Characterization of the microtubule binding domain of microtubule actin crosslinking factor (MACF): identification of a novel group of microtubule associated proteins

The molecular composition and function of desmosomes

Desmosomes are intercellular adhesive junctions that are particularly prominent in tissues experiencing mechanical stress, such as the heart and epidermis. Whereas the related adherens junction links actin to calcium-dependent adhesion molecules known as classical cadherins, desmosomes link intermediate filaments (IF) to the related subfamily of desmosomal cadherins. By tethering these stress-bearing cytoskeletal filaments to the plasma membrane, desmosomes serve as integrators of the IF cytoskeleton throughout a tissue. Recent evidence suggests that IF attachment in turn strengthens desmosomal adhesion. This collaborative arrangement results in formation of a supracellular network, which is critical for imparting mechanical integrity to tissues. Diseases and animal models targeting desmosomal components highlight the importance of desmosomes in development and tissue integrity, while the downregulation of individual protein components in cancer metastasis and wound healing suggests their importance in cell homeostasis. This chapter will provide an update on desmosome composition, function, and regulation, and will also discuss recent work which raises the possibility that desmosome proteins do more than play a structural role in tissues where they reside.

Calcium tips the balance: a microtubule plus end to lattice binding switch operates in the carboxyl terminus of BPAG1n4

Microtubules (MTs) are integral to numerous cellular functions, such as cell adhesion, differentiation and intracellular transport. Their dynamics are largely controlled by diverse MT-interacting proteins, but the signalling mechanisms that regulate these interactions remain elusive. In this report, we identify a rapid, calcium-regulated switch between MT plus end interaction and lattice binding within the carboxyl terminus of BPAG1n4. This switch is EF-hand dependent, and mutations of the EF-hands abolish this dynamic behaviour. Our study thus uncovers a new, calcium-dependent regulatory mechanism for a spectraplakin, BPAG1n4, at the MT plus end.

A molecular network for the transport of the TI-VAMP/VAMP7 vesicles from cell center to periphery

The compartmental organization of eukaryotic cells is maintained dynamically by vesicular trafficking. SNARE proteins play a crucial role in intracellular membrane fusion and need to be targeted to their proper donor or acceptor membrane. The molecular mechanisms that allow for the secretory vesicles carrying the v-SNARE TI-VAMP/VAMP7 to leave the cell center, load onto microtubules, and reach the periphery to mediate exocytosis are largely unknown. Here, we show that the TI-VAMP/VAMP7 partner Varp, a Rab21 guanine nucleotide exchange factor, interacts with GolginA4 and the kinesin 1 Kif5A. Activated Rab21-GTP in turn binds to MACF1, an actin and microtubule regulator, which is itself a partner of GolginA4. These components are required for directed movement of TI-VAMP/VAMP7 vesicles from the cell center to the cell periphery. The molecular mechanisms uncovered here suggest an integrated view of the transport of vesicles carrying a specific v-SNARE toward the cell surface.

Spectraplakins: master orchestrators of cytoskeletal dynamics

The dynamics of different cytoskeletal networks are coordinated to bring about many fundamental cellular processes, from neuronal pathfinding to cell division. Increasing evidence points to the importance of spectraplakins in integrating cytoskeletal networks. Spectraplakins are evolutionarily conserved giant cytoskeletal cross-linkers, which belong to the spectrin superfamily. Their genes consist of multiple promoters and many exons, yielding a vast array of differential splice forms with distinct functions. Spectraplakins are also unique in their ability to associate with all three elements of the cytoskeleton: F-actin, microtubules, and intermediate filaments. Recent studies have begun to unveil their role in a wide range of processes, from cell migration to tissue integrity.

Untethering the nuclear envelope and cytoskeleton: biologically distinct dystonias arising from a common cellular dysfunction

Most cases of early onset DYT1 dystonia in humans are caused by a GAG deletion in the TOR1A gene leading to loss of a glutamic acid (ΔE) in the torsinA protein, which underlies a movement disorder associated with neuronal dysfunction without apparent neurodegeneration. Mutation/deletion of the gene (Dst) encoding dystonin in mice results in a dystonic movement disorder termed dystonia musculorum, which resembles aspects of dystonia in humans. While torsinA and dystonin proteins do not share modular domain architecture, they participate in a similar function by modulating a structural link between the nuclear envelope and the cytoskeleton in neuronal cells. We suggest that through a shared interaction with the nuclear envelope protein nesprin-3α, torsinA and the neuronal dystonin-a2 isoform comprise a bridge complex between the outer nuclear membrane and the cytoskeleton, which is critical for some aspects of neuronal development and function. Elucidation of the overlapping roles of torsinA and dystonin-a2 in nuclear/endoplasmic reticulum dynamics should provide insights into the cellular mechanisms underlying the dystonic phenotype.

Microtubule stability, Golgi organization, and transport flux require dystonin-a2-MAP1B interaction

Loss of interaction between the dystonin-a2 isoform and the microtubule-associated protein MAP1B induces microtubule instability and trafficking defects that may underlie certain neuropathies.

Loss of function of dystonin cytoskeletal linker proteins causes neurodegeneration in dystonia musculorum (dt) mutant mice. Although much investigation has focused on understanding dt pathology, the diverse cellular functions of dystonin isoforms remain poorly characterized. In this paper, we highlight novel functions of the dystonin-a2 isoform in mediating microtubule (MT) stability, Golgi organization, and flux through the secretory pathway. Using dystonin mutant mice combined with isoform-specific loss-of-function analysis, we found dystonin-a2 bound to MT-associated protein 1B (MAP1B) in the centrosomal region, where it maintained MT acetylation. In dt neurons, absence of the MAP1B–dystonin-a2 interaction resulted in altered MAP1B perikaryal localization, leading to MT deacetylation and instability. Deacetylated MT accumulation resulted in Golgi fragmentation and prevented anterograde trafficking via motor proteins. Maintenance of MT acetylation through trichostatin A administration or MAP1B overexpression mitigated the observed defect. These cellular aberrations are apparent in prephenotype dorsal root ganglia and primary sensory neurons from dt mice, suggesting they are causal in the disorder.

MAP1B and clathrin are novel interacting partners of the giant cyto-linker dystonin

Dystonin is a large multidomain cytoskeletal-associated protein that plays an essential role in the nervous system. Loss of dystonin results in neuromuscular dysfunction and early death in a mouse mutant called dystonia musculorum. Conserved among related proteins, the plakin domain is a defining feature of all major dystonin isoforms, yet its interactions have not been explored in detail. The purpose of the present study was to identify novel interacting partners of the plakin domain of the neuronal isoform of dystonin (dystonin-a). Newly identified interacting proteins discovered through a pull-down assay were validated using coimmunoprecipitation, coimmunofluorescence, and proximity ligation assays. Microtubule associated protein 1B (MAP1B), a microtubule stabilizing protein, and clathrin heavy chain, the major component of the clathrin triskelion, were identified as interaction partners for dystonin-a. Increased levels of phosphorylated MAP1B suggest a misregulation of MAP1B and a potentially novel component of the dt pathology. This work will further facilitate our understanding of how cytoskeletal proteins can affect and regulate neurodegenerative disorders.

Growth-arrest-specific protein 2 inhibits cell division in Xenopus embryos

BACKGROUND

Growth-arrest-specific 2 gene was originally identified in murine fibroblasts under growth arrest conditions. Furthermore, serum stimulation of quiescent, non-dividing cells leads to the down-regulation of gas2 and results in re-entry into the cell cycle. Cytoskeleton rearrangements are critical for cell cycle progression and cell division and the Gas2 protein has been shown to co-localize with actin and microtubules in interphase mammalian cells. Despite these findings, direct evidence supporting a role for Gas2 in the mechanism of cell division has not been reported.

METHODOLOGY AND PRINCIPAL FINDINGS

To determine whether the Gas2 protein plays a role in cell division, we over-expressed the full-length Gas2 protein and Gas2 truncations containing either the actin-binding CH domain or the tubulin-binding Gas2 domain in Xenopus laevis embryos. We found that both the full-length Gas2 protein and the Gas2 domain, but not the CH domain, inhibited cell division and resulted in multinucleated cells. The observation that Gas2 domain alone can arrest cell division suggests that Gas2 function is mediated by microtubule binding. Gas2 co-localized with microtubules at the cell cortex of Gas2-injected Xenopus embryos using cryo-confocal microscopy and co-sedimented with microtubules in cytoskeleton co-sedimentation assays. To investigate the mechanism of Gas2-induced cell division arrest, we showed, using a wound-induced contractile array assay, that Gas2 stabilized microtubules. Finally, electron microscopy studies demonstrated that Gas2 bundled microtubules into higher-order structures.

CONCLUSION AND SIGNIFICANCE

Our experiments show that Gas2 inhibits cell division in Xenopus embryos. We propose that Gas2 function is mediated by binding and bundling microtubules, leading to cell division arrest.

VAB-10 spectraplakin acts in cell and nuclear migration in Caenorhabditis elegans

Cytoskeletal regulation is important in cell migration. The Caenorhabditis elegans gonadal distal tip cells (DTCs) offer a simple model with which to investigate the mechanism of cell migration in organogenesis. Here, we report that one of the spectraplakin isoforms, VAB-10B1, plays an essential role in cell and nuclear migration of DTCs by regulating the actin and microtubule (MT) cytoskeleton. In the vab-10(tk27) mutant, which lacks VAB-10B1, alignment of filamentous (F)-actin and MTs was weakly and severely disorganized, respectively, which resulted in a failure to translocate the DTC nucleus and a premature termination of DTC migration. An MT growing-tip marker, EBP-2-GFP, revealed that polarized outgrowth of MTs towards the nuclei of migrating DTCs was strikingly impaired in tk27 animals. A vab-10 mini-gene encoding only the actin- and MT-binding domains significantly rescued the gonadal defects, suggesting that VAB-10B1 has a role in linking actin and MT filaments. These results suggest that VAB-10B1/spectraplakin regulates the polarized alignment of MTs, possibly by linking F-actin and MTs, which enables normal nuclear translocation and cell migration of DTCs.

Coordinated interactions between actin and microtubules through crosslinkers in neurite retraction induced by lysophosphatidic acid

Neurite development requires rearrangement of cytoskeletal elements, which are mechanically and functionally integrated with each other. Although the process of how an extracellular signal induces rearrangement of a single element has been closely examined, the mechanisms by which the signal regulates cytoskeletal integration during cell shape changes are poorly understood. We previously reported that lysophosphatidic acid (LPA) induces actin polymerization-dependent microtubule (MT) rearrangement, leading to neurite retraction in cultured neurons. Here we examined whether the crosslinker proteins were involved in LPA-induced neurite retraction using immortalized mouse neuroblast TR cells. When the MT-binding domains of MACF (MT actin-crosslinking factor) were exogenously expressed in TR cells, MTs were found to be stabilized and become resistant to exposure to LPA. On the other hand, expression of MT-associated protein 2c showed no effect on LPA-induced neurite retraction. These findings suggest that MACF is involved in actin-dependent MT rearrangement during LPA-induced neurite retraction.

Characterization of G2L3 (GAS2-like 3), a new microtubule- and actin-binding protein related to spectraplakins

The microtubule (MT) and actin cytoskeletons are fundamental to cell integrity, because they control a host of cellular activities, including cell division, growth, polarization, and migration. Proteins involved in mediating the cross-talk between MT and actin cytoskeletons are key to many cellular processes and play important physiological roles. We identified a new member of the GAS2 family of MT-actin cross-linking proteins, named G2L3 (GAS2-like 3). We show that GAS2-like 3 is widely conserved throughout evolution and is ubiquitously expressed in human tissues. GAS2-like 3 interacts with filamentous actin and MTs via its single calponin homology type 3 domain and C terminus, respectively. Interestingly, the role of the putative MT-binding GAS2-related domain is to modulate the binding of GAS2-like 3 to both filamentous actin and MTs. This is in contrast to GAS2-related domains found in related proteins, where it functions as a MT-binding domain. Furthermore, we show that tubulin acetylation drives GAS2-like 3 localization to MTs and may provide functional insights into the role of GAS2-like 3.

Skin stem cells orchestrate directional migration by regulating microtubule-ACF7 connections through GSK3β

Homeostasis and wound healing rely on stem cells (SCs) whose activity and directed migration are often governed by Wnt signaling. In dissecting how this pathway integrates with the necessary downstream cytoskeletal dynamics, we discovered that GSK3β, a kinase inhibited by Wnt signaling, directly phosphorylates ACF7, a > 500 kDa microtubule-actin crosslinking protein abundant in hair follicle stem cells (HF-SCs). We map ACF7's GSK3β sites to the microtubule-binding domain and show that phosphorylation uncouples ACF7 from microtubules. Phosphorylation-refractile ACF7 rescues overall microtubule architecture, but phosphorylation-constitutive mutants do not. Neither mutant rescues polarized movement, revealing that phospho-regulation must be dynamic. This circuitry is physiologically relevant and depends upon polarized GSK3β inhibition at the migrating front of SCs/progeny streaming from HFs during wound repair. Moreover, only ACF7 and not GSKβ-refractile-ACF7 restore polarized microtubule-growth and SC-migration to ACF7 null skin. Our findings provide insights into how this conserved spectraplakin integrates signaling, cytoskeletal dynamics, and polarized locomotion of somatic SCs.

Plectin interacts with the rod domain of type III intermediate filament proteins desmin and vimentin

Plectin is a versatile cytolinker protein critically involved in the organization of the cytoskeletal filamentous system. The muscle-specific intermediate filament (IF) protein desmin, which progressively replaces vimentin during differentiation of myoblasts, is one of the important binding partners of plectin in mature muscle. Defects of either plectin or desmin cause muscular dystrophies. By cell transfection studies, yeast two-hybrid, overlay and pull-down assays for binding analysis, we have characterized the functionally important sequences for the interaction of plectin with desmin and vimentin. The association of plectin with both desmin and vimentin predominantly depended on its fifth plakin repeat domain and downstream linker region. Conversely, the interaction of desmin and vimentin with plectin required sequences contained within the segments 1A-2A of their central coiled-coil rod domain. This study furthers our knowledge of the interaction between plectin and IF proteins important for maintenance of cytoarchitecture in skeletal muscle. Moreover, binding of plectin to the conserved rod domain of IF proteins could well explain its broad interaction with most types of IFs.

The spectraplakin Short stop is an actin-microtubule cross-linker that contributes to organization of the microtubule network

The dynamics of actin and microtubules are coordinated in a variety of cellular and morphogenetic processes; however, little is known about the molecules mediating this cytoskeletal cross-talk. We are studying Short stop (Shot), the sole Drosophila spectraplakin, as a model actin-microtubule cross-linking protein. Spectraplakins are an ancient family of giant cytoskeletal proteins that are essential for a diverse set of cellular functions; yet, we know little about the dynamics of spectraplakins and how they bridge actin filaments and microtubules. In this study we describe the intracellular dynamics of Shot and a structure-function analysis of its role as a cytoskeletal cross-linker. We find that Shot interacts with microtubules using two different mechanisms. In the cell interior, Shot binds growing plus ends through an interaction with EB1. In the cell periphery, Shot associates with the microtubule lattice via its GAS2 domain, and this pool of Shot is actively engaged as a cross-linker via its NH(2)-terminal actin-binding calponin homology domains. This cross-linking maintains microtubule organization by resisting forces that produce lateral microtubule movements in the cytoplasm. Our results provide the first description of the dynamics of these important proteins and provide key insight about how they function during cytoskeletal cross-talk.

Nervous-tissue-specific elimination of microtubule-actin crosslinking factor 1a results in multiple developmental defects in the mouse brain

The microtubule-actin crosslinking factor 1 (MACF1) is a ubiquitous cytoskeletal linker protein with multiple spliced isoforms expressed in different tissues. The MACF1a isoform contains microtubule and actin-binding regions and is expressed at high levels in the nervous system. Macf1-/- mice are early embryonic lethal and hence the role of MACF1 in the nervous system could not be determined. We have specifically knocked out MACF1a in the developing mouse nervous system using Cre/loxP technology. Mutant mice died within 24-36h after birth of apparent respiratory distress. Their brains displayed a disorganized cerebral cortex with a mixed layer structure, heterotopia in the pyramidal layer of the hippocampus, disorganized thalamocortical and corticofugal fibers, and aplastic anterior and hippocampal commissures. Embryonic neurons showed a defect in traversing the cortical plate. Our data suggest a critical role for MACF1 in neuronal migration that is dependent on its ability to interact with both microfilaments and microtubules.

Mago Nashi, Tsunagi/Y14, and Ranshi form a complex that influences oocyte differentiation in Drosophila melanogaster

During Drosophila melanogaster oogenesis, a germline stem cell divides forming a cyst of 16 interconnected cells. One cell enters the oogenic pathway, and the remaining 15 differentiate as nurse cells. Although directed transport and localization of oocyte differentiation factors within the single cell are indispensible for selection, maintenance, and differentiation of the oocyte, the mechanisms regulating these events are poorly understood. Mago Nashi and Tsunagi/Y14, core components of the exon junction complex (a multiprotein complex assembled on spliced RNAs), are essential for restricting oocyte fate to a single cell and for localization of oskar mRNA. Here we provide evidence that Mago Nashi and Tsunagi/Y14 form an oogenic complex with Ranshi, a protein with a zinc finger-associated domain and zinc finger domains. Genetic analyses of ranshi reveal that (1) 16-cell cysts are formed, (2) two cells retain synaptonemal complexes, (3) all cells have endoreplicated DNA (as observed in nurse cells), and (4) oocyte-specific cytoplasmic markers accumulate and persist within a single cell but are not localized within the posterior pole of the presumptive oocyte. Our results indicate that Ranshi interacts with the exon junction complex to localize components essential for oocyte differentiation within the posterior pole of the presumptive oocyte.

Large gradient high magnetic field affects the association of MACF1 with actin and microtubule cytoskeleton

The intense inhomogeneous magnetic fields acting on the diamagnetic materials naturally present in cells can generate strong magnetic forces. We have developed a superconducting magnet platform with large gradient high magnetic field (LG-HMF), which can produce three magnetic force fields of -1360, 0, and 1312 T(2)/m, and three corresponding apparent gravity levels, namely 0, 1, and 2-g for diamagnetic materials. In this study, the effects of different magnetic force fields on osteoblast-like cells (MG-63 and MC3T3-E1) viability, microtubule actin crosslinking factor 1 (MACF1) expression and its association with cytoskeleton were investigated. Results showed that cell viability increased to different degrees after exposure to 0 or 1-g conditions for 24 h, but it decreased by about 30% under 2-g conditions compared with control conditions. An increase in MACF1 expression at the RNA or protein level was observed in osteoblast-like cells under the magnetic force field of -1360 T(2)/m (0-g) relative to 1312 T(2)/m (2-g). Under control conditions, anti-MACF1 staining was scattered in the cytoplasm and partially colocalized with actin filaments (AFs) or microtubules (MTs) in the majority of osteoblast-like cells. Under 0-g conditions, MACF1 labeling was concentrated at perinuclear region and colocalization was not apparent. The patterns of anti-MACF1 labeling on MTs varied with MTs' changing under LG-HMF environment. In conclusion, LG-HMF affects osteoblast-like cell viability, MACF1 distribution, expression, and its association with cytoskeleton to some extent.

Mouse ACF7 and drosophila short stop modulate filopodia formation and microtubule organisation during neuronal growth

Spectraplakins are large actin-microtubule linker molecules implicated in various processes, including gastrulation, wound healing, skin blistering and neuronal degeneration. Expression data for the mammalian spectraplakin ACF7 and genetic analyses of the Drosophila spectraplakin Short stop (Shot) suggest an important role during neurogenesis. Using three parallel neuronal culture systems we demonstrate that, like Shot, ACF7 is essential for axon extension and describe, for the first time, their subcellular functions during axonal growth. Firstly, both ACF7 and Shot regulate the organisation of neuronal microtubules, a role dependent on both the F-actin- and microtubule-binding domains. This role in microtubule organisation is probably the key mechanism underlying the roles of Shot and ACF7 in growth cone advance. Secondly, we found a novel role for ACF7 and Shot in regulating the actin cytoskeleton through their ability to control the formation of filopodia. This function in F-actin regulation requires EF-hand motifs and interaction with the translational regulator Krasavietz/eIF5C, indicating that the underlying mechanisms are completely different from those used to control microtubules. Our data provide the basis for the first mechanistic explanation for the role of Shot and ACF7 in the developing nervous system and demonstrate their ability to coordinate the organisation of both actin and microtubule networks during axonal growth.

Myosin II regulation during C. elegans embryonic elongation: LET-502/ROCK, MRCK-1 and PAK-1, three kinases with different roles

Myosin II plays a central role in epithelial morphogenesis; however, its role has mainly been examined in processes involving a single cell type. Here we analyze the structure, spatial requirement and regulation of myosin II during C. elegans embryonic elongation, a process that involves distinct epidermal cells and muscles. We developed novel GFP probes to visualize the dynamics of actomyosin remodeling, and found that the assembly of myosin II filaments, but not actin microfilaments, depends on the myosin regulatory light chain (MLC-4) and essential light chain (MLC-5, which we identified herein). To determine how myosin II regulates embryonic elongation, we rescued mlc-4 mutants with various constructs and found that MLC-4 is essential in a subset of epidermal cells. We show that phosphorylation of two evolutionary conserved MLC-4 serine and threonine residues is important for myosin II activity and organization. Finally, in an RNAi screen for potential myosin regulatory light chain kinases, we found that the ROCK, PAK and MRCK homologs act redundantly. The combined loss of ROCK and PAK, or ROCK and MRCK, completely prevented embryonic elongation, but a constitutively active form of MLC-4 could only rescue a lack of MRCK. This result, together with systematic genetic epistasis tests with a myosin phosphatase mutation, suggests that ROCK and MRCK regulate MLC-4 and the myosin phosphatase. Moreover, we suggest that ROCK and PAK regulate at least one other target essential for elongation, in addition to MLC-4.

Context-specific requirements of functional domains of the Spectraplakin Short stop in vivo

Spectraplakins are large multifunctional cytoskeletal interacting molecules implicated in various processes, including gastrulation, wound healing, skin blistering and neuronal degeneration. It has been speculated that the various functional domains and regions found in Spectraplakins are used in context-specific manners, a model which would provide a crucial explanation for the multifunctional nature of Spectraplakins. Here we tested this possibility by studying domain requirements of the Drosophila Spectraplakin Short stop (Shot) in three different cellular contexts in vivo: (1) neuronal growth, which requires dynamic actin-microtubule interaction; (2) formation and maintenance of tendon cells, which depends on highly stabilised arrays of actin filaments and microtubules, and (3) compartmentalisation in neurons, which is likely to involve cortical F-actin networks. Using these cellular contexts for rescue experiments with Shot deletion constructs in shot mutant background, a number of differential domain requirements were uncovered. First, binding of Shot to F-actin through the first Calponin domain is essential in neuronal contexts but dispensable in tendon cells. This finding is supported by our analyses of shot(kakP2) mutant embryos, which produce only endogenous isoforms lacking the first Calponin domain. Thus, our data demonstrate a functional relevance for these isoforms in vivo. Second, we provide the first functional role for the Plakin domain of Shot, which has a strong requirement for compartmentalisation in neurons and axonal growth, demonstrating that Plakin domains of long Spectraplakin isoforms are of functional relevance. Like the Calponin domain, also the Plakin domain is dispensable in tendon cells, and the currently assumed role of Shot as a linker of microtubules to the tendon cell surface may have to be reconsidered. Third, we demonstrate a function of Shot as an actin-microtubule linker in dendritic growth, thus shedding new light into principal growth mechanisms of this neurite type. Taken together, our data clearly support the view that Spectraplakins function in tissue-specific modes in vivo, and even domains believed to be crucial for Spectraplakin function can be dispensable in specific contexts.

Duplex (or quadruplet) CH domain containing human multidomain proteins: an inventory

In this paper, the inventory presented for singlet CH (calponin homology/actin binding) domain containing human multidomain proteins is extended to several duplex and one quadruplet CH containing forms. Invariably, the duplexes are located at the begin of the molecules. The regions connecting the two CH units suggest amino acid conservations which allows the placing of 18 duplex containing molecules into six groups wherein the gene for one member in each group created the others more recently by gene duplication. The ancient multidomain proteins, possibly, were primarily the result of an exon shuffling (transposition) mechanism that also guided the placing of the CH singlet or duplex domain at the amino end of the newly created proteins. A mechanism that creates pseudogenes could conceivably produce genes that encode multi-domain proteins. Intragenomic duplications (slippage) might have facilitated the occurrence of encoding repeats, thus allowing for the creation of multiple identical domains within one molecule. Gene duplication with subsequent modification and small domain gene recombination which formed multidomain proteins are important forces driving evolution.

Altered plasma proteome during an early phase of peritonitis-induced sepsis

Sepsis is a systemic response to infection commonly found in critically ill patients and is associated with multi-organ failure and high mortality rate. Its pathophysiology and molecular mechanisms are complicated and remain poorly understood. In the present study, we performed a proteomics investigation to characterize early host responses to sepsis as determined by an altered plasma proteome in a porcine model of peritonitis-induced sepsis, which simulated several clinical characteristics of human sepsis syndrome. Haemodynamics, oxygen exchange, inflammatory responses, oxidative and nitrosative stress, and other laboratory parameters were closely monitored. Plasma samples were obtained from seven pigs before and 12 h after the induction of sepsis, and plasma proteins were resolved with two-dimensional gel electrophoresis (n=7 gels/group; before being compared with during sepsis). The resolved proteins were stained with the SYPRO Ruby fluorescence dye and subjected to quantitative and comparative analyses. From approx. 1500 protein spots visualized in each gel, levels of 36 protein spots were significantly altered in the plasma of animals with sepsis (sepsis/basal ratios or degrees of change ranged from 0.07 to 21.24). Q-TOF (quadrupole–time-of-flight) MS and MS/MS (tandem MS) identified 30 protein forms representing 22 unique proteins whose plasma levels were increased, whereas six forms of five unique proteins were significantly decreased during sepsis. The proteomic results could be related to the clinical features of this animal model, as most of these altered proteins have important roles in inflammatory responses and some of them play roles in oxidative and nitrosative stress. In conclusion, these findings may lead to a better understanding of the pathophysiology and molecular mechanisms underlying the sepsis syndrome.

Arabidopsis casein kinase 1-like 6 contains a microtubule-binding domain and affects the organization of cortical microtubules,

Members of the casein kinase 1 (CK1) family are evolutionarily conserved eukaryotic protein kinases that are involved in various cellular, physiological, and developmental processes in yeast and metazoans, but the biological roles of CK1 members in plants are not well understood. Here, we report that an Arabidopsis (Arabidopsis thaliana) CK1 member named casein kinase 1-like 6 (CKL6) associates with cortical microtubules in vivo and phosphorylates tubulins in vitro. The unique C-terminal domain of CKL6 was shown to contain the signal that allows localization of CKL6 to the cortical microtubules. This domain on its own was sufficient to associate with microtubules in vivo and to bind tubulins in vitro. CKL6 was able to phosphorylate soluble tubulins as well as microtubule polymers, and its endogenous activity was found to associate with a tubulin-enriched subcellular fraction. Two major in vitro phosphorylation sites were mapped to serine-413 and serine-420 of tubulin beta. Ectopic expression of wild-type CKL6 or a kinase-inactive mutant form induced alterations in cortical microtubule organization and anisotropic cell expansion. Collectively, these results demonstrate that CKL6 is a protein kinase containing a novel tubulin-binding domain and plays a role in anisotropic cell growth and shape formation in Arabidopsis through the regulation of microtubule organization, possibly through the phosphorylation of tubulins.

ACF7 regulates cytoskeletal-focal adhesion dynamics and migration and has ATPase activity

Coordinated interactions between microtubule (MT) and actin cytoskeletons are involved in many polarized cellular processes. Spectraplakins are enormous (>500 kDa) proteins able to bind both MTs and actin filaments (F-actin) directly. To elucidate the physiological significance and functions of mammalian spectraplakin ACF7, we've conditionally targeted it in skin epidermis. Intriguingly, ACF7 deficiency compromises the targeting of microtubules along F-actin to focal adhesions (FAs), stabilizes FA-actin networks, and impairs epidermal migration. Exploring underlying mechanisms, we show that ACF7's binding domains for F-actin, MTs, and MT plus-end proteins are not sufficient to rescue the defects in FA-cytoskeletal dynamics and migration functions of ACF7 null keratinocytes. We've uncovered an intrinsic actin-regulated ATPase domain in ACF7 and demonstrate that it is both functional and essential for these roles. Our findings provide insight into the functions of this important cytoskeletal crosslinking protein in regulating dynamic interactions between MTs and F-actin to sustain directional cell movement.

The RhoGAP RGA-2 and LET-502/ROCK achieve a balance of actomyosin-dependent forces inC. elegansepidermis to control morphogenesis

Embryonic morphogenesis involves the coordinate behaviour of multiple cells and requires the accurate balance of forces acting within different cells through the application of appropriate brakes and throttles. In C. elegans, embryonic elongation is driven by Rho-binding kinase (ROCK) and actomyosin contraction in the epidermis. We identify an evolutionary conserved, actin microfilament-associated RhoGAP (RGA-2) that behaves as a negative regulator of LET-502/ROCK. The small GTPase RHO-1 is the preferred target of RGA-2 in vitro, and acts between RGA-2 and LET-502 in vivo. Two observations show that RGA-2 acts in dorsal and ventral epidermal cells to moderate actomyosin tension during the first half of elongation. First,time-lapse microscopy shows that loss of RGA-2 induces localised circumferentially oriented pulling on junctional complexes in dorsal and ventral epidermal cells. Second, specific expression of RGA-2 in dorsal/ventral, but not lateral, cells rescues the embryonic lethality of rga-2 mutants. We propose that actomyosin-generated tension must be moderated in two out of the three sets of epidermal cells surrounding the C. elegans embryo to achieve morphogenesis.

Plakins in development and disease

Plakins are large multi-domain molecules that have various functions to link cytoskeletal elements together and to connect them to junctional complexes. Plakins were first identified in epithelial cells where they were found to connect the intermediate filaments to desmosomes and hemidesmosomes [Ruhrberg, C., and Watt, F.M. (1997). The plakin family: versatile organizers of cytoskeletal architecture. Curr Opin Genet Dev 7, 392-397.]. They were subsequently found to be important for the integrity of muscle cells. Most recently, they have been found in the nervous system, where their functions appear to be more complex, including cross-linking of microtubules (MTs) and actin filaments [Leung, C.L., Zheng, M., Prater, S.M., and Liem, R.K. (2001). The BPAG1 locus: Alternative splicing produces multiple isoforms with distinct cytoskeletal linker domains, including predominant isoforms in neurons and muscles. J Cell Biol 154, 691-697., Leung, C.L., Sun, D., Zheng, M., Knowles, D.R., and Liem, R.K. (1999). Microtubule actin cross-linking factor (MACF): a hybrid of dystonin and dystrophin that can interact with the actin and microtubule cytoskeletons. J Cell Biol 147, 1275-1286.]. These plakins have also indicated their relationship to the spectrin superfamily of proteins and the plakins appear to be evolutionarily related to the spectrins, but have diverged to perform different specialized functions. In invertebrates, a single plakin is present in both Drosophila melanogaster and Caenorhabditis elegans, which resemble the more complex plakins found in mammals [Roper, K., Gregory, S.L., and Brown, N.H. (2002). The 'spectraplakins': cytoskeletal giants with characteristics of both spectrin and plakin families. J Cell Sci 115, 4215-4225.]. In contrast, there are seven plakins found in mammals and most of them have alternatively spliced forms leading to a very complex group of proteins with potential tissue specific functions [Jefferson, J.J., Leung, C.L., and Liem, R.K. (2004). Plakins: goliaths that link cell junctions and the cytoskeleton. Nat Rev Mol Cell Biol 5, 542-553.]. In this review, we will first describe the plakins, desmoplakin, plectin, envoplakin and periplakin and then describe two other mammalian plakins, Bullous pemphigoid antigen 1 (BPAG1) and microtubule actin cross-linking factor 1 (MACF1), that are expressed in multiple isoforms in different tissues. We will also describe the relationship of these two proteins to the invertebrate plakins, shortstop (shot) in Drosophila and VAB-10 in C. elegans. Finally, we will describe an unusual mammalian plakin, called epiplakin.

Molecular characterization of the genetic lesion in Dystonia musculorum (dt-Alb) mice

Dystonia musculorum (dt) is an inherited autosomal recessive neuropathy in mice. Homozygous animals display primarily sensory neurodegeneration resulting in a severe loss of coordination. Several dt strains exist, including spontaneous mutants dt-Alb (Albany), dt-J (Jackson Labs), and dt-Frk (Frankel), and a transgene insertion mutant, Tg4. They contain mutations in the gene encoding Bullous Pemphigoid Antigen 1 (BPAG1), or dystonin. BPAG1 is a member of the plakin family of cytolinker proteins. BPAG1 is alternatively spliced to produce several isoforms, including the major brain-specific isoform, BPAG1a. The neurological phenotype observed in dt-Alb mice is thought to result from the absence of BPAG1a protein in the developing nervous system. The goal of this study was to determine the precise molecular nature of the dt-Alb mutation and examine residual BPAG1 expression in homozygous dt-Alb mice. A combination of molecular biological strategies revealed that the dt-Alb lesion is a deletion-insertion eliminating a large part of the coding region of BPAG1a. The molecular lesion in the dt-Alb BPAG1 allele is expected to render it completely non-functional. Although transcripts corresponding to BPAG1 segments still remaining in homozygous dt-Alb mice could be detected by RT-PCR, there was no positive signal for BPAG1 in the brain of dt-Alb mice by Northern blotting. Western blotting with polyclonal anti-BPAG1 antibodies confirmed the absence of functional BPAG1 protein (full-length or truncated) in the dt-Alb brain. Our identification of the 5' junction of the dt-Alb insertion makes it possible to genotype dt-Alb animals by standard PCR.

The toxofilin-actin-PP2C complex of Toxoplasma: identification of interacting domains

Toxofilin is a 27 kDa protein isolated from the human protozoan parasite Toxoplasma gondii, which causes toxoplasmosis. Toxofilin binds to G-actin, and in vitro studies have shown that it controls elongation of actin filaments by sequestering actin monomers. Toxofilin affinity for G-actin is controlled by the phosphorylation status of its Ser53, which depends on the activities of a casein kinase II and a type 2C serine/threonine phosphatase (PP2C). To get insights into the functional properties of toxofilin, we undertook a structure-function analysis of the protein using a combination of biochemical techniques. We identified a domain that was sufficient to sequester G-actin and that contains three peptide sequences selectively binding to G-actin. Two of these sequences are similar to sequences present in several G- and F-actin-binding proteins, while the third appears to be specific to toxofilin. Additionally, we identified two toxofilin domains that interact with PP2C, one of which contains the Ser53 substrate. In addition to characterizing the interacting domains of toxofilin with its partners, the present study also provides information on an in vivo-based approach to selectively and competitively disrupt the protein-protein interactions that are important to parasite motility.

Genes required for osmoregulation and apical secretion in Caenorhabditis elegans

Few studies have investigated whether or not there is an interdependence between osmoregulation and vesicular trafficking. We previously showed that in Caenorhabditis elegans che-14 mutations affect osmoregulation, cuticle secretion, and sensory organ development. We report the identification of seven lethal mutations displaying che-14-like phenotypes, which define four new genes, rdy-1-rdy-4 (rod-like larval lethality and dye-filling defective). rdy-1, rdy-2, and rdy-4 mutations affect excretory canal function and cuticle formation. Moreover, rdy-1 and rdy-2 mutations reduce the amount of matrix material normally secreted by sheath cells in the amphid channel. In contrast, rdy-3 mutants have short cystic excretory canals, suggesting that it acts in a different process. rdy-1 encodes the vacuolar H+-ATPase a-subunit VHA-5, whereas rdy-2 encodes a new tetraspan protein. We suggest that RDY-1/VHA-5 acts upstream of RDY-2 and CHE-14 in some tissues, since it is required for their delivery to the epidermal, but not the amphid sheath, apical plasma membrane. Hence, the RDY-1/VHA-5 trafficking function appears essential in some cells and its proton pump function essential in others. Finally, we show that RDY-1/VHA-5 distribution changes prior to molting in parallel with that of actin microfilaments and propose a model for molting whereby actin provides a spatial cue for secretion.

Novel interaction between the human 5-HT7 receptor isoforms and PLAC-24/eIF3k

Three 5-HT(7) receptor isoforms are expressed in rat and man, which differ in the amino acid sequence of their C-terminus. Thus far, no changes have been observed in the pharmacological profile of all three isoforms. To further elucidate the signal transduction pathway specific for these receptor variants, we screened for possible interacting proteins of the C-terminus of the h5-HT(7(a)) variant in a human foetal brain cDNA library. Using a yeast two-hybrid assay, we isolated PLAC-24/eIF3k as a possible interacting candidate. The association of PLAC-24 with all three receptor variants was observed and further reconfirmed in vivo by co-immunoprecipitation of PLAC-24 with the full-length receptor isoforms in transfected COS-7 cells. Studies with different deletion mutants of the receptor showed that the interaction between PLAC-24 and the receptor is not restricted to the C-terminus of the receptor. PLAC-24/eIF3k consists of 3 domains: an N-terminal HAM domain, a central WH domain and a C-terminal tail. We generated different domain constructs of PLAC-24, which indicated that the HAM and WH domain both interact with the 5-HT(7(a)) receptor. Overexpression of PLAC-24 in HEK293 cells, stably expressing the h5-HT(7(a)) receptor, caused a threefold augmentation in the expression levels of the receptor. Co-localisation studies in COS-7 cells showed that PLAC-24 relocates from the nucleus and perinuclear sites towards the plasma membrane upon co-expression with the receptor. On the other hand, the expression of domain variants of PLAC-24 seems to block the translocation of the receptor towards the membrane. These observations suggest that PLAC-24 may play a role in the transport and the stabilisation of newly synthesised 5-HT(7) receptor towards the plasma membrane.

The role of microtubule actin cross-linking factor 1 (MACF1) in the Wnt signaling pathway

MACF1 (microtubule actin cross-linking factor 1) is a multidomain protein that can associate with microfilaments and microtubules. We found that MACF1 was highly expressed in neuronal tissues and the foregut of embryonic day 8.5 (E8.5) embryos and the head fold and primitive streak of E7.5 embryos. MACF1(-/-) mice died at the gastrulation stage and displayed developmental retardation at E7.5 with defects in the formation of the primitive streak, node, and mesoderm. This phenotype was similar to Wnt-3(-/-) and LRP5/6 double-knockout embryos. In the absence of Wnt, MACF1 associated with a complex that contained Axin, beta-catenin, GSK3beta, and APC. Upon Wnt stimulation, MACF1 appeared to be involved in the translocation and subsequent binding of the Axin complex to LRP6 at the cell membrane. Reduction of MACF1 with small interfering RNA decreased the amount of beta-catenin in the nucleus, and led to an inhibition of Wnt-induced TCF/beta-catenin-dependent transcriptional activation. Similar results were obtained with a dominant-negative MACF1 construct that contained the Axin-binding region. Reduction of MACF1 in Wnt-1-expressing P19 cells resulted in decreased T (Brachyury) gene expression, a DNA-binding transcription factor that is a direct target of Wnt/beta-catenin signaling and required for mesoderm formation. These results suggest a new role of MACF1 in the Wnt signaling pathway.

A Possible Cellular Mechanism of Neuronal Loss in the Dorsal Root Ganglia of Dystonia musculorum (dt) Mice

Dystonia musculorum (dt) is a mutant mouse with hereditary sensory neuropathy. A defective bullous pemphigoid antigen 1 (BPAG1) gene is responsible for this mutation. In the present study, we examined the distribution of neuronal intermediate filament proteins in the central and peripheral processes of the dorsal root ganglia (DRG) in adult dt mice using different approaches. We found that not only BPAG1, but also alpha-internexin was absent in the DRG neurons in adult dt mice. To study the relationship between the absence of alpha-internexin and the progressive neuronal loss in the DRG of dt mice, we further cultured DRG neurons from embryonic dt mutants. Immunocytochemical assay of cultured DRG neurons from dt embryos revealed that alpha-internexin was aggregated in the proximal region of axons and juxtanuclear region of the cytoplasma, yet the other intermediate filament proteins were widely distributed in all processes. The active caspase-3 activity was observed in the dt neuron with massive accumulation of alpha-internexin. From our observations, we suggest that the interaction between BPAG1 and alpha-internexin may be one of the key factors involved in neuronal degeneration, and abnormal accumulation of alpha-internexin may impair the axonal transport and subsequently turns on the cascade of neuronal apoptosis in dt mice.

Nesprin-3, a novel outer nuclear membrane protein, associates with the cytoskeletal linker protein plectin

Despite their importance in cell biology, the mechanisms that maintain the nucleus in its proper position in the cell are not well understood. This is primarily the result of an incomplete knowledge of the proteins in the outer nuclear membrane (ONM) that are able to associate with the different cytoskeletal systems. Two related ONM proteins, nuclear envelope spectrin repeat (nesprin)–1 and –2, are known to make direct connections with the actin cytoskeleton through their NH₂-terminal actin-binding domain (ABD). We have now isolated a third member of the nesprin family that lacks an ABD and instead binds to the plakin family member plectin, which can associate with the intermediate filament (IF) system. Overexpression of nesprin-3 results in a dramatic recruitment of plectin to the nuclear perimeter, which is where these two molecules are colocalized with both keratin-6 and -14. Importantly, plectin binds to the integrin α6β4 at the cell surface and to nesprin-3 at the ONM in keratinocytes, suggesting that there is a continuous connection between the nucleus and the extracellular matrix through the IF cytoskeleton.

Genetic alterations at the Bpag1 locus in dt mice and their impact on transcript expression

The dystonin/Bpag1 gene encodes several tissue-specific alternatively spliced transcripts that encode cytoskeletal binding proteins. These various isoforms are necessary for maintaining the structural integrity of epithelial, neural, and muscle tissues. Mutations in the dystonin/Bpag1 gene cause dystonia musculorum (dt), a hereditary neuropathy of the mouse characterized by the progressive degeneration of sensory neurons. Several dt mutant alleles exist, most of which have arisen through spontaneous mutations. In this article we demonstrate that the dt locus encodes 107 exons spanning 400 kb. The high frequency of occurrence of spontaneous dt mutants may therefore be a result of the large size of the gene. Analysis of genomic DNA from several dt spontaneous mutant alleles, dt(24J), dt(27J), dt(Alb), and dt(Frk), shows a deletion of the central portion of the gene in dt(Alb) but no large rearrangements or deletions in the other alleles. These other alleles likely have small deletions or rearrangements, or point mutations. To determine the impact of the known and unknown mutations on transcript levels, RT-PCR was performed to detect various coding regions of the dystonin/Bpag1 transcripts in brain and muscle from multiple dt alleles: dt(Tg4), dt(Alb), dt(24J), dt(27J), and dt(Frk). With the exception of dt(Frk), reduced transcript levels were observed for all alleles tested. Such alterations likely result in reduced or absent dystonin/Bpag1 protein levels. Thus, distinct genetic defects lead to a common outcome of reduced transcript expression causing the same phenotype in multiple dt alleles.

Microtubule actin crosslinking factor 1b: a novel plakin that localizes to the Golgi complex

MACF1 (microtubule actin crosslinking factor), also called ACF7 (actin crosslinking family 7) is a cytoskeletal linker protein that can associate with both actin filaments and microtubules. We have identified a novel alternatively spliced isoform of MACF1. We named this isoform MACF1b and renamed the original isoform MACF1a. MACF1b is identical to MACF1a, except that it has a region containing plakin (or plectin) repeats in the middle of the molecule. MACF1b is ubiquitously expressed in adult tissues with especially high levels in the lung. We studied the subcellular localization of MACF1b proteins in mammalian cell lines. In two lung cell lines, MACF1b was chiefly localized to the Golgi complex. Upon treatments that disrupt the Golgi complex, MACF1b redistributed into the cytosol, but remained co-localized with the dispersed Golgi ministacks. MACF1b proteins can be detected in the enriched Golgi fraction by western blotting. The domain of MACF1b that targets it to the Golgi was found at the N-terminal part of the region that contains the plakin repeats. Reducing the level of MACF1 proteins by small-interfering RNA resulted in the dispersal of the Golgi complex.

Doublecortin Association with Actin Filaments Is Regulated by Neurabin II

Mutations in the human Doublecortin (DCX) gene cause X-linked lissencephaly, a neuronal migration disorder affecting the neocortex and characterized by mental retardation and epilepsy. Because dynamic cellular asymmetries such as those seen in cell migration critically depend on a cooperation between the microtubule and actin cytoskeletal filament systems, we investigated whether Dcx, a microtubule-associated protein, is engaged in cytoskeletal cross-talk. We now demonstrate that Dcx co-sediments with actin filaments (F-actin), and using light and electron microscopy and spin down assays, we show that Dcx induces bundling and cross-linking of microtubules and F-actin in vitro. It has recently been shown that binding of Dcx to microtubules is negatively regulated by phosphorylation of the Dcx at Ser-47 or Ser-297. Although the phosphomimetic green fluorescent protein (GFP)-Dcx(S47E) transfected into COS-7 cells had a reduced affinity for microtubules, we found that pseudophosphorylation was not sufficient to cause Dcx to bind to F-actin. When cells were co-transfected with neurabin II, a protein that binds F-actin as well as Dcx, GFP-Dcx and to an even greater extent GFP-Dcx(S47E) became predominantly associated with filamentous actin. Thus Dcx phosphorylation and neurabin II combinatorially enhance Dcx binding to F-actin. Our findings raise the possibility that Dcx acts as a molecular link between microtubule and actin cytoskeletal filaments that is regulated by phosphorylation and neurabin II.

Structural determinants for EB1-mediated recruitment of APC and spectraplakins to the microtubule plus end

EB1 is a member of a conserved protein family that localizes to growing microtubule plus ends. EB1 proteins also recruit cell polarity and signaling molecules to microtubule tips. However, the mechanism by which EB1 recognizes cargo is unknown. Here, we have defined a repeat sequence in adenomatous polyposis coli (APC) that binds to EB1's COOH-terminal domain and identified a similar sequence in members of the microtubule actin cross-linking factor (MACF) family of spectraplakins. We show that MACFs directly bind EB1 and exhibit EB1-dependent plus end tracking in vivo. To understand how EB1 recognizes APC and MACFs, we solved the crystal structure of the EB1 COOH-terminal domain. The structure reveals a novel homodimeric fold comprised of a coiled coil and four-helix bundle motif. Mutational analysis reveals that the cargo binding site for MACFs maps to a cluster of conserved residues at the junction between the coiled coil and four-helix bundle. These results provide a structural understanding of how EB1 binds two regulators of microtubule-based cell polarity.

Disruption of the actin network enhances MAP-2c and Fyn-induced process outgrowth

We investigated the interaction of MAP-2c and Fyn in the initiation of process outgrowth in COS7 cells. Single transfections of Fyn and MAP-2c resulted in a dramatic decrease in flat, rounded COS7 cells, and a significant increase in both the number of cells with multiple short, spike-like processes, and cells with longer processes. Co-transfection of Fyn and MAP-2c resulted in an additive increase in the number of cells with more than two processes and discrete sites of co-localization within processes. When single or double transfected cells were treated with cytochalasin D or lantrunculin there was a dramatic increase in the number of cells with more than two processes. In addition, there was an increase in the length of the processes, both in single and double transfected cells, suggesting that the actin meshwork provides a barrier for MT-based process extension. When co-transfected cells were post-treated with nocodazole, Fyn was not associated with MAP-2c and acetylated, stable tubulin. Although some Fyn/MAP-2c co-localization was retained, punctate staining of MAP-2c and Fyn were observed at the cell periphery, in areas devoid of stable MTs. Mutations in either tyrosine 67 (Tyr67), a site on human MAP-2c phosphorylated by Fyn, or a second tyrosine residue (Tyr50), did not alter the ability of MAP-2c and Fyn to induce process outgrowth. These studies suggest that independent of one another MAP-2c and Fyn are able to induce process outgrowth and in concert can initiate and enhance process outgrowth in an additive manner.

Intermediate filament associated proteins

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural crosslinkers and bundlers but also include chaperones, enzymes, adapters, and receptors. IF networks can therefore be considered scaffolding upon which associated proteins are organized and regulated to control metabolic activities and maintain cell homeostasis.

Interaction between p230 and MACF1 is associated with transport of a glycosyl phosphatidyl inositol-anchored protein from the Golgi to the cell periphery

The molecular basis by which proteins are transported along cytoskeletal tracts from the trans-Golgi network (TGN) to the cell periphery remains poorly understood. Previously, using human autoimmune sera, we identified and characterized a TGN protein, p230/Golgin-245, an extensively coiled-coil protein with flexible amino- and carboxyl-terminal ends, that is anchored to TGN membranes and TGN-derived vesicles by its carboxyl-terminal GRIP domain. To identify molecules that interact with the flexible amino-terminal end of p230, we used this domain as bait to screen a human brain cDNA library in a yeast two-hybrid assay. We found that this domain interacts with the carboxyl-terminal domain of MACF1, a protein that cross-links microtubules to the actin cytoskeleton. The interaction was confirmed by co-immunoprecipitation, an in vitro binding assay, double immunofluorescence images demonstrating overlapped localization in HeLa cells, and co-localization of FLAG-tagged constructs containing the interacting domains of these two proteins with their endogenous partners. Expression in HeLa cells of FLAG-tagged constructs containing the interacting domains of p230 and MACF1 disrupted transport of the glycosyl phosphatidyl inositol-anchored marker protein conjugated with yellow fluorescent protein (YFP-SP-GPI), while trafficking of the transmembrane marker protein, vesicular stomatitis virus glycoprotein conjugated with YFP (VSVG3-GL-YFP), was unaffected. Our results suggest that p230, through its interaction with MACF1, provides the molecular link for transport of GPI-anchored proteins along the microtubule and actin cytoskeleton from the TGN to the cell periphery.

Identification of neurabin II as a novel doublecortin interacting protein

The neuronal migration protein doublecortin (DCX) that associates with microtubules through a tandem DCX repeat, is required for the development of the complex architecture of the human cerebral cortex. Using a yeast two-hybrid screen with Dcx as bait, we have isolated neurabin II/spinophilin, an F-actin binding protein known to play a role in dendritic spine formation. The coiled-coil domain of neurabin II binds to a DCX region encompassing the C-terminal portion of the second DCX repeat and the N-terminal portion of the Ser/Pro-rich domain. Immunoprecipitation experiments with brain extracts show that neurabin II and Dcx interact in vivo. Several Dcx constructs that mimic human DCX mutant alleles failed to interact with neurabin II. Since Dcx and neurabin II colocalized in the developing and adult brain, a neurabin II-DCX heterodimer may be involved in neuronal migration and dendritic spine formation.

EB1 and APC bind to mDia to stabilize microtubules downstream of Rho and promote cell migration

Lysophosphatidic acid (LPA) stimulates Rho GTPase and its effector, the formin mDia, to capture and stabilize microtubules in fibroblasts. We investigated whether mammalian EB1 and adenomatous polyposis coli (APC) function downstream of Rho-mDia in microtubule stabilization. A carboxy-terminal APC-binding fragment of EB1 (EB1-C) functioned as a dominant-negative inhibitor of microtubule stabilization induced by LPA or active mDia. Knockdown of EB1 with small interfering RNAs also prevented microtubule stabilization. Expression of either full-length EB1 or APC, but not an APC-binding mutant of EB1, was sufficient to stabilize microtubules. Binding and localization studies showed that EB1, APC and mDia may form a complex at stable microtubule ends. Furthermore, EB1-C, but not an APC-binding mutant, inhibited fibroblast migration in an in vitro wounding assay. These results show an evolutionarily conserved pathway for microtubule capture, and suggest that mDia functions as a scaffold protein for EB1 and APC to stabilize microtubules and promote cell migration.

Two Protein 4.1 Domains Essential for Mitotic Spindle and Aster Microtubule Dynamics and Organization in Vitro

Multifunctional structural proteins belonging to the 4.1 family are components of nuclei, spindles, and centrosomes in vertebrate cells. Here we report that 4.1 is critical for spindle assembly and the formation of centrosome-nucleated and motor-dependent self-organized microtubule asters in metaphase-arrested Xenopus egg extracts. Immunodepletion of 4.1 disrupted microtubule arrays and mislocalized the spindle pole protein NuMA. Remarkably, assembly was completely rescued by supplementation with a recombinant 4.1R isoform. We identified two 4.1 domains critical for its function in microtubule polymerization and organization utilizing dominant negative peptides. The 4.1 spectrin-actin binding domain or NuMA binding C-terminal domain peptides caused morphologically disorganized structures. Control peptides with low homology or variant spectrin-actin binding domain peptides that were incapable of binding actin had no deleterious effects. Unexpectedly, the addition of C-terminal domain peptides with reduced NuMA binding caused severe microtubule destabilization in extracts, dramatically inhibiting aster and spindle assembly and also depolymerizing preformed structures. However, the mutant C-terminal peptides did not directly inhibit or destabilize microtubule polymerization from pure tubulin in a microtubule pelleting assay. Our data showing that 4.1 is a crucial factor for assembly and maintenance of mitotic spindles and self-organized and centrosome-nucleated microtubule asters indicates that 4.1 is involved in regulating both microtubule dynamics and organization. These investigations underscore an important functional context for protein 4.1 in microtubule morphogenesis and highlight a previously unappreciated role for 4.1 in cell division.

Co-assembly of Envoplakin and Periplakin into Oligomers and Ca2+-dependent Vesicle Binding

Plakin family members envoplakin and periplakin have been shown to be part of the cornified cell envelope in terminally differentiating stratified squamous epithelia. In the present study, purified recombinant human envoplakin and periplakin were used to investigate their properties and interactions. We found that envoplakin was insoluble at physiological conditions in vitro, and co-assembly with periplakin was required for its solubility. Envoplakin and periplakin formed soluble complexes with equimolar stoichiometry. Chemical cross-linking revealed that the major soluble form of all periplakin constructs and of envoplakin/periplakin rod domains was a dimer, although co-assembly of the full-length proteins resulted in formation of higher order oligomers. Electron microscopy of rotary-shadowed periplakin demonstrated thin flexible molecules with an average contour length of 88 nm for the rod-plus-tail fragment, and immunolabeling EM confirmed the molecule as a parallel, in-register, dimer. Both periplakin and envoplakin/periplakin oligomers were able to bind synthetic lipid vesicles whose composition mimicked the cytoplasmic side of the plasma membrane of eukaryotic cells. This binding was dependent on anionic phospholipids and Ca(2+). These findings raise the possibility that envoplakin and periplakin bind to the plasma membrane upon elevation of intracellular [Ca(2+)] in differentiating keratinocytes, where they serve as a scaffold for cornified cell envelope assembly.