A spectrin membrane skeleton of the Golgi complex

SLC4A2 anion exchanger promotes tumour cell malignancy via enhancing net acid efflux across golgi membranes

Proper functioning of each secretory and endocytic compartment relies on its unique pH micro-environment that is known to be dictated by the rates of V-ATPase-mediated H⁺ pumping and its leakage back to the cytoplasm via an elusive "H⁺ leak" pathway. Here, we show that this proton leak across Golgi membranes is mediated by the AE2a (SLC4A2a)-mediated bicarbonate-chloride exchange, as it is strictly dependent on bicarbonate import (in exchange for chloride export) and the expression level of the Golgi-localized AE2a anion exchanger. In the acidic Golgi lumen, imported bicarbonate anions and protons then facilitate a common buffering reaction that yields carbon dioxide and water before their egress back to the cytoplasm via diffusion or water channels. The flattened morphology of the Golgi cisternae helps this process, as their high surface-volume ratio is optimal for water and gas exchange. Interestingly, this net acid efflux pathway is often upregulated in cancers and established cancer cell lines, and responsible for their markedly elevated Golgi resting pH and attenuated glycosylation potential. Accordingly, AE2 knockdown in SW-48 colorectal cancer cells was able to restore these two phenomena, and at the same time, reverse their invasive and anchorage-independent growth phenotype. These findings suggest a possibility to return malignant cells to a benign state by restoring Golgi resting pH.

Crumbs organizes the transport machinery by regulating apical levels of PI(4,5)P2 in Drosophila

An efficient vectorial intracellular transport machinery depends on a well-established apico-basal polarity and is a prerequisite for the function of secretory epithelia. Despite extensive knowledge on individual trafficking pathways, little is known about the mechanisms coordinating their temporal and spatial regulation. Here, we report that the polarity protein Crumbs is essential for apical plasma membrane phospholipid-homeostasis and efficient apical secretion. Through recruiting βHeavy-Spectrin and MyosinV to the apical membrane, Crumbs maintains the Rab6-, Rab11- and Rab30-dependent trafficking and regulates the lipid phosphatases Pten and Ocrl. Crumbs knock-down results in increased apical levels of PI(4,5)P2 and formation of a novel, Moesin- and PI(4,5)P2-enriched apical membrane sac containing microvilli-like structures. Our results identify Crumbs as an essential hub required to maintain the organization of the apical membrane and the physiological activity of the larval salivary gland.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

Actin acting at the Golgi

The organization, assembly and remodeling of the actin cytoskeleton provide force and tracks for a variety of (endo)membrane-associated events such as membrane trafficking. This review illustrates in different cellular models how actin and many of its numerous binding and regulatory proteins (actin and co-workers) participate in the structural organization of the Golgi apparatus and in trafficking-associated processes such as sorting, biogenesis and motion of Golgi-derived transport carriers.

βIII spectrin regulates the structural integrity and the secretory protein transport of the Golgi complex

A spectrin-based cytoskeleton is associated with endomembranes, including the Golgi complex and cytoplasmic vesicles, but its role remains poorly understood. Using new generated antibodies to specific peptide sequences of the human βIII spectrin, we here show its distribution in the Golgi complex, where it is enriched in the trans-Golgi and trans-Golgi network. The use of a drug-inducible enzymatic assay that depletes the Golgi-associated pool of PI4P as well as the expression of PH domains of Golgi proteins that specifically recognize this phosphoinositide both displaced βIII spectrin from the Golgi. However, the interference with actin dynamics using actin toxins did not affect the localization of βIII spectrin to Golgi membranes. Depletion of βIII spectrin using siRNA technology and the microinjection of anti-βIII spectrin antibodies into the cytoplasm lead to the fragmentation of the Golgi. At ultrastructural level, Golgi fragments showed swollen distal Golgi cisternae and vesicular structures. Using a variety of protein transport assays, we show that the endoplasmic reticulum-to-Golgi and post-Golgi protein transports were impaired in βIII spectrin-depleted cells. However, the internalization of the Shiga toxin subunit B to the endoplasmic reticulum was unaffected. We state that βIII spectrin constitutes a major skeletal component of distal Golgi compartments, where it is necessary to maintain its structural integrity and secretory activity, and unlike actin, PI4P appears to be highly relevant for the association of βIII spectrin the Golgi complex.

Splice variant-specific cellular function of the formin INF2 in maintenance of Golgi architecture

INF2 is a unique formin that can both polymerize and depolymerize actin. One INF2 splice variant localizes in an actin-dependent, web-like network in cytoplasm, whereas a second isoform is ER bound. Suppression of the first isoform causes Golgi dispersion. These findings denote isoform-specific cellular functions for INF2.

INF2 is a unique formin that can both polymerize and depolymerize actin filaments. Mutations in INF2 cause the kidney disease focal and segmental glomerulosclerosis. INF2 can be expressed as two C-terminal splice variants: CAAX and non-CAAX. The CAAX isoform contains a C-terminal prenyl group and is tightly bound to endoplasmic reticulum (ER). The localization pattern and cellular function of the non-CAAX isoform have not been studied. Here we find that the two isoforms are expressed in a cell type–dependent manner, with CAAX predominant in 3T3 fibroblasts and non-CAAX predominant in U2OS, HeLa, and Jurkat cells. Although INF2-CAAX is ER localized in an actin-independent manner, INF2–non-CAAX localizes in an actin-dependent meshwork pattern distinct from ER. INF2–non-CAAX is loosely attached to this meshwork, being extracted by brief digitonin treatment. Suppression of INF2–non-CAAX causes fragmentation of the Golgi apparatus. This effect is counteracted by treatment with the actin monomer–sequestering drug latrunculin B. We also find discrete patches of actin filaments in the peri-Golgi region, and these patches are reduced upon INF2 suppression. Our results suggest that the non-CAAX isoform of INF2 serves a distinct cellular function from that of the CAAX isoform.

Involvement of the Rho-mDia1 pathway in the regulation of Golgi complex architecture and dynamics

A study of the role of actin cytoskeleton regulation in Golgi organization and function shows that Rho regulates Golgi fragmentation into ministacks, as well as formation of Rab6-positive Golgi-derived vesicles, via mDia1 formin activation. The Rho–mDia1 pathway affects the Golgi complex by controlling fusion and fission of Golgi membranes.

In mammalian cells, the Golgi apparatus is a ribbon-like, compact structure composed of multiple membrane stacks connected by tubular bridges. Microtubules are known to be important to Golgi integrity, but the role of the actin cytoskeleton in the maintenance of Golgi architecture remains unclear. Here we show that an increase in Rho activity, either by treatment of cells with lysophosphatidic acid or by expression of constitutively active mutants, resulted in pronounced fragmentation of the Golgi complex into ministacks. Golgi dispersion required the involvement of mDia1 formin, a downstream target of Rho and a potent activator of actin polymerization; moreover, constitutively active mDia1, in and of itself, was sufficient for Golgi dispersion. The dispersion process was accompanied by formation of dynamic F-actin patches in the Golgi area. Experiments with cytoskeletal inhibitors (e.g., latrunculin B, blebbistatin, and Taxol) revealed that actin polymerization, myosin-II–driven contractility, and microtubule-based intracellular movement were all involved in the process of Golgi dispersion induced by Rho–mDia1 activation. Live imaging of Golgi recovery revealed that fusion of the small Golgi stacks into larger compartments was repressed in cells with active mDia1. Furthermore, the formation of Rab6-positive transport vesicles derived from the Golgi complex was enhanced upon activation of the Rho–mDia1 pathway. Transient localization of mDia1 to Rab6-positive vesicles was detected in cells expressing active RhoA. Thus, the Rho–mDia1 pathway is involved in regulation of the Golgi structure, affecting remodeling of Golgi membranes.

Osmoprotective proteome adjustments in mouse kidney papilla

The papilla of the mammalian kidney must tolerate greatly varying degrees of hyperosmotic stress during urine concentration and depending on whole organism hydration state. To identify proteome adaptations supporting cell function and survival in such a harsh environment we compared the proteome of a) the hyperosmotic renal papilla with that of adjacent iso-osmotic cortex tissue and b) the renal papilla of diuretic versus that of anti-diuretic mice. Though functionally distinct the papilla is in close physical proximity to the renal cortex, an iso-osmotic region. Proteomic differences between the papilla and cortex of C57BL6 mice were identified using two-dimensional gel electrophoresis and MALDI-TOF/TOF mass spectrometry. We found 37 different proteins characteristic of the cortex and 16 proteins over-represented in the papilla. Regional specificity was confirmed by Western blot and further substantiated by immunohistochemistry for selected proteins. Proteins that are characteristic of the renal papilla include αB crystallin, Hsp beta-1, Hsp90, 14-3-3 protein, glutathione S-transferase, aldose reductase, actin and tropomyosin. Gene ontology analysis confirmed a significant increase in molecular functions associated with protein chaperoning and cell stabilization. Proteins over-represented in the cortex were largely related to routine metabolism. During antidiuresis 15 different proteins changed significantly while 18 different proteins changed significantly during diuresis relative to normally hydrated controls. Changes were confirmed by Western blot for selected proteins. Proteins that are significantly altered by diuretic state are associated with cell structure (actin, tubulin), signaling (Rho GDP dissociation inhibitor, abhydrolase domain-containing protein 14B), chaperone functioning (Hsp beta-1, αB crystallin, T complex protein-1) and anti-oxidant functions (α-enolase, GAPDH and LDH). Taken together our study reveals that specific proteins involved in protein folding, cytoskeletal stabilization, antioxidant responses, and stress signaling contribute greatly to the unique hyperosmotic stress resistant phenotype of the kidney papilla.

Calpain-mediated breakdown of cytoskeletal proteins contributes to cholecystokinin-induced damage of rat pancreatic acini

The cytosolic cysteine protease calpain is implicated in a multitude of cellular functions but also plays a role in cell damage. Our previous results suggest that an activation of calpain accompanied by a decrease in its endogenous inhibitor calpastatin may contribute to pancreatic damage during cerulein-induced acute pancreatitis. The present study aimed at the time course of secretagogue-induced calpain activation and cellular substrates of the protease. Isolated rat pancreatic acini were incubated with a supramaximal concentration of cholecystokinin (0.1 microM CCK) for 30 min in the presence or absence of the calpain inhibitor Z-Val-Phe methyl ester (100 microM ZVP). The activation of calpain and the expression of calpastatin and the actin cytoskeleton-associated proteins alphaII-spectrin, E-cadherin and vinculin were studied by immunoblotting. The cell damage was assessed by lactate dehydrogenase release and ultrastructural analysis including fluorescence-labelled actin filaments. Immediately after administration, CCK led to activation of both calpain isoforms, mu- and m-calpain. The protease activation was accompanied by a decrease in the E-cadherin level and formation of calpain-specific breakdown products of alphaII-spectrin. A calpain-specific cleavage product of vinculin appeared concomitantly with changes in the actin filament organization. No effect of CCK on calpastatin was found. Inhibition of calpain by ZVP reduced CCK-induced damage of the actin-associated proteins and the cellular ultrastructure including the actin cytoskeleton. The results suggest that CCK-induced acinar cell damage requires activation of calpain and that the actin cytoskeleton belongs to the cellular targets of the protease.

Conformational change of erythroid alpha-spectrin at the tetramerization site upon binding beta-spectrin

We previously determined the solution structures of the first 156 residues of human erythroid alpha-spectrin (SpalphaI-1-156, or simply Spalpha). Spalpha consists of the tetramerization site of alpha-spectrin and associates with a model beta-spectrin protein (Spbeta) with an affinity similar to that of native alpha- and beta-spectrin. Upon alphabeta-complex formation, our previous results indicate that there is an increase in helicity in the complex, suggesting conformational change in either Spalpha or Spbeta or in both. We have now used isothermal titration calorimetry, circular dichroism, static and dynamic light scattering, and solution NMR methods to investigate properties of the complex as well as the conformation of Spalpha in the complex. The results reveal a highly asymmetric complex, with a Perrin shape parameter of 1.23, which could correspond to a prolate ellipsoid with a major axis of about five and a minor axis of about one. We identified 12 residues, five prior to and seven following the partial domain helix in Spalpha that moved freely relative to the structural domain in the absence of Spbeta but when in the complex moved with a mobility similar to that of the structural domain. Thus, it appears that the association with Spbeta induced an unstructured-to-helical conformational transition in these residues to produce a rigid and asymmetric complex. Our findings may provide insight toward understanding different association affinities of alphabeta-spectrin at the tetramerization site for erythroid and non-erythroid spectrin and a possible mechanism to understand some of the clinical mutations, such as L49F of alpha-spectrin, which occur outside the functional partial domain region.

Brain proteins interacting with the tetramerization region of non-erythroid alpha spectrin

The N-terminal region of non-erythroid alpha spectrin (SpαII) is responsible for interacting with its binding partner, beta spectrin, to form functional spectrin tetramers. We used a yeast-two-hybrid system, with an N-terminal segment of alpha spectrin representing the functional tetramerization site, as a bait to screen human brain c-DNA library for proteins that interact with the alpha spectrin segment. In addition to several beta spectrin isoforms, we identified 14 proteins that interact with SpαII. Seven of the 14 were matched to 6 known proteins: Duo protein, Lysyl-tRNA synthetase, TBP associated factor 1, two isoforms (b and c) of a protein kinase A interacting protein and Zinc finger protein 333 (2 different segments). Four of the 6 proteins are located primarily in the nucleus, suggesting that spectrin plays important roles in nuclear functions. The remaining 7 proteins were unknown to the protein data base. Structural predictions show that many of the 14 proteins consist of a large portion of unstructured regions, suggesting that many of these proteins fold into a rather flexible conformation. It is interesting to note that all but 3 of the 14 proteins are predicted to consist of one to four coiled coils (amphiphilic helices). A mutation in SpαII, V22D, which interferes with the coiled coil bundling of SpαII with beta spectrin, also affects SpαII interaction with Duo protein, TBP associated factor 1 and Lysyl-tRNA synthetase, suggesting that they may compete with beta spectrin for interaction with SpαII. Future structural and functional studies of these proteins to provide interaction mechanisms will no doubt lead to a better understanding of brain physiology and pathophysiology.

Ankyrin-dependent and -independent mechanisms orchestrate axonal compartmentalization of L1 family members neurofascin and L1/neuron-glia cell adhesion molecule

Axonal initial segments (IS) and nodes of Ranvier are functionally important membrane subdomains in which the clustering of electrogenic channels enables action potential initiation and propagation. In addition, the initial segment contributes to neuronal polarity by serving as a diffusion barrier. To study the mechanisms of axonal compartmentalization, we focused on two L1 family of cell adhesion molecules (L1-CAMs) [L1/neuron-glia cell adhesion molecule (L1/NgCAM) and neurofascin (NF)] and two neuronal ankyrins (ankB and ankG). NF and ankG accumulate specifically at the initial segment, whereas L1/NgCAM and ankB are expressed along the entire lengths of axons. We find that L1/NgCAM and NF show distinct modes of steady-state accumulation during axon outgrowth in cultured hippocampal neurons. Despite their different steady-state localizations, both L1/NgCAM and NF show slow diffusion and low detergent extractability specifically in the initial segment but fast diffusion and high detergent extractability in the distal axon. We propose that L1-CAMs do not strongly bind ankB in the distal axon because of spatial regulation of ankyrin affinity by phosphorylation. NF, conversely, is initially enriched in an ankyrin-independent manner in the axon generally and accumulates progressively in the initial segment attributable to preferential binding to ankG. Our results suggest that NF and L1/NgCAM accumulate in the axon by an ankyrin-independent pathway, but retention at the IS requires ankyrin binding.

Spectrin mutations in spinocerebellar ataxia (SCA)

Recently, betaIII spectrins have been recognized as ataxia disease genes, with the identification by Ikeda and co-workers of pathogenic mutations in the SPTBN2 gene in three large (and mapped) SCA5 families of American and European origin.((1)) With their discovery, the large "Lincoln" family has been traced back to the underlying genetic defect for the slowly progressive cerebellar ataxia. In addition, the involvement of this component of the cytoskeleton directs attention towards the possible role of organelle stability during neurodegeneration. The findings suggest that the mechanical properties of neurons and their dynamics may be as important as altered Ca(2+) homeostasis, transcriptional dysregulation, and impaired protein degradation in neurodegeneration conditions.

Identification and characterization of GSRP-56, a novel Golgi-localized spectrin repeat-containing protein

Spectrin repeat (SR)-containing proteins are important for regulation of integrity of biomembranes, not only the plasma membrane but also those of intracellular organelles, such as the Golgi, nucleus, endo/lysosomes, and synaptic vesicles. We identified a novel SR-containing protein, named GSRP-56 (Golgi-localized SR-containing protein-56), by a yeast two-hybrid method, using a member of the transient receptor potential channel family, TRPV2, as bait. GSRP-56 is an isoform derived from a giant SR-containing protein, Syne-1 (synaptic nuclear envelope protein-1, also referred to as Nesprin-1 or Enaptin), predicted to be produced by alternative splicing. Immunological analysis demonstrated that this isoform is a 56-kDa protein, which is localized predominantly in the Golgi apparatus in cardiomyocytes and C2C12 myoblasts/myotubes, and we found that two SR domains were required both for Golgi targeting and for interaction with TRPV2. Interestingly, overexpression of GSRP-56 resulted in a morphological change in the Golgi structure, characterized by its enlargement of cis-Golgi marker antibody-staining area, which would result partly from fragmentation of Golgi membranes. Our findings indicate that GSRP-56 is a novel, particularly small Golgi-localized member of the spectrin family, which possibly play a role in maintenance of the Golgi structure.

Complexes of syndapin II with dynamin II promote vesicle formation at the trans-Golgi network

The role of dynamin and so-called accessory proteins in endocytosis is well established. However, molecular details of the function(s) of dynamin II at the Golgi are largely unclear. We demonstrate that the ubiquitously expressed syndapin II isoform interacts with the proline-rich domain (PRD) of dynamin II through its Src-homology 3 (SH3) domain. Co-immunoprecipitation of endogenous syndapin II and dynamin II, and successful reconstitutions of such complexes at membranes in COS-7 cells, show the in vivo relevance of the interaction. Syndapin II can associate with Golgi membranes and this association increases upon Golgi exit block. Brefeldin A treatment clearly shows that the observed perinuclear localization of syndapin II co-localizing with syntaxin 6 reflects the Golgi complex and that it requires functional integrity of the Golgi. Syndapins are crucial for Golgi vesicle formation because anti-syndapin antibodies, used either in in vitro reconstitutions or in living cells, inhibited this process. Both types of assays additionally revealed the essential role of syndapin II SH3 interactions with the dynamin II PRD in vesicle formation. An excess of the syndapin SH3 domain strongly inhibited budding from Golgi membranes in vitro. Likewise, overexpression of the syndapin SH3 domain or of a dynamin II variant incapable of associating with syndapin II (dynamin IIΔPRD) impaired trafficking of vesicular stomatitis virus glycoprotein (VSVG)-GFP in vivo. By contrast, full-length syndapin II-l had no negative effect, and instead promoted VSVG-GFP export from the Golgi. Importantly, a cytosolic fraction containing endogenous syndapin-dynamin complexes was sufficient to promote vesicle formation from Golgi membranes in a syndapin-dependent manner. Thus, syndapin-dynamin complexes are crucial and sufficient to promote vesicle formation from the trans-Golgi network.

Spectrins and the Golgi

Several isoforms of spectrin membrane skeleton proteins have been localized to the Golgi complex. Golgi-specific membrane skeleton proteins associate with the Golgi as a detergent-resistant cytoskeletal structure that likely undergoes a dynamic assembly process that accommodates Golgi membrane dynamics. This review discusses the potential roles for this molecule in Golgi functions. In particular, it will focus on a recently identified distant cousin to conventional erythroid spectrin variously named Syne-1, Nesprin, myne, Enaptin, MSP-300, and Ank-1. Syne-1 has the novel ability to bind to both the Golgi and the nuclear envelope, a property that raises several intriguing and novel insights into Golgi structure and function. These include (1) the facilitation of interactions between Golgi and transitional ER sites on the nuclear envelope of muscle cells, and (2) an ability to impart localized specificity to the secretory pathway within large multinucleate syncytia such as skeletal muscle fibers.

Mutational effects at the tetramerization site of nonerythroid alpha spectrin

Spectrin, a prominent cytoskeletal protein, exerts its fundamental role in cellular function by forming a sub-membrane filamentous network. An essential aspect of spectrin network formation is the tetramerization of spectrin alphabeta heterodimers. We used laboratory methods, the yeast two-hybrid system and random mutagenesis, to investigate, for the first time, effects of amino acid mutations on tetramerization of nonerythroid (brain) spectrin (fodrin). Based on high sequence homology with erythroid spectrin, we assume the putative tetramerization region of nonerythroid alpha-spectrin at the N-terminal region. We introduced mutations in the region consisting of residues 1-45 and studied mutational effects on spectrin alphabeta association to form tetramers. We detected single, double, and triple mutations involving 24 residues in this region. These amino acid mutations of nonerythroid alpha-spectrin exhibit full, partial, or no effect on the association with nonerythroid beta-spectrin. Single amino acid mutations in the region of residues 1-9 (D2Y, G5V, V6D, and V8M) did not affect the association. However, seven single mutations (I15F, I15N, R18G, V22D, R25P, Y26N, and R28P) affected the alphabeta association. These mutations were clustered in the region predicted by sequence alignment to be crucial in nonerythroid alpha-spectrin for tetramerization, a region that spanned residues 12-36, corresponding to the partial domain Helix C' (residues 21-45) in erythroid alpha-spectrin. In addition, two other mutations, one upstream and one downstream of this region at positions 10 (E10D) and 37 (R37P), also affected the alphabeta association. Our results implied nonerythroid alpha-spectrin partial domain helix may be longer than Helix C' (residues 21-45 and a total of 25 residues) in erythroid alpha-spectrin and spanned at least residues 10-37. It is interesting to note that seven out of these nine single mutations (I15F, I15N, R18G, V22D, R25P, Y26N, R37P) were at the a, d, e or g heptad positions based on sequence alignment with erythroid alpha-spectrin. Four of the mutated residues (I15, R18, V22, R25) are conserved in both erythroid and nonerythroid spectrin. These positions were previously identified as hot spots in erythroid alpha-spectrin that lead to severe hematological symptoms. This study clearly demonstrated that single mutation in a region predicted to be critical functionally in nonerythroid alpha-spectrin indeed leads to functional abnormalities and may lead to neurological disorders.

Golgi structure in stress sensing and apoptosis

The Golgi complex in mammalian cells is composed of polarized stacks of flattened cisternal membranes. Stacks are connected by tubules forming a reticular network of membranes closely associated with the microtubule-organizing center. While the Golgi structure is important for the efficient processing of secretory cargo, the organization of the mammalian Golgi complex may indicate potential functions in addition to the processing and sorting of cargo. Similar to the endoplasmic reticulum stress response pathway, the Golgi complex may initiate signaling pathways to alleviate stress, and if irreparable, trigger apoptosis. Here, we review recent experimental evidence suggesting that the elaborate structure of the Golgi complex in mammalian cells may have evolved to sense and transduce stress signals.

Ankyrin-B targets beta2-spectrin to an intracellular compartment in neonatal cardiomyocytes

Ankyrin-B is a spectrin-binding protein that is required for localization of inositol 1,4,5-trisphosphate receptor and ryanodine receptor in neonatal cardiomyocytes. This work addresses the interaction between ankyrin-B and beta(2)-spectrin in these cells. Ankyrin-B and beta(2)-spectrin are colocalized in an intracellular striated compartment overlying the M-line and distinct from T-tubules, sarcoplasmic reticulum, Golgi, endoplasmic reticulum, lysosomes, and endosomes. Beta(2)-Spectrin is absent in ankyrin-B-null cardiomyocytes and is restored to a normal striated pattern by rescue with green fluorescent protein-220-kDa ankyrin-B. We identified two mutants (A1000P and DAR976AAA) located in the ZU5 domain which eliminate spectrin binding activity of ankyrin-B. Ankyrin-B mutants lacking spectrin binding activity are normally targeted but do not reestablish beta(2)-spectrin in ankyrin-B(+/-) cardiomyocytes. However, both mutant forms of ankyrin-B are still capable of restoring inositol 1,4,5-trisphosphate receptor localization and normal contraction frequency of cardiomyocytes. Therefore, direct binding of beta(2)-spectrin to ankyrin-B is required for the normal targeting of beta(2)-spectrin in neonatal cardiomyocytes. In contrast, ankyrin-B localization and function are independent of beta(2)-spectrin. In summary, this work demonstrates that interaction between members of the ankyrin and beta-spectrin families previously established in erythrocytes and axon initial segments also occurs in neonatal cardiomyocytes with ankyrin-B and beta(2)-spectrin. This work also establishes a functional hierarchy in which ankyrin-B determines the localization of beta(2)-spectrin and operates independently of beta(2)-spectrin in its role in organizing membrane-spanning proteins.

The syndapin protein family: linking membrane trafficking with the cytoskeleton

Syndapins – also called PACSINs – are highly conserved Src-homology 3 (SH3)-domain-containing proteins that seem to exist in all multicellular eukaryotes. They interact with the large GTPase dynamin and several other proteins implicated in vesicle trafficking. Syndapin-dynamin complexes appear to play an important role in vesicle fission at different donor membranes, including the plasma membrane (endocytosis) and Golgi membranes. In addition, syndapins are implicated in later steps of vesicle cycling in neuronal and non-neuronal cells. Syndapins also interact with N-WASP, a potent activator of the Arp2/3 complex that forms a critical part of the actin polymerization machinery. Syndapin oligomers can thereby couple bursts of actin polymerization with the vesicle fission step involving dynamins. This allows newly formed vesicles to move away from the donor membrane driven by actin polymerization. Syndapins also engage in additional interactions with molecules involved in several signal transduction pathways, producing crosstalk at the interface between membrane trafficking and the cytoskeleton. Given the distinct expression patterns of the different syndapins and their splice forms, these proteins could have isoform-specific functions.

The spectrin family member Syne-1 functions in retrograde transport from Golgi to ER

To address the function of the Golgi- and nuclear envelope-localized spectrin family member synaptic nuclear envelope protein-1 (Syne-1), we expressed two separate recombinant fragments derived from the central portion of the molecule. Both of these fragments were predicted to act as dominant negative inhibitors of Syne-1 function at the Golgi. One of the fragments was previously shown to bind the Golgi complex. The other fragment was found to form microtubule-associated puncta that sequester endogenous Syne-1. Expression of either fragment resulted in a cell type-specific alteration in the structure of the Golgi complex, which appeared to collapse into a compact juxtanuclear structure in some cell types but not others. These fragments were expressed in cultured cells and their effects on Golgi function were examined. Expression of both dominant negative Syne-1 fragments blocked recycling of the endoplasmic reticulum (ER) resident protein disulfide isomerase (PDI), which accumulated in the Golgi complex. In addition, we found that fragment expression altered the distribution of the KDEL receptor and the COP-I coat protein beta-COP, two proteins known to be involved in regulating the retrograde pathway. We conclude that these results indicate a role for Syne-1 in facilitating retrograde vesicular trafficking from the Golgi to the ER.

Fanconi anemia cell lines deficient in alphaII spectrin express normal levels of alphaII spectrin mRNA

Fanconi anemia is a genetic disorder characterized by hypersensitivity to DNA interstrand cross-linking agents and a defect in the ability to repair this type of damage. This deficiency correlates with reduced levels of alphaII spectrin, a structural protein involved in the repair of DNA interstrand cross-links. The present study addresses the question of whether the reduced levels of alphaII spectrin in FA-A, FA-C, and FA-G cells are due to reduced expression of this protein and/or due to differences in the three regions of alternate splicing of alphaII spectrin mRNA. Relative quantitative RT-PCR showed that levels of alphaII spectrin mRNA in the three FA cell lines were similar to normal as were the sites of alternative mRNA splicing. These results indicate that decreased levels of alphaII spectrin in these FA cell lines are not due to reduced expression of alphaII spectrin mRNA or due to differences in regions of alternate splicing of these transcripts, but rather appear to be related to reduced stability of alphaII spectrin in these cell lines.

Golgi localization of Syne-1

We have previously identified a Golgi-localized spectrin isoform by using an antibody to the beta-subunit of erythrocyte spectrin. In this study, we show that a screen of a lambdagt11 expression library resulted in the isolation of an approximately 5-kb partial cDNA from a Madin-Darby bovine kidney (MDBK) cell line, which encoded a polypeptide of 1697 amino acids with low, but detectable, sequence homology to spectrin (37%). A blast search revealed that this clone overlaps with the 5' end of a recently identified spectrin family member Syne-1B/Nesprin-1beta, an alternately transcribed gene with muscle-specific forms that bind acetylcholine receptor and associate with the nuclear envelope. By comparing the sequence of the MDBK clone with sequence data from the human genome database, we have determined that this cDNA represents a central portion of a very large gene ( approximately 500 kb), encoding an approximately 25-kb transcript that we refer to as Syne-1. Syne-1 encodes a large polypeptide (8406 amino acids) with multiple spectrin repeats and a region at its amino terminus with high homology to the actin binding domains of conventional spectrins. Golgi localization for this spectrin-like protein was demonstrated by expression of epitope-tagged fragments in MDBK and COS cells, identifying two distinct Golgi binding sites, and by immunofluorescence microscopy by using several different antibody preparations. One of the Golgi binding domains on Syne-1 acts as a dominant negative inhibitor that alters the structure of the Golgi complex, which collapses into a condensed structure near the centrosome in transfected epithelial cells. We conclude that the Syne-1 gene is expressed in a variety of forms that are multifunctional and are capable of functioning at both the Golgi and the nuclear envelope, perhaps linking the two organelles during muscle differentiation.

Fragmentation of the Golgi apparatus. A role for beta III spectrin and synthesis of phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) synthesis has been implicated in maintaining the function of the Golgi apparatus. Here we demonstrate that the inhibition of PtdIns(4,5)P(2) synthesis in vitro in response to primary alcohol treatment and the kinetics of Golgi fragmentation in vivo were very rapid and tightly coupled. Preloading Golgi membranes with short chain phosphatidic acid abrogated the alcohol-mediated inhibition of PtdIns(4,5)P(2) synthesis in vitro. We also show that fragmentation of the Golgi apparatus in response to diminished PtdIns(4,5)P(2) synthesis correlated with both the phosphorylation of a Golgi form of beta III spectrin, a PtdIns(4,5)P(2)-interacting protein, and changes in its intracellular redistribution. The data are consistent with a model suggesting that the decreased PtdIns(4,5)P(2) synthesis and the phosphorylation state of beta III spectrin modulate the structural integrity of the Golgi apparatus.

The spectrin-ankyrin skeleton controls CD45 surface display and interleukin-2 production

With T cell receptor stimulation, intracellular pools of CD45 and spectrin move to the surface. These processes are coupled. In both peripheral lymphocytes and Jurkat T cells, betaI spectrin and ankyrin associate with CD45. In Jurkat T cells, betaI spectrin peptides suppress surface recruitment of CD45 and CD3 and abrogate T cell activation. Other glycoproteins such as CD43 are not altered by the spectrin peptides. Spectrin's effects are mediated by ankyrin, which binds directly to the cytoplasmic domain of CD45 (K(d) = 4.3 +/- 3.0 nM). These data reveal a novel and unexpected contribution of the spectrin-ankyrin skeleton to the control of T lymphocyte function.

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.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

Viscoelastic properties of actin-coated membranes

In living cells, cytoskeletal filaments interact with the plasma membrane to form structures that play a key role in cell shape and mechanical properties. To study the interaction between these basic components, we designed an in vitro self-assembled network of actin filaments attached to the outer surface of giant unilamellar vesicles. Optical tweezers and single-particle tracking experiments are used to study the rich dynamics of these actin-coated membranes (ACM). We show that microrheology studies can be carried out on such an individual microscopic object. The principle of the experiment consists in measuring the thermally excited position fluctuations of a probe bead attached biochemically to the membrane. We propose a model that relates the power spectrum of these thermal fluctuations to the viscoelastic properties of the membrane. The presence of the actin network modifies strongly the membrane dynamics with respect to a fluid, lipid bilayer one. It induces first a finite (omega=0) two-dimensional (2D) shear modulus G(0)(2D) approximately 0.5 to 5 microN/m in the membrane plane. Moreover, the frequency dependence at high frequency of the shear modulus [G(')(2D)(f ) approximately f(0.85+/-0.07)] and of the bending modulus (kappa(ACM)(f) approximately f(0.55+/-0.21)) demonstrate the viscoelastic behavior of the composite membrane. These results are consistent with a common exponent of 0.75 for both moduli as expected from our model and from prior measurements on actin solutions.

betaIV spectrin, a new spectrin localized at axon initial segments and nodes of ranvier in the central and peripheral nervous system

We report the identification of betaIV spectrin, a novel spectrin isolated as an interactor of the receptor tyrosine phosphatase-like protein ICA512. The betaIV spectrin gene is located on human and mouse chromosomes 19q13.13 and 7b2, respectively. Alternative splicing of betaIV spectrin generates at least four distinct isoforms, numbered betaIVSigma1-betaIVSigma4 spectrin. The longest isoform (betaIVSigma1 spectrin) includes an actin-binding domain, followed by 17 spectrin repeats, a specific domain in which the amino acid sequence ERQES is repeated four times, several putative SH3-binding sites and a pleckstrin homology domain. betaIVSigma2 and betaIVSigma3 spectrin encompass the NH(2)- and COOH-terminal halves of betaIVSigma1 spectrin, respectively, while betaIVSigma4 spectrin lacks the ERQES and the pleckstrin homology domain. Northern blots revealed an abundant expression of betaIV spectrin transcripts in brain and pancreatic islets. By immunoblotting, betaIVSigma1 spectrin is recognized as a protein of 250 kD. Anti-betaIV spectrin antibodies also react with two additional isoforms of 160 and 140 kD. These isoforms differ from betaIVSigma1 spectrin in terms of their distribution on subcellular fractionation, detergent extractability, and phosphorylation. In islets, the immunoreactivity for betaIV spectrin is more prominent in alpha than in beta cells. In brain, betaIV spectrin is enriched in myelinated neurons, where it colocalizes with ankyrin(G) 480/270-kD at axon initial segments and nodes of Ranvier. Likewise, betaIV spectrin is concentrated at the nodes of Ranvier in the rat sciatic nerve. In the rat hippocampus, betaIVSigma1 spectrin is detectable from embryonic day 19, concomitantly with the appearance of immunoreactivity at the initial segments. Thus, we suggest that betaIVSigma1 spectrin interacts with ankyrin(G) 480/270-kD and participates in the clustering of voltage-gated Na(+) channels and cell-adhesion molecules at initial segments and nodes of Ranvier.

Lava lamp, a novel peripheral golgi protein, is required for Drosophila melanogaster cellularization

Drosophila cellularization and animal cell cytokinesis rely on the coordinated functions of the microfilament and microtubule cytoskeletal systems. To identify new proteins involved in cellularization and cytokinesis, we have conducted a biochemical screen for microfilament/microtubule-associated proteins (MMAPs). 17 MMAPs were identified; seven have been previously implicated in cellularization and/or cytokinesis, including KLP3A, Anillin, Septins, and Dynamin. We now show that a novel MMAP, Lava Lamp (Lva), is also required for cellularization. Lva is a coiled-coil protein and, unlike other proteins previously implicated in cellularization or cytokinesis, it is Golgi associated. Our functional analysis shows that cellularization is dramatically inhibited upon injecting anti-Lva antibodies (IgG and Fab) into embryos. In addition, we show that brefeldin A, a potent inhibitor of membrane trafficking, also inhibits cellularization. Biochemical analysis demonstrates that Lva physically interacts with the MMAPs Spectrin and CLIP190. We suggest that Lva and Spectrin may form a Golgi-based scaffold that mediates the interaction of Golgi bodies with microtubules and facilitates Golgi-derived membrane secretion required for the formation of furrows during cellularization. Our results are consistent with the idea that animal cell cytokinesis depends on both actomyosin-based contraction and Golgi-derived membrane secretion.

Kinectin-kinesin binding domains and their effects on organelle motility

Intracellular organelle motility involves motor proteins that move along microtubules or actin filaments. One of these motor proteins, kinesin, was proposed to bind to kinectin on membrane organelles during movement. Whether kinectin is the kinesin receptor on organelles with a role in organelle motility has been controversial. We have characterized the sites of interaction between human kinectin and conventional kinesin using in vivo and in vitro assays. The kinectin-binding domain on the kinesin tail partially overlaps its head-binding domain and the myosin-Va binding domain. The kinesin-binding domain on kinectin resides near the COOH terminus and enhances the microtubule-stimulated kinesin-ATPase activity, and the overexpression of the kinectin-kinesin binding domains inhibited kinesin-dependent organelle motility in vivo. These data, when combined with other studies, suggest a role for kinectin in organelle motility.

ADPKD: a human disease altering Golgi function and basolateral exocytosis in renal epithelia

Epithelial cells explanted from autosomal dominant polycystic kidney disease (ADPKD) tissue exhibit impaired exocytosis, specifically between the Golgi and basolateral membrane (Charron A, Nakamura B, Bacallo R, Wandinger-Ness A. Compromised cytoarchitecture and polarized trafficking in autosomal dominant polycystic kidney disease cells. J Cell Biol 2000; 148: 111-124.). Here the defect is shown to result in the accumulation of the basolateral transport marker vesicular stomatitis virus (VSV) G protein in the Golgi complex. Golgi complex morphology is consequently altered in the disease cells, evident in the noticeable fenestration and dilation of the cisternae. Further detailed microscopic evaluation of normal kidney and ADPKD cells revealed that ineffective basolateral exocytosis correlated with modulations in the localization of select post-Golgi transport effectors. The cytosolic coat proteins p200/myosin II and caveolin exhibited enhanced association with the cytoskeleton or the Golgi of the disease cells, respectively. Most cytoskeletal components with known roles in vesicle translocation or formation were normally arrayed with the exception of Golgi beta-spectrin, which was less prevalent on vesicles. The rab8 GTPase, important for basolateral vesicle targeting, was redistributed from the perinuclear Golgi region to disperse vesicles in ADPKD cells. At the basolateral membrane of ADPKD cells, there was a notable loss of the exocyst components sec6/sec8 and an unidentified syntaxin. It is postulated that dysregulated basolateral transport effector function precipitates the disruption of basolateral exocytosis and dilation of the ADPKD cell Golgi as basolateral cargo accumulates within the cisternae.

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.

Integrating centrosome structure with protein composition and function in animal cells

The centrosome found in animal cells is a complex and dynamic organelle that functions as the major microtubule organizing center. Structural studies over the past several decades have defined the primary structural features of the centrosome but recent studies are now beginning to reveal structural detail previously unknown. Concurrent with these studies has been an explosion in the identification of the proteins that reside within the centrosome. Our growing understanding of how protein composition integrates with centrosome structure and hence with function is the focus of this review.

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.

New insights into functions of erythroid proteins in nonerythroid cells

This article presents new insights into potential roles that three erythrocyte cytoskeletal proteins, protein 4.1, ankyrin, and spectrin, may play in nonerythroid nucleated cells. Each of these proteins is encoded by several closely related genes characterized by complex alternative splicing of their pre-mRNA, thus resulting in the cellular expression of a broad repertoire of isoforms that can adopt tissue- and cell-specific distribution. This could account for the presence of skeletal networks in intracellular organelles such as lysosomes, the Golgi apparatus, or the nucleus. In addition to providing structural support to cell membranes, these cytoskeletal proteins regulate the functions of various transmembrane proteins they interact with, in particular ion channels, as well as the activity of membrane-bound enzymes. Thus, they appear to be key players in major unsuspected cell functions such as protein sorting, dynamics of nuclear architecture during mitosis, or regulation of signal transduction pathways.

Dynamic association of cytoplasmic dynein heavy chain 1a with the Golgi apparatus and intermediate compartment

Microtubule motors, such as the minus end-directed motor, cytoplasmic dynein, play an important role in maintaining the integrity, intracellular location, and function of the Golgi apparatus, as well as in the translocation of membrane between the endoplasmic reticulum and Golgi apparatus. We have immunolocalised conventional cytoplasmic dynein heavy chain to the Golgi apparatus in cultured vertebrate cells. In addition, we present evidence that cytoplasmic dynein heavy chain cycles constitutively between the endoplasmic reticulum and Golgi apparatus: it colocalises partially with the intermediate compartment, it is found on nocodazole-induced peripheral Golgi elements and, most strikingly, on Brefeldin A-induced tubules that are moving towards microtubule plus ends. The direction of movement of membrane between the endoplasmic reticulum and Golgi apparatus is therefore unlikely to be regulated by controlling motor-membrane interactions: rather, the motors probably remain bound throughout the whole cycle, with their activity being modulated instead. We also report that the overexpression of p50/dynamitin results in the loss of cytoplasmic dynein heavy chain from the membrane of peripheral Golgi elements. These results explain how dynamitin overexpression causes the inhibition of endoplasmic reticulum-to-Golgi transport complex movement towards the centrosomal region, and support the general model that an intact dynactin complex is required for cytoplasmic dynein binding to all cargoes.

Ankyrin-B is required for intracellular sorting of structurally diverse Ca2+ homeostasis proteins

This report describes a congenital myopathy and major loss of thymic lymphocytes in ankyrin-B (-/-) mice as well as dramatic alterations in intracellular localization of key components of the Ca(2+) homeostasis machinery in ankyrin-B (-/-) striated muscle and thymus. The sarcoplasmic reticulum (SR) and SR/T-tubule junctions are apparently preserved in a normal distribution in ankyrin-B (-/-) skeletal muscle based on electron microscopy and the presence of a normal pattern of triadin and dihydropyridine receptor. Therefore, the abnormal localization of SR/ER Ca ATPase (SERCA) and ryanodine receptors represents a defect in intracellular sorting of these proteins in skeletal muscle. Extrapolation of these observations suggests defective targeting as the basis for abnormal localization of ryanodine receptors, IP3 receptors and SERCA in heart, and of IP3 receptors in the thymus of ankyrin-B (-/-) mice. Mis-sorting of SERCA 2 and ryanodine receptor 2 in ankyrin-B (-/-) cardiomyocytes is rescued by expression of 220-kD ankyrin-B, demonstrating that lack of the 220-kD ankyrin-B polypeptide is the primary defect in these cells. Ankyrin-B is associated with intracellular vesicles, but is not colocalized with the bulk of SERCA 1 or ryanodine receptor type 1 in skeletal muscle. These data provide the first evidence of a physiological requirement for ankyrin-B in intracellular targeting of the calcium homeostasis machinery of striated muscle and immune system, and moreover, support a catalytic role that does not involve permanent stoichiometric complexes between ankyrin-B and targeted proteins. Ankyrin-B is a member of a family of adapter proteins implicated in restriction of diverse proteins to specialized plasma membrane domains. Similar mechanisms involving ankyrins may be essential for segregation of functionally defined proteins within specialized regions of the plasma membrane and within the Ca(2+) homeostasis compartment of the ER.

Cytoplasmic dynein and microtubule transport in the axon: the action connection

The neuron uses two families of microtubule-based motors for fast axonal transport, kinesin, and cytoplasmic dynein. Cytoplasmic dynein moves membranous organelles from the distal regions of the axon to the cell body. Because dynein is synthesized in the cell body, it must first be delivered to the axon tip. It has recently been shown that cytoplasmic dynein is moved from the cell body along the axon by two different mechanisms. A small amount is associated with fast anterograde transport, the membranous organelles moved by kinesin. Most of the dynein is transported in slow component b, the actin-based transport compartment. Dynactin, a protein complex that binds dynein, is also transported in slow component b. The dynein in slow component b binds to microtubules in an ATP-dependent manner in vitro, suggesting that this dynein is enzymatically active. The finding that functionally active dynein, and dynactin, are associated with the actin-based transport compartment suggests a mechanism whereby dynein anchored to the actin cytoskeleton via dynactin provides the motive force for microtubule movement in the axon.

Morphological changes in the Golgi complex correlate with actin cytoskeleton rearrangements

In this report we have studied the morphological changes of the Golgi complex (GC) that specifically accompany F-actin reorganizations. In starved rat RBL-2H3 tumor mast cells, the GC, that was visualized at immunofluorescence level with antibodies raised against the Golgi-resident proteins giantin, mannosidase II, or TGN-38, showed a compacted morphology with a supranuclear positioning. Concomitant to membrane ruffle formation induced by epidermal growth factor (EGF) or phorbol 12-myristate 13-acetate (PMA), and stress fiber formation induced by lysophosphatidic acid (LPA), specific GC morphological changes were observed. When cells were stimulated with EGF or PMA, the compacted GC morphology was transformed into a reticular network that was extended towards the cell periphery. When cells were incubated with LPA, the GC acquired a characteristic ring-shaped morphology. Brefeldin A (BFA) did not affect the PMA- or LPA-induced membrane ruffling and stress fiber formation, respectively, indicating that actin rearrangements occurred independent of the presence of the GC. Upon BFA removal, the presence of PMA or LPA during the recovery process induced the GC to acquire the morphological appearance described above for each agent. Moreover, the PMA- but not the LPA-induced GC rearrangements were sensitive to the actin perturbing agents cytochalasin D and jasplakinolide. When cells were preincubated with the phosphatidylinositide 3-kinase (PI3K) inhibitors wortmannin or LY294002, the PMA-induced GC morphological changes were inhibited but not membrane ruffles. Finally, the PMA-induced increase in the post-Golgi transport of glycosaminoglycans to the cell surface was not altered by cytochalasin D or jasplakinolide. Altogether, these data suggest that: (1) the shape of the GC is influenced by the 3D arrangement of actin microfilaments; (2) PI3K regulates the association of the GC with actin microfilaments; and (3) actin microfilaments are not essential for the post-Golgi transport to the plasma membrane.

Fusogenic domains of golgi membranes are sequestered into specialized regions of the stack that can be released by mechanical fragmentation

A well-characterized cell-free assay that reconstitutes Golgi transport is shown to require physically fragmented Golgi fractions for maximal activity. A Golgi fraction containing large, highly stacked flattened cisternae associated with coatomer-rich components was inactive in the intra-Golgi transport assay. In contrast, more fragmented hepatic Golgi fractions of lower purity were highly active in this assay. Control experiments ruled out defects in glycosylation, the presence of excess coatomer or inhibitory factors, as well as the lack or consumption of limiting diffusible factors as responsible for the lower activity of intact Golgi fractions. Neither Brefeldin A treatment, preincubation with KCl (that completely removed associated coatomer) or preincubation with imidazole buffers that caused unstacking, activated stacked fractions for transport. Only physical fragmentation promoted recovery of Golgi fractions active for transport in vitro. Rate-zonal centrifugation partially separated smaller transport-active Golgi fragments with a unique v-SNARE pattern, away from the bulk of Golgi-derived elements identified by their morphology and content of Golgi marker enzymes (N-acetyl glucosaminyl and galactosyl transferase activities). These fragments released during activation likely represent intra-Golgi continuities involved in maintaining the dynamic redistribution of resident enzymes during rapid anterograde transport of secretory cargo through the Golgi in vivo.