Accumulation Of Vesicles Research Articles

A lipid-managing program maintains a stout Spitzenkörper.

The Spitzenkörper (SPK) is an accumulation of vesicles interleaved with actin microfilaments present at the cytosolic side of the apical plasma membrane (PM) of hyphal tips of many species of filamentous fungi. The physiological role of the SPK has captivated fungal biologists over the years, but only very recently this 'organelle' is starting to be understood in the molecular terminology used for cell biological models. One aspect that has received little attention is the role of cellular membrane asymmetry in the organization of membrane traffic, in particular in the genetic and cell biological model Aspergillus nidulans. The paper by Schultzhaus et al. (2015) in this issue breaks the ice, providing original insight that may foster research in phospholipid composition in the context of intracellular traffic and the organization of the SPK. Notably, it shows that like the stout Neurospora crassa SPK, the much slimmer one of A. nidulans, appears to be formed by different strata, altogether suggesting that the SPK might be a mosaic of exocytic carriers with different functional specializations, and a major sorting hub for intracellular membranes.

Expression ofMalus xiaojinensisIRT1 (MxIRT1) protein in transgenic yeast cells leads to degradation through autophagy in the presence of excessive iron

Iron is essential for plants, but highly toxic when present in excess. Consequently, iron uptake by root transporters must be finely tuned to avoid excess uptake from soil under iron excess. The iron-regulated transporter of Malus xiaojinensis (MxIRT1), induced in roots under iron deficiency, is a highly effective iron(II) transporter. Here, we investigated how the presence of excessive iron leads to MxIRT1 degradation in yeast expressing this plant iron transporter protein. To determine the relationship between iron abundance and MxIRT1 degradation, relative levels of autophagy-related gene-8 (ATG8) mRNA and the active ATG8-phosphatidylethanolamine-conjugated (PE) protein were measured in wild-type yeast and the autophagic mutant strains atg1∆, atg5∆, atg7∆, ypt7∆ and tor1∆ under normal and excessive iron conditions. The data showed that the exposure of MxIRT1-eGFP-transformed wild-type and tor1∆ strains to excessive iron led to significantly increased levels of ATG8 transcript and ATG8-PE protein, which resulted in enhanced MxIRT1 degradation. Co-localization of mCherry-ATG8 and MxIRT1-eGFP provided evidence that these proteins interact during autophagy in yeast. While inhibition of autophagic initiation, autophagosome formation and vacuole fusion all decreased MxIRT1 degradation. PMSF inhibition of autophagy prevented degradation, leading to the accumulation of MxIRT1-containing vesicles in the vacuoles. MxIRT1-vesicles were sorted into autophagosomes for iron-induced degradation in yeast, whereas the endogenous iron(II) transporter Fet4 was degraded in an autophagy-independent manner. Moreover, immunoprecipitation showed that multimono-ubiquitins provided MxIRT1 with the ubiquitination signal. Together, three factors, iron excess, autophagy and mono-ubiquitination, affect the functional activity and stability of exogenous MxIRT1 in yeast, thereby preventing iron uptake via this root transporter.

Facile Synthesis of Rod–Coil Block Copolymers with Chiral, Helical Polycarbodiimide Segments via Postpolymerization CuAAC “Click” Coupling of Functional End Groups

Using the living nickel(II)-mediated polymerization of carbodiimides, the chiral (R)- or (S)-N-1-phenethyl-N′-methylcarbodiimide (PMC) monomers were polymerized with a new TIPS protected alkyne functional nickel initiator forming PPMC with an excess single-handed screw sense and the alkyne moiety covalently attached to the terminus of the polymer, as confirmed by 1H NMR and MALDI-TOF MS. After deprotection, the alkyne end groups of rigid-rod PPMC-2 were coupled with azide-terminated, random-coil PS and PEG homopolymers forming a novel class of helical-b-coil block copolymers. In the thin-film, all synthesized diblock copolymers formed interesting nanofibular morphologies when subject to specific conditions. The triblock RCP-4, however, adopted unique macroporous morphology as identified by AFM and SEM with average pore diameters of ca. 832 ± 194 nm. The origin of this was found to be associated with the ordering of large, hollow vesicle aggregates upon solvent evaporation followed by the melting of these aggregates filling in the hollow interior forming the submicron pores observed. Furthermore, the size of these aggregates can be easily modulated in a linear fashion from 272 to 1648 nm simply by increasing the concentration of RCP-4 in THF. Finally, the three PPMC–PEG copolymers synthesized were found to adopt lyotropic cholesteric mesophases in concentrated toluene solutions (ca. 30 wt %).

Soft aggregates formed by a nonionic phytosterol ethoxylate and β-cyclodextrin in aqueous solution

The self-assembling behavior of a nonionic phytosterol ethoxylate surfactant (BPS-30), mixed with beta-cyclodextrin (beta-CD) at a 1:1 molar ratio, has been investigated in aqueous solutions. The anisotropic and birefringent aggregates obtained with the mixtures present vesicle and lamellar morphologies, which are rarely found in other surfactant/beta-CD mixture systems. The observed aggregate morphologies are determined mainly by the surfactant concentrations as evidenced through observations using scanning or cryo-transmission electron microscopy. The vesicle aggregates appear in solutions of low BPS-30 concentrations ( 12 wt%). Additional aggregate structural details are explored through small-angle X-ray scattering measurements. Different with other surfactant/beta-CD mixture systems, beta-CD appears to function as a cosurfactant in this research, as shown by the H-1-H-1 NOESY nuclear magnetic resonance spectroscopy and the surface tension measurements. Control experiments with urea molecules were conducted to demonstrate that the main driving force of aggregation is a synergistic effect of the H-bonding and hydrophobic interactions. The obtained results on the nonionic mixture of biosurfactant/beta-CD in this paper further demonstrate the potential expanded applications of soft matter construction.

Association between cationic liposomes and low molecular weight hyaluronic acid.

This work presents a study of the association between low molecular weight hyaluronic acid (16 kDa HA) and cationic liposomes composed of egg phosphatidylcholine (EPC), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP). The cationic liposome/HA complexes were evaluated to determine their mesoscopic structure, average size, zeta potential, and morphology as a function of the amount of HA in the system. Small angle X-ray scattering results revealed that neighboring cationic liposomes either stick together after a partial coating of low concentration HA or disperse completely in excess of HA, but they never assemble as multilamellar vesicles. Cryo-transmission electron microscopy images confirm the existence of unilamellar vesicles and large aggregates of unilamellar vesicles for HA fractions up to 80% (w/w). High concentrations of HA (> 20% w/w) proved to be efficient for coating extruded liposomes, leading to particle complexes with sizes in the nanoscale range and a negative zeta potential.

CCM-3/STRIPAK promotes seamless tube extension through endocytic recycling.

The mechanisms governing apical membrane assembly during biological tube development are poorly understood. Here, we show that extension of the C. elegans excretory canal requires cerebral cavernous malformation 3 (CCM-3), independent of the CCM1 orthologue KRI-1. Loss of ccm-3 causes canal truncations and aggregations of canaliculular vesicles, which form ectopic lumen (cysts). We show that CCM-3 localizes to the apical membrane, and in cooperation with GCK-1 and STRIPAK, promotes CDC-42 signalling, Golgi stability and endocytic recycling. We propose that endocytic recycling is mediated through the CDC-42-binding kinase MRCK-1, which interacts physically with CCM-3-STRIPAK. We further show canal membrane integrity to be dependent on the exocyst complex and the actin cytoskeleton. This work reveals novel in vivo roles of CCM-3·STRIPAK in regulating tube extension and membrane integrity through small GTPase signalling and vesicle dynamics, which may help explain the severity of CCM3 mutations in patients.

Cytotoxic and genotoxic characterization of aluminum and silicon oxide nanoparticles in macrophages

ObjectiveAlthough aluminum oxide and silicon oxide nanoparticles are currently available as dental materials, there is a lack of basic information concerning their biocompatibility. This study evaluates the biological responses of cultured macrophages (RAW264) to aluminum oxide (Al2O3NPs) and silicon oxide nanoparticles (SiO2NPs) by analyzing cytotoxicity and genotoxicity. MethodsThe nanoparticles are amorphous and spherical, with diameters of 13nm for the Al2O3NPs and 12nm for the SiO2NPs. The cultured RAW264 are exposed to the nanoparticles (NPs) and examined for cytotoxicity using the WST-8 cell viability and Hoechst/PI apoptosis assay, for genotoxicity by micronucleus analysis, for changes in nuclear shape (deformed nuclei) and for comet assay using confocal microscopy, and micromorphological analysis is done using scanning and transmission electron microscopes. ResultsNuclei and DNA damage because of exposure to both types of NPs is observed by inmunostaining genotoxicity testing. The cytotoxicity and genotoxicity are well correlated in this study. Numerous NPs are observed as large aggregates in vesicles, but less or nonexistent NP internalization is seen in the nucleus or cytoplasm. These morphological results suggest that a primary cause of cell disruption is the chemical changes of the NPs in the low pH of vesicles (i.e., ionization of Al2O3 or SiO2) for both types of oxide NPs. SignificanceAlthough further research on the elution of NP concentrations on cell or tissue activity under simulated clinical conditions is required, NP concentrations over 200μg/mL are large enough to induce cytotoxic and genotoxic effects to cells.

Investigation of the cytotoxicity of aluminum oxide nanoparticles and nanowires and their localization in L929 fibroblasts and RAW264 macrophages.

The biological responses of aluminum oxide (Al2 O3 ) nanoparticles (NPs) and nanowires (NWs) in cultured fibroblasts (L929) and macrophages (RAW264) were evaluated from their cytotoxicities and micromorphologic properties. Cultured cells were exposed to Al2 O3 NPs (13 nm diameter) and Al2 O3 NWs (2-6 × 200-400 nm). Cytotoxicity and genotoxicity were examined by immunostaining with fluorescence microscopy, and nanomaterial localization was studied by using scanning electron microscopy and transmission electron microscopy. The NPs were cytotoxic and genotoxic, whereas the NWs were not. The scanning electron microscopy images showed that the NPs aggregate more on the cell surface than do the NWs. The transmission electron microscopy images showed that the NPs were internalized into the vesicle and nuclei, for both cell types. In contrast, numerous solid NWs were observed as large aggregates in vesicles, but not in nuclei. Nuclear damage was confirmed by measuring cell viability and by immunostaining for NPs. The chemical changes induced by the NPs in the vesicles or cells may cause cell damage because of their large surface area per volume. The extent of NW entrapment was not sufficient to lower the viability of either cell type.

Kinetics and mechanism of electron transfer reaction of single and double chain surfactant cobalt(III) complex by Fe2+ ions in liposome (dipalmitoylphosphotidylcholine) vesicles: effects of phase transition

In this study, we report the kinetics of reduction reactions of single and double chain surfactant cobalt(III) complexes of octahedral geometry, cis-[Co(en)2(4AMP)(DA)](ClO4)3 and cis-[Co(dmp)2(C12H25NH2)2](ClO4)3 (en = ethylenediamine, dmp = 1,3-diaminopropane, 4AMP = 4-aminopropane, C12H25NH2 = dodecylamine) by Fe2+ ion in dipalmitoylphosphotidylcholine (DPPC) vesicles at different temperatures under pseudo first-order conditions. The kinetics of these reactions is followed by spectrophotometry method. The reactions are found to be second order and the electron transfer is postulated as outer sphere. The remarkable findings in the present investigation are that, below the phase transition temperature of DPPC, the rate decreases with an increase in the concentration of DPPC, while above the phase transition temperature the rate increases with an increase in the concentration of DPPC. The main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction between vesicles surface and hydrophobic part of the surfactant complexes. Besides, comparing the values of rate constants of these outer-sphere electron transfer reactions in the absence and in the presence of DPPC, the rate constant values in the presence of DPPC are always found to be greater than in the absence of DPPC. This is ascribed to the double hydrophobic fatty acid chain in the DPPC that gives the molecule an overall tubular shape due to the intervesicular hydrophobic interaction between vesicles surface and hydrophobic part of the surfactant complexes more suitable for vesicle aggregation which facilitates lower activation energy, and consequently higher rate is observed in the presence of DPPC. The activation parameters (ΔS# and ΔH#) of the reactions at different temperatures have been calculated which corroborate the kinetics of the reaction.

Dissecting molecular aspects of cell interactions using glycodendrimersomes with programmable glycan presentation and engineered human lectins.

Glycodendrimersomes with programmable surface display of glycan, together with artificially engineered galectins, were used to understand the physiological significance of human lectins with homodimeric and tandem-repeat-type displays. The mode of topological surface presentation and the density of glycan affected vesicle aggregation mediated by multivalent carbohydrate-protein interactions. The cross-linking capacity of homodimeric lectins was enhanced by covalent connection of the two carbohydrate-binding sites. These findings highlight the value of glycodendrimersomes as versatile cell membrane mimetics, and assays provide diagnostic tools for protein functionality. This work also provides guidelines for the design of cell separators, bioactive matrices, bioeffectors, and other biomedical applications.

The GTPase Rab26 links synaptic vesicles to the autophagy pathway.

Small GTPases of the Rab family not only regulate target recognition in membrane traffic but also control other cellular functions such as cytoskeletal transport and autophagy. Here we show that Rab26 is specifically associated with clusters of synaptic vesicles in neurites. Overexpression of active but not of GDP-preferring Rab26 enhances vesicle clustering, which is particularly conspicuous for the EGFP-tagged variant, resulting in a massive accumulation of synaptic vesicles in neuronal somata without altering the distribution of other organelles. Both endogenous and induced clusters co-localize with autophagy-related proteins such as Atg16L1, LC3B and Rab33B but not with other organelles. Furthermore, Atg16L1 appears to be a direct effector of Rab26 and binds Rab26 in its GTP-bound form, albeit only with low affinity. We propose that Rab26 selectively directs synaptic and secretory vesicles into preautophagosomal structures, suggesting the presence of a novel pathway for degradation of synaptic vesicles.

Functional analysis of optineurin and some of its disease-associated mutants.

Optineurin is a multifunctional protein involved in a variety of cellular functions such as protein trafficking by vesicles, autophagy, and signal transduction. Certain mutations in optineurin (gene OPTN) are associated with neurodegenerative diseases like glaucoma and amyotrophic lateral sclerosis (ALS). Optineurin is also seen in pathological structures present in several other neurodegenerative diseases. In glaucoma, loss of vision occurs due to progressive degeneration of retinal ganglion cells, and perhaps loss of photoreceptor cone cells as well. Most of the glaucoma-associated mutations of optineurin are heterozygous missense mutations, whereas the ALS-associated mutations include deletion, truncation, and missense mutations. Optineurin mediates its functions by interacting with various proteins, often acting as an adaptor to provide a link between two or more proteins. Disease-causing mutations alter these interactions leading to functional defects in membrane vesicle trafficking, autophagy, signaling, aggregate formation, and other processes. Some of these functional defects, caused by glaucoma-associated mutants of optineurin, led to retinal cell death mediated by apoptosis and therefore may contribute to pathogenesis directly. Other mutations are likely to cause glaucoma by indirect mechanisms involving other cell types. Mechanisms of ALS pathogenesis by optineurin mutations are yet to be investigated in detail; however, some ALS-associated mutants cause defects in signaling, autophagy, and ubiquitin binding, which might contribute to pathogenesis.

Histones Cause Aggregation and Fusion of Lipid Vesicles Containing Phosphatidylinositol-4-Phosphate

In a previous article, we demonstrated that histones (H1 or histone octamers) interact with negatively charged bilayers and induce extensive aggregation of vesicles containing phosphatidylinositol-4-phosphate (PIP) and, to a lesser extent, vesicles containing phosphatidylinositol (PI). Here, we found that vesicles containing PIP, but not those containing PI, can undergo fusion induced by histones. Fusion was demonstrated through the observation of intervesicular mixing of total lipids and inner monolayer lipids, and by ultrastructural and confocal microscopy studies. Moreover, in both PI- and PIP-containing vesicles, histones caused permeabilization and release of vesicular aqueous contents, but the leakage mechanism was different (all-or-none for PI and graded release for PIP vesicles). These results indicate that histones could play a role in the remodeling of the nuclear envelope that takes place during the mitotic cycle.

Supramolecular nanoassemblies of an amphiphilic porphyrin-cyclodextrin conjugate and their morphological transition from vesicle to network.

An amphiphilic compound, 5-(4'-dodecyloxyphenyl)-10,15,20-tri(permethyl-β-CD)-modified Zn(II)-porphyrin (1; β-CD = β-cyclodextrin), was synthesized by means of the click reaction of an alkylated Zn-porphyrin derivative with 6-deoxy-6-azidopermethyl-β-CD. The complexation between 1 and tetrasodium tetraphenylporphyrintetrasulfonate (5) with different molar ratios led to the formation of two distinctly different nanoarchitectures, which were proven to be vesicle and network aggregates, respectively, by using dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. On the basis of the results of the time-dependent TEM studies, fluorescence, and NMR spectroscopic measurements, we have determined that the mechanism of the morphology transition from vesicles to networks is controlled by the stepwise complexation of 1 with 5. Furthermore, these supramolecular nanoarchitectures show the controlled- release property of doxorubicin as potential candidates for drug delivery.

Sucrose induces vesicle accumulation and autophagy.

It has been shown that the treatment of mammalian cells with sucrose leads to vacuole accumulation associated with lysosomes and upregulation of lysosomal enzyme expression and activity. Autophagy is an evolutionarily conserved homeostatic process by which cells deliver cytoplasmic material for degradation into lysosomes, thus it is probable that sucrose affects the autophagic activity. The role of sucrose in autophagy is unknown; however, another disaccharide, trehalose has been shown to induce autophagy. In the current study, we used mouse embryonic fibroblasts to investigate whether sucrose induces autophagy and whether vesicle formation is associated with autophagy. The results showed that sucrose induces autophagy while being accumulated within the endosomes/lysosomes. These vesicles were swollen and packed within the cytoplasm. Furthermore, trehalose and the trisaccharide raffinose, which are not hydrolyzed in mammalian cells, increased the rate of vesicles accumulation and LC3-II level (a protein marker of autophagy). However, fructose and maltose did not show the same effects. The correlation between the two processes, vesicle accumulation and autophagy induction, was confirmed by treatment of cells with sucrose plus invertase, or maltose plus acarbose-the α-glucosidase inhibitor-and by sucrose deprivation. Results also showed that vesicle accumulation was not affected by autophagy inhibition. Therefore, the data suggest that sucrose-induced autophagy through accumulation of sucrose-containing vesicles is caused by the absence of hydrolysis enzymes.

Chronic desipramine prevents acute stress-induced reorganization of medial prefrontal cortex architecture by blocking glutamate vesicle accumulation and excitatory synapse increase.

Background:Although a clear negative influence of chronic exposure to stressful experiences has been repeatedly demonstrated, the outcome of acute stress on key brain regions has only just started to be elucidated. Although it has been proposed that acute stress may produce enhancement of brain plasticity and that antidepressants may prevent such changes, we still lack ultrastructural evidence that acute stress-induced changes in neurotransmitter physiology are coupled with structural synaptic modifications.Methods:Rats were pretreated chronically (14 days) with desipramine (10mg/kg) and then subjected to acute foot-shock stress. By means of serial section electron microscopy, the structural remodeling of medial prefrontal cortex glutamate synapses was assessed soon after acute stressor cessation and stress hormone levels were measured.Results:Foot-shock stress induced a remarkable increase in the number of docked vesicles and small excitatory synapses, partially and strongly prevented by desipramine pretreatment, respectively. Acute stress-induced corticosterone elevation was not affected by drug treatment.Conclusions:Since desipramine pretreatment prevented the stress-induced structural plasticity but not the hormone level increase, we hypothesize that the preventing action of desipramine is located on pathways downstream of this process and/or other pathways. Moreover, because enhancement of glutamate system remodeling may contribute to overexcitation dysfunctions, this aspect could represent a crucial component in the pathophysiology of stress-related disorders.

Testing the vesicular morphology to destruction: birth and death of diblock copolymer vesicles prepared via polymerization-induced self-assembly.

Small angle X-ray scattering (SAXS), electrospray ionization charge detection mass spectrometry (CD-MS), dynamic light scattering (DLS), and transmission electron microscopy (TEM) are used to characterize poly(glycerol monomethacrylate)55-poly(2-hydroxypropyl methacrylate)x (G55-Hx) vesicles prepared by polymerization-induced self-assembly (PISA) using a reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization formulation. A G55 chain transfer agent is utilized to prepare a series of G55-Hx diblock copolymers, where the mean degree of polymerization (DP) of the membrane-forming block (x) is varied from 200 to 2000. TEM confirms that vesicles with progressively thicker membranes are produced for x = 200–1000, while SAXS indicates a gradual reduction in mean aggregation number for higher x values, which is consistent with CD-MS studies. Both DLS and SAXS studies indicate minimal change in the overall vesicle diameter between x = 400 and 800. Fitting SAXS patterns to a vesicle model enables calculation of the membrane thickness, degree of hydration of the membrane, and the mean vesicle aggregation number. The membrane thickness increases at higher x values, hence the vesicle lumen must become smaller if the external vesicle dimensions remain constant. Geometric considerations indicate that this growth mechanism lowers the total vesicle interfacial area and hence reduces the free energy of the system. However, it also inevitably leads to gradual ingress of the encapsulated water molecules into the vesicle membrane, as confirmed by SAXS analysis. Ultimately, the highly plasticized membranes become insufficiently hydrophobic to stabilize the vesicle morphology when x exceeds 1000, thus this PISA growth mechanism ultimately leads to vesicle “death”.

Investigation on the phase behaviors of aqueous surfactant two-phase systems in a mixture of N-dodecyl-N-methylpiperidinium bromide (C12MDB) and sodium dodecyl sulfate (SDS)

Abstract To study the phase behavior and formation mechanism of an aqueous surfactant two-phase system (ASTP), which was composed of a cationic surfactant N-dodecyl-N-methylpiperidinium bromide (C12MDB) and an anionic surfactant sodium dodecyl sulfate (SDS), an investigation was conducted. The phase diagram for C12MDB/SDS aqueous solutions was mapped, indicating that the ASTP forms between the homogeneous and heterogeneous phase regions in a narrow concentration range. By measurements of surface tension, transmission electron microscopy, dynamic light scattering, and zeta potential, the formation mechanism of the ASTP was proposed, which is attributed to the aggregation of vesicles. Different aggregation behaviors of vesicles lead to phase separation and the formation of the ASTP.

RhoA inhibition by C3 prevents dopaminergic neuronal degeneration in rotenone induced neurotoxicity

Parkinson disease (PD) is a complex neurodegenerative disorder characterized by a progressive loss of dopaminergic neurons. Mitochondrial dysfunction, oxidative stress or protein misfolding may underlie this process. Rotenone, a natural substance widely used as a pesticide, produces selective degeneration of dopaminergic neurons. Rotenone is a membrane-permeable compound that has two known molecular targets in the cell: it inhibits complex I in the mitochondrial respiratory chain and depolymerizes microtubules. Microtubule depolymerization would disrupt vesicular transport and cause the accumulation of vesicles in the soma. RhoA, a member of the Rho GTP-binding proteins, activates several effector proteins which target actin, actomyosin and microtubules. Here, we use an in vitro rotenone-induced PD mouse model to verify the neuroprotective effects of C3, an exoenzyme produced by Clostridium botulinum that inactivates RhoA by ADP-ribosylation. Mesencephalic cell cultures from CD1 mice were performed, at D.I.V. 6 the cells were treated with or without 2.5 μg/ml of C3; 4 hours later 25 nM of Rotenone was added. 24 hours later [3H] dopamine uptake and immunocytochemistry with anti-TH and anti-RhoA antibodies were performed. In TH+ neurons Rotenone reduced dopamina uptake to 45% and activated RhoA that was found at level of the cell membrane. Pre-treatment with C3 was able to protect mesencephalic cells from rotenone toxicity to about 40% preventing the traslocation of RhoA from the cytoplasm to the plasma membrane as observed by confocal analysis of immunostained cells. Our data indicate that the inhibition of RhoA by C3 is able to partially rescue the rotenone induced damage by probably preventing microtubules depolymerization.

How Rab proteins determine Golgi structure.

Rab proteins, small GTPases, are key regulators of mammalian Golgi apparatus organization. Based on the effect of Rab activation state, Rab proteins fall into two functional classes. In Class1, inactivation induces Golgi ribbon fragmentation and/or redistribution of Golgi enzymes to the Endoplasmic Reticulum, while overexpression of wild type or activation has little, if any, effect on Golgi ribbon organization. In Class 2, the reverse is true. We give emphasis to Rab6, the most abundant Golgi-associated Rab protein. Rab6 depletion in HeLa cells causes an increase in Golgi cisternal number, longer, more continuous cisternae, and a pronounced accumulation of vesicles; the effect of Rab6 on Golgi ribbon organization is probably through regulation of vesicle transport. In effector studies, motor proteins and their regulators are found to be key Rab6 effectors. A related Rab, Rab41, affects Golgi ribbon organization in a contrasting manner. The balance between minus- and plus-end directed motor recruitment may well be the major Rab-dependent factor in Golgi ribbon organization.