Molecular motors and a spectrin matrix associate with Golgi membranes in vitro

Small Sized Yet Powerful: Nuclear Distribution C Proteins in Plants

The family of Nuclear Distribution C (NudC) proteins plays a pivotal and evolutionarily conserved role in all eukaryotes. In animal systems, these proteins influence vital cellular processes like cell division, protein folding, nuclear migration and positioning, intracellular transport, and stress response. This review synthesizes past and current research on NudC family members, focusing on their growing importance in plants and intricate contributions to plant growth, development, and stress tolerance. Leveraging information from available genomic databases, we conducted a thorough characterization of NudC family members, utilizing phylogenetic analysis and assessing gene structure, motif organization, and conserved protein domains. Our spotlight on two Arabidopsis NudC genes, BOB1 and NMig1, underscores their indispensable roles in embryogenesis and postembryonic development, stress responses, and tolerance mechanisms. Emphasizing the chaperone activity of plant NudC family members, crucial for mitigating stress effects and enhancing plant resilience, we highlight their potential as valuable targets for enhancing crop performance. Moreover, the structural and functional conservation of NudC proteins across species suggests their potential applications in medical research, particularly in functions related to cell division, microtubule regulation, and associated pathways. Finally, we outline future research avenues centering on the exploration of under investigated functions of NudC proteins in plants.

Fluorescence microscopy applied to intracellular transport by microtubule motors

Long-distance transport of many organelles inside eukaryotic cells is driven by the dynein and kinesin motors on microtubule filaments. More than 30 years since the discovery of these motors, unanswered questions include motor- organelle selectivity, structural determinants of processivity, collective behaviour of motors and track selection within the complex cytoskeletal architecture, to name a few. Fluorescence microscopy has been invaluable in addressing some of these questions. Here we present a review of some efforts to understand these sub-microscopic machines using fluorescence.

Regulation of intracellular trafficking and secretion of adiponectin by myosin II

Adiponectin is a protein secreted by white adipocytes that plays an important role in insulin action, energy homeostasis and the development of atherosclerosis. The intracellular localization and trafficking of GLUT4 and leptin in adipocytes has been well studied, but little is known regarding the intracellular trafficking of adiponectin. Recent studies have demonstrated that constitutive adiponectin secretion is dependent on PIP2 levels and the integrity of cortical F-actin. Non-muscle myosin II is an actin-based motor that is associated with membrane vesicles and participates in vesicular trafficking in mammalian cells. Therefore, we investigated the role of myosin II in the trafficking and secretion of adiponectin in 3T3-L1 adipocytes. Confocal microscopy revealed that myosin IIA and IIB were dispersed throughout the cytoplasm of the adipocyte. Both myosin isoforms were localized in the Golgi/TGN region as evidenced by colocalization with the cis-Golgi marker, p115 and the trans-Golgi marker, γ-adaptin. Inhibition of myosin II activity by blebbistatin or actin depolymerization by latrunculin B dispersed myosin IIA and IIB towards the periphery while significantly inhibiting adiponectin secretion. Therefore, the constitutive trafficking and secretion of adiponectin in 3T3-L1 adipocytes occurs by an actin-dependent mechanism that involves the actin-based motors, myosin IIA and IIB.

Iterative sorting of apical and basolateral cargo in Madin-Darby canine kidney cells

A novel assay quantitatively distinguishes different cargo pairs by their degree of colocalization at the TGN and the evolution of colocalization during their TGN-to-surface transport. Apical NTRp75 and basolateral VSVG in MDCK cells undergo continuous sorting between TGN exit and surface arrival.

For several decades, the trans-Golgi network (TGN) was considered the most distal stop and hence the ultimate protein-sorting station for distinct apical and basolateral transport carriers that reach their respective surface domains in the direct trafficking pathway. However, recent reports of apical and basolateral cargoes traversing post-Golgi compartments accessible to endocytic ligands before their arrival at the cell surface and the post-TGN breakup of large pleomorphic membrane fragments that exit the Golgi region toward the surface raised the possibility that compartments distal to the TGN mediate or contribute to biosynthetic sorting. Here we describe the development of a novel assay that quantitatively distinguishes different cargo pairs by their degree of colocalization at the TGN and by the evolution of colocalization during their TGN-to-surface transport. Keys to the high resolution of our approach are 1) conversion of perinuclear organelle clustering into a two-dimensional microsomal spread and 2) identification of TGN and post-TGN cargo without the need for a TGN marker that universally cosegregates with all cargo. Using our assay, we provide the first evidence that apical NTRp75 and basolateral VSVG in Madin–Darby canine kidney cells still undergo progressive sorting after they exit the TGN toward the cell surface.

Isoforms of Spectrin and Ankyrin Reflect the Functional Topography of the Mouse Kidney

The kidney displays specialized regions devoted to filtration, selective reabsorption, and electrolyte and metabolite trafficking. The polarized membrane pumps, channels, and transporters responsible for these functions have been exhaustively studied. Less examined are the contributions of spectrin and its adapter ankyrin to this exquisite functional topography, despite their established contributions in other tissues to cellular organization. We have examined in the rodent kidney the expression and distribution of all spectrins and ankyrins by qPCR, Western blotting, immunofluorescent and immuno electron microscopy. Four of the seven spectrins (αΙΙ, βΙ, βΙΙ, and βΙΙΙ) are expressed in the kidney, as are two of the three ankyrins (G and B). The levels and distribution of these proteins vary widely over the nephron. αΙΙ/βΙΙ is the most abundant spectrin, found in glomerular endothelial cells; on the basolateral membrane and cytoplasmic vesicles in proximal tubule cells and in the thick ascending loop of Henle; and less so in the distal nephron. βΙΙΙ spectrin largely replaces βΙΙ spectrin in podocytes, Bowman’s capsule, and throughout the distal tubule and collecting ducts. βΙ spectrin is only marginally expressed; its low abundance hinders a reliable determination of its distribution. Ankyrin G is the most abundant ankyrin, found in capillary endothelial cells and all tubular segments. Ankyrin B populates Bowman’s capsule, podocytes, the ascending thick loop of Henle, and the distal convoluted tubule. Comparison to the distribution of renal protein 4.1 isoforms and various membrane proteins indicates a complex relationship between the spectrin scaffold, its adapters, and various membrane proteins. While some proteins (e.g. ankyrin B, βΙΙΙ spectrin, and aquaporin 2) tend to share a similar distribution, there is no simple mapping of different spectrins or ankyrins to most membrane proteins. The implications of this data are discussed.

Gene Electrotransfer: A Mechanistic Perspective

Gene electrotransfer is a powerful method of DNA delivery offering several medical applications, among the most promising of which are DNA vaccination and gene therapy for cancer treatment. Electroporation entails the application of electric fields to cells which then experience a local and transient change of membrane permeability. Although gene electrotransfer has been extensively studied in in vitro and in vivo environments, the mechanisms by which DNA enters and navigates through cells are not fully understood. Here we present a comprehensive review of the body of knowledge concerning gene electrotransfer that has been accumulated over the last three decades. For that purpose, after briefly reviewing the medical applications that gene electrotransfer can provide, we outline membrane electropermeabilization, a key process for the delivery of DNA and smaller molecules. Since gene electrotransfer is a multipart process, we proceed our review in describing step by step our current understanding, with particular emphasis on DNA internalization and intracellular trafficking. Finally, we turn our attention to in vivo testing and methodology for gene electrotransfer.

Axonal transport and neurodegenerative disease: vesicle-motor complex formation and their regulation

The process of axonal transport serves to move components over very long distances on microtubule tracks in order to maintain neuronal viability. Molecular motors - kinesin and dynein - are essential for the movement of neuronal cargoes along these tracks; defects in this pathway have been implicated in the initiation or progression of some neurodegenerative diseases, suggesting that this process may be a key contributor in neuronal dysfunction. Recent work has led to the identification of some of the motor-cargo complexes, adaptor proteins, and their regulatory elements in the context of disease proteins. In this review, we focus on the assembly of the amyloid precursor protein, huntingtin, mitochondria, and the RNA-motor complexes and discuss how these may be regulated during long-distance transport in the context of neurodegenerative disease. As knowledge of these motor-cargo complexes and their involvement in axonal transport expands, insight into how defects in this pathway contribute to the development of neurodegenerative diseases becomes evident. Therefore, a better understanding of how this pathway normally functions has important implications for early diagnosis and treatment of diseases before the onset of disease pathology or behavior.

Protein partners of dynamin-1 in the retina

Dynamin proteins are involved in vesicle generation, providing mechanical force to excise newly formed vesicles from membranes of cellular compartments. In the brain, dynamin-1, dynamin-2, and dynamin-3 have been well studied; however, their function in the retina remains elusive. A retina-specific splice variant of dynamin-1 interacts with the photoreceptor-specific protein Tubby-like protein 1 (Tulp1), which when mutated causes an early onset form of autosomal recessive retinitis pigmentosa. Here, we investigated the role of the dynamins in the retina, using immunohistochemistry to localize dynamin-1, dynamin-2, and dynamin-3 and immunoprecipitation followed by mass spectrometry to explore dynamin-1 interacting proteins in mouse retina. Dynamin-2 is primarily confined to the inner segment compartment of photoreceptors, suggesting a role in outer segment protein transport. Dynamin-3 is present in the terminals of photoreceptors and dendrites of second-order neurons but is most pronounced in the inner plexiform layer where second-order neurons relay signals from photoreceptors. Dynamin-1 appears to be the dominant isoform in the retina and is present throughout the retina and in multiple compartments of the photoreceptor cell. This suggests that it may function in multiple cellular pathways. Surprisingly, dynamin-1 expression and localization did not appear to be disrupted in tulp1−/− mice. Immunoprecipitation experiments reveal that dynamin-1 associates primarily with proteins involved in cytoskeletal-based membrane dynamics. This finding is confirmed by western blot analysis. Results further implicate dynamin-1 in vesicular protein transport processes relevant to synaptic and post-Golgi pathways and indicate a possible role in photoreceptor stability.

Cdc42 regulates microtubule-dependent Golgi positioning

The molecular mechanisms underlying cytoskeleton-dependent Golgi positioning are poorly understood. In mammalian cells, the Golgi apparatus is localized near the juxtanuclear centrosome via dynein-mediated motility along microtubules. Previous studies implicate Cdc42 in regulating dynein-dependent motility. Here we show that reduced expression of the Cdc42-specific GTPase-activating protein, ARHGAP21, inhibits the ability of dispersed Golgi membranes to reposition at the centrosome following nocodazole treatment and washout. Cdc42 regulation of Golgi positioning appears to involve ARF1 and a binding interaction with the vesicle-coat protein coatomer. We tested whether Cdc42 directly affects motility, as opposed to the formation of a trafficking intermediate, using a Golgi capture and motility assay in permeabilized cells. Disrupting Cdc42 activation or the coatomer/Cdc42 binding interaction stimulated Golgi motility. The coatomer/Cdc42-sensitive motility was blocked by the addition of an inhibitory dynein antibody. Together, our results reveal that dynein and microtubule-dependent Golgi positioning is regulated by ARF1-, coatomer-, and ARHGAP21-dependent Cdc42 signaling.

Targeted deletion of betaIII spectrin impairs synaptogenesis and generates ataxic and seizure phenotypes

The spectrin membrane skeleton controls the disposition of selected membrane channels, receptors, and transporters. In the brain betaIII spectrin binds directly to the excitatory amino acid transporter (EAAT4), the glutamate receptor delta, and other proteins. Mutations in betaIII spectrin link strongly to human spinocerebellar ataxia type 5 (SCA5), correlating with alterations in EAAT4. We have explored the mechanistic basis of this phenotype by targeted gene disruption of Spnb3. Mice lacking intact betaIII spectrin develop normally. By 6 months they display a mild nonprogressive ataxia. By 1 year most Spnb3(-/-) animals develop a myoclonic seizure disorder with significant reductions of EAAT4, EAAT1, GluRdelta, IP3R, and NCAM140. Other synaptic proteins are normal. The cerebellum displays increased dark Purkinje cells (PC), a thin molecular layer, fewer synapses, a loss of dendritic spines, and a 2-fold expansion of the PC dendrite diameter. Membrane and expanded Golgi profiles fill the PC dendrite and soma, and both regions accumulate EAAT4. Correlating with the seizure disorder are enhanced hippocampal levels of neuropeptide Y and EAAT3 and increased calpain proteolysis of alphaII spectrin. It appears that betaIII spectrin disruption impairs synaptogenesis by disturbing the intracellular pathways selectively regulating protein trafficking to the synapse. The mislocalization of these proteins secondarily disrupts glutamate transport dynamics, leading to seizures, neuronal damage, and compensatory changes in EAAT3 and neuropeptide Y.

The mammalian NudC-like genes: a family with functions other than regulating nuclear distribution

Nuclear distribution gene C homolog (NudC) is a highly conserved gene. It has been identified in different species from fungi to mammals. The high degree of conservation, in special in the nudC domain, suggests that they are genes with essential functions. Most of the identified genes in the family have been implicated in cell division through the regulation of cytoplasmic dynein. As for mammalian genes, human NUDC has been implicated in the migration and proliferation of tumor cells and has therefore been considered a possible therapeutic target. There is evidence suggesting that mammalian NudC is also implicated in the regulation of the inflammatory response and in thrombopoiesis. The presence of these other functions not related to the interaction with molecular motors agrees with that these genes and their products are larger in size than their microbial orthologous, indicating that they have evolved to convey additional features.

The unique architecture of Bunyamwera virus factories around the Golgi complex

Viral factories are novel structures built by viruses in infected cells. During their construction organelles are recruited and build a large scaffold for viral replication and morphogenesis. We have studied how a bunyavirus uses the Golgi to build the factory. With the help of confocal and 3D ultrastructural imaging together with molecular mapping in situ and in vitro we have characterized a tubular structure that harbours the viral replication complexes in a globular domain. Numerous ribonucleoproteins were released from purified tubes disrupted in vitro. Actin and myosin I were identified by peptide mass fingerprinting in isolated tubes while actin and the viral NSm non-structural protein were detected in the tubes' internal proteinaceous scaffold by immunogold labelling. Studies with NSm deletion mutants and drugs affecting actin showed that both NSm and actin are key factors for tube and virus assembly in Golgi. Three-dimensional reconstructions based on oriented serial sections of infected cells showed that tubes anchor cell organelles to Golgi stacks and make contacts with intracellular viruses. We propose that this new structure, unique among enveloped viruses, assembles in association with the most stable component of Golgi stacks, the actin-containing matrix scaffold, connecting viral replication and morphogenesis inside viral factories.

Ankyrin facilitates intracellular trafficking of alpha1-Na+-K+-ATPase in polarized cells

Defects in ankyrin underlie many hereditary disorders involving the mislocalization of membrane proteins. Such phenotypes are usually attributed to ankyrin's role in stabilizing a plasma membrane scaffold, but this assumption may not be accurate. We found in Madin-Darby canine kidney cells and in other cultured cells that the 25-residue ankyrin-binding sequence of alpha(1)-Na(+)-K(+)-ATPase facilitates the entry of alpha(1),beta(1)-Na(+)-K(+)-ATPase into the secretory pathway and that replacement of the cytoplasmic domain of vesicular stomatitis virus G protein (VSV-G) with this ankyrin-binding sequence bestows ankyrin dependency on the endoplasmic reticulum (ER) to Golgi trafficking of VSV-G. Expression of the ankyrin-binding sequence of alpha(1)-Na(+)-K(+)-ATPase alone as a soluble cytosolic peptide acts in trans to selectively block ER to Golgi transport of both wild-type alpha(1)-Na(+)-K(+)-ATPase and a VSV-G fusion protein that includes the ankyrin-binding sequence, whereas the trafficking of other proteins remains unaffected. Similar phenotypes are also generated by small hairpin RNA-mediated knockdown of ankyrin R or the depletion of ankyrin in semipermeabilized cells. These data indicate that the adapter protein ankyrin acts not only at the plasma membrane but also early in the secretory pathway to facilitate the intracellular trafficking of alpha(1)-Na(+)-K(+)-ATPase and presumably other selected proteins. This novel ankyrin-dependent assembly pathway suggests a mechanism whereby hereditary disorders of ankyrin may be manifested as diseases of membrane protein ER retention or mislocalization.

In vitro reassembly of nuclear envelopes and organelles in Xenopus egg extracts

We reconstituted bilayer nuclear membranes, multilayer membranes, and organelles from mixtures of Xenopus laevis egg extracts and demembranated Xenopus sperm nuclei. Varying proportions of the cytosolic and vesicular fractions from the eggs were used in the reconstitution mixtures. A cytosol:vesicle ratio of 10:1 promoted reassembly of the normal bilayer nuclear membrane with inserted nuclear pore complexes around the decondensed Xenopus sperm chromatin. A cytosol:vesicle ratio of 5:1 caused decondensed and dispersed sperm chromatin to be either surrounded by or divided by unusual multilayer membrane structures with inlaid pore complexes. A cytosol:vesicle ratio of 2.5:1 promoted reconstitution of mitochondria, endoplasmic reticulum networks, and Golgi apparatus. During reassembly of the endoplasmic reticulum and Golgi apparatus, vesicular fragments of the corresponding organelles fused together and changed their shape to form flattened cisternae, which were then stacked one on top of another.

Critical interactions between TGF-beta signaling/ELF, and E-cadherin/beta-catenin mediated tumor suppression

Inactivation of the transforming growth factor-beta (TGF-beta) pathway occurs often in malignancies of the gastrointestinal (GI) system. However, only a fraction of sporadic GI tumors exhibit inactivating mutations in early stages of cancer formation, suggesting that other mechanisms play a critical role in the inactivation of this pathway. Here, we show a wide range of GI tumors, including those of the stomach, liver and colon in elf+/- and elf+/- / Smad4+/- mutant mice. We found that embryonic liver fodrin (ELF), a beta-Spectrin originally identified in endodermal stem/progenitor cells committed to foregut lineage, possesses potent antioncogenic activity and is frequently inactivated in GI cancers. Specifically, E-cadherin accumulation at cell-cell contacts and E-cadherin-beta-catenin-dependent epithelial cell-cell adhesion is disrupted in elf+/- / Smad4+/- mutant gastric epithelial cells, and could be rescued by ectopic expression of full-length elf, but not Smad3 or Smad4. Subcellular fractionation revealed that E-cadherin is expressed mainly at the cell membrane after TGF-beta stimulation. In contrast, elf+/- / Smad4+/- mutant tissues showed abnormal distribution of E-cadherin that could be rescued by overexpression of ELF but not Smad3 or Smad4. Our results identify a group of common lethal malignancies in which inactivation of TGF-beta signaling, which is essential for tumor suppression, is disrupted by inactivation of the ELF adaptor protein.

Cytoskeletal Transport in the Aging Brain: Focus on the Cholinergic System

There is now compelling evidence for the aging-related breakdown of cytoskeletal support in neurons. Similarly affected are the principal components of the intracellular microtubule system, the transport units involved in active shuttle of organelles and molecules in an antero- and retrograde manner, and the proteins stabilizing the cytoskeleton and providing trophic support. Here, we review the basic organization of the cytoskeleton, and describe its elements and their interactions. We then critically assess the role of these cytoskeletal proteins in physiological aging and aging-related malfunction. Our focus is on the microtubule-associated protein tau, for which comprehensive investigations suggest a critical role in neurodegenerative diseases, for instance tauopathies. These diseases frequently lead to cognitive decline and are often paralleled by reductions in cholinergic neurotransmission. We propose this reduction to be due to destabilization of the cytoskeleton and protein transport mechanisms in these neurons. Therefore, maintenance of the neuronal cytoskeleton during aging may prevent or delay neurodegeneration as well as cognitive decline during physiological aging.

Inactivation of ELF/TGF-beta signaling in human gastrointestinal cancer

TGF-beta/Smads regulate a wide variety of biological responses through transcriptional regulation of target genes. ELF, a beta-spectrin, plays a key role in the transmission of TGF-beta-mediated transcriptional response through Smads. ELF was originally identified as a key protein involved in endodermal stem/progenitor cells committed to foregut lineage. Also, as a major dynamic adaptor and scaffolding protein, ELF is important for the generation of functionally distinct membranes, protein sorting and the development of polarized differentiated epithelial cells. Disruption of elf results in the loss of Smad3/Smad4 activation and, therefore, a disruption of the TGF-beta pathway. These observations led us to pursue the function of ELF in gastrointestinal (GI) epithelial cell-cell adhesion and tumor suppression. Here, we show a significant loss of ELF and reduced Smad4 expression in human gastric cancer tissue samples. Also, of the six human gastric cancer cell lines examined, three show deficient ELF expression. Furthermore, we demonstrate the rescue of E-cadherin-dependent homophilic cell-cell adhesion by ectopic expression of full-length elf. Our results suggest that ELF has an essential role in tumor suppression in GI cancers.

Rab Proteins, Connecting Transport and Vesicle Fusion

Small GTPases of the Rab family control timing of vesicle fusion. Fusion of two vesicles can only occur when they have been brought into close contact. Transport by microtubule- or actin-based motor proteins will facilitate this process in vivo. Ideally, transport and vesicle fusion are linked activities. Active, GTP-bound Rab proteins dock on specific compartments and are therefore perfect candidates to control transport of the different compartments. Recently, a number of Rab proteins were identified that control motor protein recruitment to their specific target membranes. By cycling through inactive and active states, Rab proteins are able to control motor protein-mediated transport and subsequent fusion of intracellular structures in both spatial and timed manners.

Characterization of myosin-II binding to Golgi stacks in vitro

In addition to important roles near the actin-rich cell cortex, ample evidence indicates that multiple myosins are also involved in membrane movements in the endomembrane system. Nonmuscle myosin-II has been shown to have roles in anterograde and retrograde trafficking at the Golgi. Myosin-II is present on Golgi stacks isolated from intestinal epithelial cells and has been localized to the Golgi in several polarized and unpolarized cell lines. An understanding of roles of myosin-II in Golgi physiology will be facilitated by understanding the molecular arrangement of myosin-II at the Golgi. Salt-washing removes endogenous myosin-II from isolated Golgi and purified brush border myosin-II can bind in vitro. Brush border myosin-II binds to a tightly bound Golgi peripheral membrane protein with a K(1/2) of 75 nM and binding is saturated at 0.7 pmol myosin/microg Golgi. Binding studies using papain cleavage fragments of brush border myosin-II show that the 120-kDa rod domain, but not the head domain, of myosin heavy chain can bind directly to Golgi stacks. The 120-kDa domain does not bind to Golgi membranes when phosphorylated in vitro with casein kinase-II. These results suggest that phosphorylation in the rod domain may regulate the binding and/or release of myosin-II from the Golgi. These data support a model in which myosin-II is tethered to the Golgi membrane by its tail and actin filaments by its head. Thus, translocation along actin filaments may extend Golgi membrane tubules and/or vesicles away from the Golgi complex.

Large scale protein identification in intracellular aquaporin-2 vesicles from renal inner medullary collecting duct

Vasopressin acts on renal collecting duct cells to stimulate translocation of aquaporin-2 (AQP2)-containing membrane vesicles from throughout the cytoplasm to the apical region. The vesicles fuse with the plasma membrane to increase water permeability. To identify the intracellular membrane compartments that contain AQP2, we carried out LC-MS/MS-based proteomic analysis of immunoisolated AQP2-containing intracellular vesicles from rat inner medullary collecting duct. Immunogold electron microscopy and immunoblotting confirmed heavy AQP2 labeling of immunoisolated vesicles. Vesicle proteins were separated by SDS-PAGE followed by in-gel trypsin digestion in consecutive gel slices and identification by LC-MS/MS. Identification of Rab GTPases 4, 5, 18, and 21 (associated with early endosomes); Rab7 (late endosomes); and Rab11 and Rab25 (recycling endosomes) indicate that a substantial fraction of intracellular AQP2 is present in endosomal compartments. In addition, several endosome-associated SNARE proteins were identified including syntaxin-7, syntaxin-12, syntaxin-13, Vti1a, vesicle-associated membrane protein 2, and vesicle-associated membrane protein 3. Rab3 was not found, however, either by mass spectrometry or immunoblotting, suggesting a relative lack of AQP2 in secretory vesicles. Additionally, we identified markers of the trans-Golgi network, components of the exocyst complex, and several motor proteins including myosin 1C, non-muscle myosins IIA and IIB, myosin VI, and myosin IXB. Beyond this, identification of multiple endoplasmic reticulum-resident proteins and ribosomal proteins indicated that a substantial fraction of intracellular AQP2 is present in rough endoplasmic reticulum. These results show that AQP2-containing vesicles are heterogeneous and that intracellular AQP2 resides chiefly in endosomes, trans-Golgi network, and rough endoplasmic reticulum.

Coatomer-bound Cdc42 regulates dynein recruitment to COPI vesicles

Cytoskeletal dynamics at the Golgi apparatus are regulated in part through a binding interaction between the Golgi-vesicle coat protein, coatomer, and the regulatory GTP-binding protein Cdc42 (Wu, W.J., J.W. Erickson, R. Lin, and R.A. Cerione. 2000. Nature. 405:800–804; Fucini, R.V., J.L. Chen, C. Sharma, M.M. Kessels, and M. Stamnes. 2002. Mol. Biol. Cell. 13:621–631). The precise role of this complex has not been determined. We have analyzed the protein composition of Golgi-derived coat protomer I (COPI)–coated vesicles after activating or inhibiting signaling through coatomer-bound Cdc42. We show that Cdc42 has profound effects on the recruitment of dynein to COPI vesicles. Cdc42, when bound to coatomer, inhibits dynein binding to COPI vesicles whereas preventing the coatomer–Cdc42 interaction stimulates dynein binding. Dynein recruitment was found to involve actin dynamics and dynactin. Reclustering of nocodazole-dispersed Golgi stacks and microtubule/dynein-dependent ER-to-Golgi transport are both sensitive to disrupting Cdc42 mediated signaling. By contrast, dynein-independent transport to the Golgi complex is insensitive to mutant Cdc42. We propose a model for how proper temporal regulation of motor-based vesicle translocation could be coupled to the completion of vesicle formation.

Microtubule organization and function in epithelial cells

Microtubules are essential for many aspects of polarity in multicellular organisms, ranging from the asymmetric distribution of cell-fate determinants in the one-cell embryo to the transient polarity generated in migrating fibroblasts. Epithelial cells exhibit permanent cell polarity characterized by apical and basolateral surface domains of distinct protein and lipid composition that are segregated by tight junctions. They are also endowed with a microtubule network that reflects the asymmetry of their cell surface: microtubule minus-ends face the apical- and microtubule plus-ends the basal domain. Strikingly, the formation of distinct surface domains during epithelial differentiation is accompanied by the re-organization of microtubules from a uniform array focused at the centrosome to the noncentrosomal network that aligns along the apico-basolateral polarity axis. The significance of this coincidence for epithelial morphogenesis and the signaling mechanisms that drive microtubule repolymerization in developing epithelia remain major unresolved questions that we are only beginning to address. Studies in cultured polarized epithelial cells have established that microtubules serve as tracks that facilitate targeted vesicular transport. Novel findings suggest, moreover, that microtubule-based transport promotes protein sorting, and even the generation of transport carriers in the endo- and exocytic pathways.

Secretory trafficking in neuronal dendrites

The neuronal secretory pathway represents the intracellular route for proteins involved in synaptic transmission and plasticity, as well as lipids required for outgrowth and remodelling of dendrites and axons. Although neurons use the same secretory compartments as other eukaryotic cells, the enormous distances involved, as well as the unique morphology of the neuron and its signalling requirements, challenge canonical models of secretory pathway organization. Here, we review evidence for a distributed secretory pathway in neurons, suggest mechanisms that may regulate secretory compartment distribution, and discuss the implications of a distributed secretory pathway for neuronal morphogenesis and neural-circuit plasticity.

The AE2 anion exchanger is necessary for the structural integrity of the Golgi apparatus in mammalian cells

The structural integrity of the Golgi apparatus is known to be dependent on multiple factors, including the organizational status of microtubules, actin and the ankyrin/spectrin-based Golgi membrane skeleton, as well as vesicular trafficking and pH homeostasis. In this respect, our recently identified Golgi-associated anion exchanger, AE2, may also be of importance, since it potentially acts as a Golgi pH regulator and as a novel membrane anchor for the spectrin-based Golgi membrane skeleton. Here, we show that inhibition (>75%) of AE2 expression by antisense oligonucleotides in COS-7 cells results in the fragmentation of the juxtanuclear Golgi apparatus and in structural disorganization of the Golgi stacks, the cisternae becoming generally shorter, distorted, vesiculated and/or swollen. These structural changes occurred without apparent dissociation of the Golgi membrane skeletal protein Ankyrin(195), but were accompanied by the disappearance of the well-focused microtubule-organizing center (MTOC), suggesting the involvement of microtubule reorganization. Similar changes in Golgi structure and assembly of the MTOC were also observed upon transient overexpression of the EGFP-AE2 fusion protein. These data implicate a clear structural role for the AE2 protein in the Golgi and in its cytological positioning around the MTOC.

A Role for Spectrin in Dynactin-dependent Melanosome Transport in Xenopus laevis Melanophores

The bi-directional movement of pigment granules in frog melanophores involves the microtubule-based motors cytoplasmic dynein, which is responsible for aggregation, and kinesin II and myosin V, which are required for dispersion of pigment. It was recently shown that dynactin acts as a link between dynein and kinesin II and melanosomes, but it is not fully understood how this is regulated and if more proteins are involved. Here, we suggest that spectrin, which is known to be associated with Golgi vesicles as well as synaptic vesicles in a number of cells, is of importance for melanosome movements in Xenopus laevis melanophores. Large amounts of spectrin were found on melanosomes isolated from both aggregated and dispersed melanophores. Spectrin and two components of the oligomeric dynactin complex, p150(glued) and Arp1/centractin, co-localized with melanosomes during aggregation and dispersion, and the proteins were found to interact as determined by co-immunoprecipitation. Spectrin has been suggested as an important link between cargoes and motor proteins in other cell types, and our new data indicate that spectrin has a role in the specialized melanosome transport processes in frog melanophores, in addition to a more general vesicle transport.

Cytosol-derived proteins are sufficient for Arp2/3 recruitment and ARF/coatomer-dependent actin polymerization on Golgi membranes

The actin cytoskeleton has been implicated in protein trafficking at the Golgi apparatus and in Golgi orientation and morphology. Actin dynamics at the Golgi are regulated in part by recruiting Cdc42 or Rac to the membrane through a binding interaction with the coatomer-coated (COPI)-vesicle coat protein, coatomer. This leads to actin polymerization through the effector, N-WASP and the Arp2/3 complex. Here, we have used reconstitution of vesicle budding to test whether Arp2/3 is recruited to membranes during the formation of COPI vesicles. Our results revealed that ARF1 activation leads to greatly increased Arp3 levels on the membranes. Coatomer-bound Cdc42 and pre-existing F-actin are important for Arp2/3 binding. ARF1-dependent Arp2/3 recruitment and actin polymerization can be reconstituted on liposomal membranes, indicating that no membrane proteins are necessary. These results show that activated ARF1 can stimulate Arp2/3 recruitment to Golgi membranes through coatomer, Cdc42 or Rac, and N-WASP.

Herpes simplex virus type 1 infection of polarized epithelial cells requires microtubules and access to receptors present at cell–cell contact sites

Mucosal epithelia are invaded from the apical surface during a primary infection by herpes simplex virus type 1 (HSV-1). HSV-1 progeny virus, synthesized from latently infected peripheral neurons that innervate such epithelia, reinfects the epithelia most likely from the basolateral surface. The epithelial cell lines MDCK and Caco-2 can be inducedin vitroto differentiate into polarized cells with distinct apical and plasma membrane domains separated by tight junctions if they are cultured on porous membrane filters. Our data using these culture systems showed that highly polarized epithelial cells were not susceptible to apical HSV-1 infection. However, HSV-1 infected these cells if added from the basolateral surface or if a depletion of extracellular Ca2+had weakened the strength of the cell–cell contacts. Basolateral infection and apical infection after the Ca2+switch required an intact microtubule network for genome targeting to the nucleus. This system can be used to identify the microtubule motors that HSV-1 uses during virus entry in polarized epithelial cells.

Membrane dynamics and the regulation of epithelial cell polarity

Plasma membranes of epithelial cells consist of two domains, an apical and a basolateral domain, the surfaces of which differ in composition. The separation of these domains by a tight junction and the fact that specific transport pathways exist for intracellular communication between these domains and distinct intracellular compartments relevant to cell polarity development, have triggered extensive research on issues that focus on how the polarity is generated and maintained. Apart from proper assembly of tight junctions, their potential functioning as landmark for the transport machinery, cell-cell adhesion is obviously instrumental in barrier formation. In recent years, distinct endocytic compartments, defined as subapical compartment or common endosome, were shown to play a prominent role in regulating membrane trafficking to and from polarized membrane domains. Sorting devices remain to be determined but likely include distinct rab proteins, and evidence is accumulating to indicate that signaling events may direct intracellular membrane transport, intimately involved in the biogenesis and maintenance of polarized membrane domains and hence the development of cell polarity.

Roles of the cytoskeleton and motor proteins in endocytic sorting

After internalization, endocytic material is actively transported through the cytoplasm, predominantly by microtubule motor proteins. Microtubule-based endocytic transport facilitates sorting of endocytic contents, vesicle fusion and fission, delivery to lysosomes, cytosolic dispersal, as well as nuclear uptake and cytosolic egress of pathogens. Endosomes, like most organelles, move bidirectionally through the cytosol and regulate their cellular location by controlling the activity of motor proteins, and potentially by controlling microtubule and actin polymerization. Control of motor protein activity is manifest by increased microtubule "run lengths", and the binding of motor proteins to organelles can be regulated by motor protein receptors. A mechanistic understanding of how organelles control motor protein activity to allow for endocytic sorting presents an exciting avenue for future research.

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.

Analysis of microtubule movement on isolated Xenopus egg cortices provides evidence that the cortical rotation involves dynein as well as Kinesin Related Proteins and is regulated by local microtubule polymerisation

In amphibians, the cortical rotation, a translocation of the egg cortex relative to the cytoplasm, specifies the dorsoventral axis. The cortical rotation involves an array of subcortical microtubules whose alignment is mediated by Kinesin-related proteins (KRPs), and stops as M-phase promoting factor (MPF) activation propagates across the egg. To dissect the role of different motor proteins in the cortical rotation and to analyse their regulation, we have developed an open cell assay system involving reactivation of microtubule movement on isolated cortices. Microtubule movements were dependent on ATP and consisted mainly of wriggling and flailing without net displacement, consistent with a tethering of microtubules to the cortex. Reactivated movements were inhibited by anti-KRP and anti-dynein antibodies perfused together but not separately, the KRP antibody alone becoming fixed to the cortex. Neither antibody could inhibit movement in the presence of MPF, indicating that arrest of the cortical rotation is not due to MPF-dependent inhibition of motor molecules. In contrast, D(2)O treatment of live eggs to protect microtubules from progressive depolymerisation prolonged the cortical rotation. We conclude that the cortical rotation probably involves cytoplasmic dynein as well as cortical KRPs and terminates as a result of local MPF-dependent microtubule depolymerisation.

Cargo-carrying motor vehicles on the neuronal highway: Transport pathways and neurodegenerative disease

Within axons vital cargoes must be transported over great distances along microtubule tracks to maintain neuronal viability. Essential to this system are the molecular motors, kinesin and dynein, which transport a variety of neuronal cargoes. Elucidating the transport pathways, the identity of the cargoes transported, and the regulation of motor-cargo complexes are areas of intense investigation. Evidence suggests that essential components, including signaling proteins, neuroprotective and repair molecules, and vesicular and cytoskeletal components are all transported. In addition newly emerging data indicate that defects in axonal transport pathways may contribute to the initiation or progression of chronic neuronal dysfunction. In this review we concentrate on microtubule-based motor proteins, their linkers, and cargoes and discuss how factors in the axonal transport pathway contribute to disease states. As additional cargo complexes and transport pathways are identified, an understanding of the role these pathways play in the development of human disease will hopefully lead to new diagnostic and treatment strategies.

Neural cell adhesion molecule promotes accumulation of TGN organelles at sites of neuron-to-neuron contacts

Transformation of a contact between axon and dendrite into a synapse is accompanied by accumulation of the synaptic machinery at this site, being delivered in intracellular organelles mainly of TGN origin. Here, we report that in cultured hippocampal neurons, TGN organelles are linked via spectrin to clusters of the neural cell adhesion molecule (NCAM) in the plasma membrane. These complexes are translocated along neurites and trapped at sites of initial neurite-to-neurite contacts within several minutes after initial contact formation. The accumulation of TGN organelles at contacts with NCAM-deficient neurons is reduced when compared with wild-type cells, suggesting that NCAM mediates the anchoring of intracellular organelles in nascent synapses.

Motoring around the Golgi

The Golgi apparatus is a dynamic organelle through which nascent secretory and transmembrane proteins are transported, post-translationally modified and finally packaged into carrier vesicles for transport along the cytoskeleton to a variety of destinations. In the past decade, studies have shown that a number of 'molecular motors' are involved in maintaining the proper structure and function of the Golgi apparatus. Here, we review just some of the many functions performed by these mechanochemical enzymes - dyneins, kinesins, myosins and dynamin - in relation to the Golgi apparatus.

ELF a beta-spectrin is a neuronal precursor cell marker in developing mammalian brain; structure and organization of the elf/beta-G spectrin gene

Spectrins play a pivotal role in axonal transport, neurite extension, the organization of synaptic vesicles, as well as for protein sorting in the Golgi apparatus and cell membrane. Among spectrins there is great variability in sequence composition, tissue distribution, and function, with two known genes encoding the alpha-chain, and at least five encoding the beta-chain. It remains unclear as to whether novel beta-spectrins such as elf1-4 are distinct genes or beta-G-spectrin isoforms. The role for ELF in the developing nervous system has not been identified to date. In this study we demonstrate the genomic structure of elf-3, as well as the expression of ELF in the developing mouse brain using a peptide specific antibody against its distinctive amino-terminal end. Full genomic structural analyses reveal that elf-3 is composed of 31 exons spanning approximately 67 kb, and confirm that elf and mouse brain beta-G-spectrin share multiple exons, with a complex form of exon/intron usage. In embryonic stages, E9-12, anti-ELF localized to the primary brain vesicular cells that also labeled strongly with anti-nestin but not anti-vimentin. At E12-14, anti-ELF localized to axonal sprouts in the developing neuroblasts of cortex and purkinje cell layer of the cerebellum, as well as in cell bodies in the diencephalon and metencephalon. Double labeling identified significant co-localization of anti-ELF, nestin and dystrophin in sub ventricular zone cells and in stellate-like cells of the developing forebrain. These studies define clearly the expression of ELF, a new isoform of beta-G-spectrin in the developing brain. Based on its expression pattern, ELF may have a role in neural stem cell development and is a marker of axonal sprouting in mid stages of embryonic development.

Regulating the actin cytoskeleton during vesicular transport

Although the actin cytoskeleton is widely believed to play an important role in intracellular protein transport, this role is poorly understood. Recently, progress has been made toward identifying specific actin-binding proteins and signaling molecules involved in regulating actin structures that function in the secretory pathway. Studies on coat protomer I (COPI)-mediated transport at the Golgi apparatus and on clathrin-mediated endocytosis have been particularly informative in identifying such mechanisms. Important similarities between actin regulation at the Golgi and at the plasma membrane have been uncovered. The studies reveal that ADP-ribosylation factor and vesicle coat proteins are able to act through the Rho-family GTP-binding proteins, Cdc42 and Rac, and several specific actin-binding proteins to direct actin assembly through the Arp2/3 complex. Efficient function of the secretory pathway is likely to require precise temporal regulation among transport-vesicle assembly, vesicle scission, and the targeting machinery. It is proposed that numerous actin regulatory mechanisms and the connections between actin signaling and vesicle-coat formation are employed to provide such temporal regulation.

Motor-cargo interactions: the key to transport specificity

Eukaryotic cells organize their cytoplasm by moving different organelles and macromolecular complexes along microtubules and actin filaments. These movements are powered by numerous motor proteins that must recognize their respective cargoes in order to function. Recently, several proteins that interact with motors have been identified by yeast two-hybrid and biochemical analyses, and their roles in transport are now being elucidated. In several cases, analysis of the binding partners helped to identify new transport pathways, new types of cargo, and transport regulated at the level of motor-cargo binding. We discuss here how different motors of the kinesin, dynein and myosin families recognize their cargo and how motor-cargo interactions are regulated.

Actin dependence of polarized receptor recycling in Madin-Darby canine kidney cell endosomes

Mammalian epithelial cell plasma membrane domains are separated by junctional complexes supported by actin. The extent to which actin acts elsewhere to maintain cell polarity remains poorly understood. Using latrunculin B (Lat B) to depolymerize actin filaments, several basolateral plasma membrane proteins were found to lose their polarized distribution. This loss of polarity did not reflect lateral diffusion through junctional complexes because a low-density lipoprotein receptor mutant lacking a functional endocytosis signal remained basolateral after Lat B treatment. Furthermore, Lat B treatment did not facilitate membrane diffusion across the tight junction as observed with ethylenediaminetetraacetic acid or dimethyl sulfoxide treatment. Detailed analysis of transferrin recycling confirmed Lat B depolarized recycling of transferrin from endosomes to the basolateral surface. Kinetic analysis suggested sorting was compromised at both basolateral early endosomes and perinuclear recycling endosomes. Despite loss of function, these two endosome populations remained distinct from each other and from early endosomes labeled by apically internalized ligand. Furthermore, apical and basolateral early endosomes were functionally distinct populations that directed traffic to a single common recycling endosomal compartment even after Lat B treatment. Thus, filamentous actin may help to guide receptor traffic from endosomes to the basolateral plasma membrane.

KIFC3, a microtubule minus end-directed motor for the apical transport of annexin XIIIb-associated Triton-insoluble membranes

We have identified and characterized a COOH-terminal motor domain-type kinesin superfamily protein (KIFC), KIFC3, in the kidney. KIFC3 is a minus end-directed microtubule motor protein, therefore it accumulates in regions where minus ends of microtubules assemble. In polarized epithelial cells, KIFC3 is localized on membrane organelles immediately beneath the apical plasma membrane of renal tubular epithelial cells in vivo and polarized MDCK II cells in vitro. Flotation assay, coupled with detergent extraction, demonstrated that KIFC3 is associated with Triton X-100-insoluble membrane organelles, and that it overlaps with apically transported TGN-derived vesicles. This was confirmed by immunoprecipitation and by GST pulldown experiments showing the specific colocalization of KIFC3 and annexin XIIIb, a previously characterized membrane protein for apically transported vesicles (Lafont, F., S. Lecat, P. Verkade, and K. Simons. 1998. J. Cell Biol. 142:1413-1427). Furthermore, we proved that the apical transport of both influenza hemagglutinin and annexin XIIIb was partially inhibited or accelerated by overexpression of motor-domainless (dominant negative) or full-length KIFC3, respectively. Absence of cytoplasmic dynein on these annexin XIIIb-associated vesicles and distinct distribution of the two motors on the EM level verified the existence of KIFC3-driven transport in epithelial cells.

beta III spectrin binds to the Arp1 subunit of dynactin

Cytoplasmic dynein is an intracellular motor responsible for endoplasmic reticulum-to-Golgi vesicle trafficking and retrograde axonal transport. The accessory protein dynactin has been proposed to mediate the association of dynein with vesicular cargo. Dynactin contains a 37-nm filament made up of the actin-related protein, Arp1, which may interact with a vesicle-associated spectrin network. Here, we demonstrate that Arp1 binds directly to the Golgi-associated betaIII spectrin isoform. We identify two Arp1-binding sites in betaIII spectrin, one of which overlaps with the actin-binding site conserved among spectrins. Although conventional actin binds weakly to betaIII spectrin, Arp1 binds robustly in the presence of excess F-actin. Dynein, dynactin, and betaIII spectrin co-purify on vesicles isolated from rat brain, and betaIII spectrin co-immunoprecipitates with dynactin from rat brain cytosol. In interphase cells, betaIII spectrin and dynactin both localize to cytoplasmic vesicles, co-localizing most significantly in the perinuclear region of the cell. In dividing cells, betaIII spectrin and dynactin co-localize to the developing cleavage furrow and mitotic spindle, a novel localization for betaIII spectrin. We hypothesize that the interaction between betaIII spectrin and Arp1 recruits dynein and dynactin to intracellular membranes and provides a direct link between the microtubule motor complex and its membrane-bounded cargo.

Microtubule- and dynein-mediated movement of Orientia tsutsugamushi to the microtubule organizing center

The host cell microfilaments and microtubules (MTs) are known to play a critical role in the life cycles of several pathogenic intracellular microbes by providing for successful invasion and promoting movement of the pathogen once inside the host cell cytoplasm. Orientia tsutsugamushi, an obligate intracellular bacterium, enters host cells by induced phagocytosis, escapes to the cytosol, and then replicates in the cytosol. ECV304 cells infected with O. tsutsugamushi revealed the colocalization of the MT organizing center (MTOC) and cytosolic orientiae by indirect immunofluorescence assay. Using immunofluorescence microscopy in the presence and absence of MT-depolymerizing agents (colchicine and nocodazole), it was shown that the cytosolic oriential movement was mediated by MTs. By transfection study (overexpression of dynamitin [also called p50], which is known to associate with dynein-dependent movement), the movement of O. tsutsugamushi to the MTOC was also mediated by dynein, the minus-end-directed MT-related motor. Although the significance of this movement in the life cycle of O. tsutsugamushi was not proven, we propose that the cytosolic O. tsutsugamushi bacteria use MTs and dyneins to propel themselves from the cell periphery to the MTOC.

Immunolocalization of cytoplasmic dynein and dynactin subunits in cultured macrophages: enrichment on early endocytic organelles

Cytoplasmic dyneins and their cofactor, dynactin, work together to mediate the movement of numerous cargo organelles toward the minus-ends of microtubules. In many cases, there is compelling evidence that dynactin functions in part to attach dyneins to cargo organelles, but this may not always be the case. We have localized three dynactin subunits (Arp1, p62 and p150(Glued)) and two subunits of conventional cytoplasmic dynein (dynein intermediate chain and dynein heavy chain 1) in murine macrophages using immunogold labeling of thawed cryosections. Using stereological techniques, we have quantified the relative distributions of each of these subunits on specific membrane organelles to generate a comprehensive analysis of the distribution of these proteins in a single cell type. Our results show that each of the subunits tested exhibits the same distribution with respect to different membrane organelles, with highest levels present on early endosomes, and lower levels present on later endocytic organelles, the mitochondrial outer membrane, the plasma membrane and vesicles in the Golgi region. An additional pool of punctate dynactin labeling was detected in the cell periphery, in the absence of dynein labeling. Even when examined closely, membrane organelles could not be detected in association with these dynactin-positive sites; however, double labeling with anti-tubulin antibody revealed that at least some of these sites represent the ends of microtubules. The similarities among the labeling profiles with respect to membrane organelles suggest that dynein and dynactin bind to membrane organelles as an obligate unit. In contrast, our results show that dynactin can associate with microtubule ends in the absence of dynein, perhaps providing sites for subsequent organelle and dynein association to form a functional motility complex.

Spectrin tethers and mesh in the biosynthetic pathway

The paradox of how the Golgi and other organelles can sort a continuous flux of protein and lipid but maintain temporal and morphological stability remains unresolved. Recent discoveries highlight a role for the cytoskeleton in guiding the structure and dynamics of organelles. Perhaps one of the more striking, albeit less expected, of these discoveries is the recognition that a spectrin skeleton associates with many organelles and contributes to the maintenance of Golgi structure and the efficiency of protein trafficking in the early secretory pathway. Spectrin interacts directly with phosphoinositides and with membrane proteins. The small GTPase ARF, a key player in Golgi dynamics, regulates the assembly of the Golgi spectrin skeleton through its ability to control phosphoinositide levels in Golgi membranes, whereas adapter molecules such as ankyrin link spectrin to other membrane proteins. Direct interactions of spectrin with actin and centractin (ARP1) provide a link to dynein, myosin and presumably other motors involved with intracellular transport. Building on the recognized ability of spectrin to organize macromolecular complexes of membrane and cytosolic proteins into a multifaceted scaffold linked to filamentous structural elements (termed linked mosaics), recent evidence supports a similar role for spectrin in organelle function and the secretory pathway. Two working models accommodate much of the available data: the Golgi mesh hypothesis and the spectrin ankyrin adapter protein tethering system (SAATS) hypothesis.

Caenorhabditis elegans beta-G spectrin is dispensable for establishment of epithelial polarity, but essential for muscular and neuronal function

The Caenorhabditis elegans genome encodes one alpha spectrin subunit, a beta spectrin subunit (beta-G), and a beta-H spectrin subunit. Our experiments show that the phenotype resulting from the loss of the C. elegans alpha spectrin is reproduced by tandem depletion of both beta-G and beta-H spectrins. We propose that alpha spectrin combines with the beta-G and beta-H subunits to form alpha/beta-G and alpha/beta-H heteromers that perform the entire repertoire of spectrin function in the nematode. The expression patterns of nematode beta-G spectrin and vertebrate beta spectrins exhibit three striking parallels including: (1) beta spectrins are associated with the sites of cell-cell contact in epithelial tissues; (2) the highest levels of beta-G spectrin occur in the nervous system; and (3) beta spectrin-G in striated muscle is associated with points of attachment of the myofilament apparatus to adjacent cells. Nematode beta-G spectrin associates with plasma membranes at sites of cell-cell contact, beginning at the two-cell stage, and with a dramatic increase in intensity after gastrulation when most cell proliferation has been completed. Strikingly, depletion of nematode beta-G spectrin by RNA-mediated interference to undetectable levels does not affect the establishment of structural and functional polarity in epidermis and intestine. Contrary to recent speculation, beta-G spectrin is not associated with internal membranes and depletion of beta-G spectrin was not associated with any detectable defects in secretion. Instead beta-G spectrin-deficient nematodes arrest as early larvae with progressive defects in the musculature and nervous system. Therefore, C. elegans beta-G spectrin is required for normal muscle and neuron function, but is dispensable for embryonic elongation and establishment of early epithelial polarity. We hypothesize that heteromeric spectrin evolved in metazoans in response to the needs of cells in the context of mechanically integrated tissues that can withstand the rigors imposed by an active organism.

Uniform apicobasal polarity of microtubules and apical location of gamma-tubulin in polarized intestinal epithelium in situ

Polarized differentiation of the intestinal epithelium has been previously shown to depend on an intact microtubular system that is essential for vectorial delivery of apical membrane proteins to the apical cell surfaces. Uniform alignment and polarization of microtubules have been suggested to provide the ultrastructural basis for vectorial transport between the Golgi apparatus and the apical cell surface. In the present study we applied the hook decoration technique to analyse the polarity of microtubules in the rat jejunal epithelium. By immunocytochemistry we studied the subcellular location of gamma-tubulin, an essential component of microtubule-organizing centers. Microtubules were found to be mainly aligned parallel to the apicobasal axis of the cells and to extend from the subterminal space underneath the apical terminal web down to the cellular basis. We found that 98% out of 1122 decorated microtubules displayed uniform apicobasal polarity with the growing ends (plus ends) pointing basally and the non-growing ends (minus ends) pointing towards the cellular apex. No differences were observed with respect to microtubular polarity between the apical, perinuclear and infranuclear cellular portions. Immunostaining specific for gamma-tubulin was restricted to the apical subterminal space underneath the rootlets of microvilli. These findings indicate that the apical subterminal space of enterocytes serves as the predominant if not exclusive microtubule-organizing compartment from which uniformly polarized microtubules grow out with their plus ends towards the cellular basis.

The golgi-associated COPI-coated buds and vesicles contain beta/gamma -actin

It has been shown previously that the morphology and subcellular positioning of the Golgi complex is controlled by actin microfilaments. To further characterize the association between actin microfilaments and the Golgi complex, we have used the Clostridium botulinum toxins C2 and C3, which specifically inhibit actin polymerization and cause depolymerization of F-actin in intact cells by the ADP ribosylation of G-actin monomers and the Rho small GTP-binding protein, respectively. Normal rat kidney cells treated with C2 showed that disruption of the actin and the collapse of the Golgi complex occurred concomitantly. However, when cells were treated with C3, the actin disassembly was observed without any change in the organization of the Golgi complex. The absence of the involvement of Rho was further confirmed by the treatment with lysophosphatidic acid or microinjection with the constitutively activated form of RhoA, both of which induced the stress fiber formation without affecting the Golgi complex. Immunogold electron microscopy in normal rat kidney cells revealed that beta- and gamma-actin isoforms were found in Golgi-associated COPI-coated buds and vesicles. Taken together, the results suggest that the Rho signaling pathway does not directly regulate Golgi-associated actin microfilaments, and that beta- and gamma-actins might be involved in the formation and/or transport of Golgi-derived vesicular or tubular intermediates.

Unconventional myosins: anchors in the membrane traffic relay

The family of unconventional myosins is ever growing and the functions attributed to them seem to expand in parallel. These actin-based motor proteins have been implicated in processes as seemingly diverse as endocytosis and exocytosis, the transport of organelles, in spermatogenesis and in neurosensory functions such as hearing and sight. A common myosin function may underlie them all--the regulation of intracellular membrane traffic.