a small fraction of Arp3-labeled tubulo-vesicular elements showed TGN46 labeling. Active Cdc42 (GTP-bound form) induced relocation of N-WASP and Arp3 to the lateral rims of Golgi cisternae. These results show that the actin nucleation and polymerization signaling pathway governed by Cdc42/N-WASP/Arp operates in the Golgi complex of mammalian cells, further implicating actin dynamics in Golgi-associated membrane trafficking.     

Scale biogenesis in the green alga,Mesostigma viride

Summary The unicellular green algal flagellate,Mesostigma viride, is characterized by an extracellular matrix of multiple layers of scales. These scales are processed within the Golgi apparatus (GA). The GA consists of 11–13 closely stacked cisternae. The cis cisternae are highly fenestrated and grow via vesicles from adjacent transition ER. Medial-trans cisternae are plate-like with swollen peripheries. The calcified basket scales are produced in the peripheries of GA cisternae, usually first observable in the medial zone of the cisternal stack. Cisternal membrane closely conforms to the precise architecture of the developing scale. Antimonate labeling reveals that a population of smooth cytoplasmic vacuoles situated near the GA contains a store of calcium, perhaps used for scale processing. Vesicles carry calcium from these vacuoles to the cisternal loci where basket scale ontogenesis is occurring. The smaller scale types are produced within the central areas of the GA. A discussion concerning membrane flow through the GA is provided.     

Transformation of Golgi membrane into the envelope of herpes simplex virus in rat anterior pituitary cells

Envelopment of herpes simplex virus type-1 (HSV-1) was investigated in relation to membrane differentiation in dissociated anterior pituitary cells. The number of cells stained positively with anti-HSV-1 serum was increased from 16 h to 31 h post infection. During this period, electron microscopy revealed that a number of nucleocapsids (unenveloped particles) were accumulated in the Golgi area, where they frequently became surrounded by a double membrane of short Golgi cisternae or by one with a Golgi associated endoplasmic reticulum lysosome (GERL)-like structure. The inner membrane of the cisterna surrounding the nucleocapsids showed regional specialization which was characterized by increased thickness and electron opacity. Acid phosphatase activity, a marker for GERL or trans Golgi cisternae, appeared in the cytoplasmic short cisternae surrounding the nucleocapsids, whereas glucose-6-phosphatase activity, a marker for the nuclear envelope or for endoplasmic reticulum, was not demonstrated in such cisternae. Monoclonal antibody against glycoprotein gD revealed that gD was localized in the trans Golgi membrane as well as in the envelope of the virion. The antibody-binding sites were highly concentrated in the area where Golgi membranes showed increased opacity. Furthermore, nucleocapsids were surrounded exclusively by gD-positive cisternal (Golgi or Golgi-derived) membranes. Thus, our results indicate that the envelope of HSV is derived from trans Golgi cisterna (GERL), and that some viral components, including gD, destined for the envelope may be assembled initially in the Golgi membrane, which is thereby transformed into the envelope of the virus.     

Modeling morphological instabilities in lipid membranes with anchored amphiphilic polymers

Anchoring molecules, like amphiphilic polymers, are able to dynamically regulate membrane morphology. Such molecules insert their hydrophobic groups into the bilayer, generating a local membrane curvature. In order to minimize the elastic energy penalty, a dynamic shape instability may occur, as in the case of the curvature-driven pearling instability or the polymer-induced tubulation of lipid vesicles. We review recent works on modeling of such instabilities by means of a mesoscopic dynamic model of the phase-field kind, which take into account the bending energy of lipid bilayers.     

Golgi cisternal unstacking stimulates COPI vesicle budding and protein transport

The Golgi apparatus in mammalian cells is composed of flattened cisternae that are densely packed to form stacks. We have used the Golgi stacking protein GRASP65 as a tool to modify the stacking state of Golgi cisternae. We established an assay to measure protein transport to the cell surface in post-mitotic cells in which the Golgi was unstacked. Cells with an unstacked Golgi showed a higher transport rate compared to cells with stacked Golgi membranes. Vesicle budding from unstacked cisternae in vitro was significantly increased compared to stacked membranes. These results suggest that Golgi cisternal stacking can directly regulate vesicle formation and thus the rate of protein transport through the Golgi. The results further suggest that at the onset of mitosis, unstacking of cisternae allows extensive and rapid vesiculation of the Golgi in preparation for its subsequent partitioning.     

A role for the vesicle tethering protein, p115, in the post-mitotic stacking of reassembling Golgi cisternae in a cell-free system.

During telophase, Golgi cisternae are regenerated and stacked from a heterogeneous population of tubulovesicular clusters. A cell-free system that reconstructs these events has revealed that cisternal regrowth requires interplay between soluble factors and soluble N-ethylmaleimide (NEM)-sensitive fusion protein (NSF) attachment protein receptors (SNAREs) via two intersecting pathways controlled by the ATPases, p97 and NSF. Golgi reassembly stacking protein 65 (GRASP65), an NEM-sensitive membrane-bound component, is required for the stacking process. NSF-mediated cisternal regrowth requires a vesicle tethering protein, p115, which we now show operates through its two Golgi receptors, GM130 and giantin. p97-mediated cisternal regrowth is p115-independent, but we now demonstrate a role for p115, in conjunction with its receptors, in stacking p97 generated cisternae. Temporal analysis suggests that p115 plays a transient role in stacking that may be upstream of GRASP65-mediated stacking. These results implicate p115 and its receptors in the initial alignment and docking of single cisternae that may be an important prerequisite for stack formation.     

Mechanisms of negative membrane curvature sensing and generation by ESCRT III subunit Snf7

Certain proteins have the propensity to bind to negatively curved membranes and generate negative membrane curvature. The mechanism of action of these proteins is much less studied and understood than those that sense and generate positive curvature. In this work, we use implicit membrane modeling to explore the mechanism of an important negative curvature sensing and generating protein: the main ESCRT III subunit Snf7. We find that Snf7 monomers alone can sense negative curvature and that curvature sensitivity increases for dimers and trimers. We have observed spontaneous bending of Snf7 oligomers into circular structures with preferred radius of ~20 nm. The preferred curvature of Snf7 filaments is further confirmed by the simulations of preformed spirals on a cylindrical membrane surface. Snf7 filaments cannot bind with the same interface to flat and curved membranes. We find that even when a filament has the preferred radius, it is always less stable on the flat membrane surface than on the interior cylindrical membrane surface. This provides an additional energy for membrane bending which has not been considered in the spiral spring model. Furthermore, the rings on the cylindrical spirals are bridged together by helix 4 and hence are extra stabilized compared to the spirals on the flat membrane surface.     

The tubular network of the Golgi apparatus

 Golgi apparatus of both plant and animal cells are characterized by an extensive system of approximately 30 nm diameter peripheral tubules. The total surface area of the tubules and associated fenestrae is thought to be approximately equivalent to that of the flattened portions of cisternae. The tubules may extend for considerable distances from the stacks. The tubules are continuous with the peripheral edges of the stacked cisternae, but the way they interconnect differs across the stack. In plant cells, for example, tubules associated with the near-cis and mid cisternae often begin to anastomose close to the peripheral edges of the stacked cisternae, whereas the tubules of the trans cisternae are less likely to anastomose and are more likely to be directly continuous with the peripheral edges of the stacked cisternae. Additionally, the tubules may blend gradually into fenestrae that surround some of the stack cisternae. Because of the large surface area occupied by tubules and fenestrae, it is reasonable to suppose that these components of the Golgi apparatus play a significant role in Golgi apparatus function. Tubules clearly interconnect closely adjacent stacks of the Golgi apparatus and may represent a communication channel to synchronize stack function within the cell. A feasible hypothesis is that tubules may be a potentially static component of the Golgi apparatus in contrast to the stacked cisternal plates which may turn over continuously. The coated buds associated with tubules may represent the means whereby adjacent Golgi apparatus stacks exchange carbohydrate-processing enzymes or where resident Golgi apparatus proteins are introduced into and out of the stack during membrane flow differentiation. The limited gradation of tubules from cis to medial to trans offers additional possibilities for functional specialization of Golgi apparatus in keeping with the hypothesis that tubules are repositories of resident Golgi apparatus proteins protected from turnover during the flow differentiation of the flattened saccules of the Golgi apparatus stack. Accepted: 3 November 1997