Functional coupling of microtubules to membranes - implications for membrane structure and dynamics

Architecture of microtubule cytoskeleton in the hindgut cells of Porcellioscaber

The distribution and orientation of microtubules were investigated in cells of distinct shapes from different hindgut regions of adult Porcellioscaber Latreille, 1804 and during hindgut morphogenesis in late embryonic and early postembryonic development. All hindgut cells of adult P.scaber contain abundant apico-basal microtubules organized in extensive bundles, but the architecture of bundles is specific for distinct cells. In the anterior chamber the architecture of microtubule bundles closely coincides with different shapes of the cells in this hindgut region and are most prominent in hindgut cells associated with extensive muscles. The shape of cells that form the typhlosole and typhlosole channels is particularly complex. In the papillate region the microtubule bundles protrude between the infoldings of apical plasma membrane and the mitochondria are closely aligned along the microtubules, thus the microtubule bundles in the papillate region are likely involved in the stabilization of the apical labyrinth and positioning of mitochondria. During hindgut morphogenesis the apico-basal microtubule bundles are established relatively late, mainly during early postembryonic development. Morphogenesis of the typhlosole is characterized by coinciding changes in cell shape and microtubule arrangement.

The roles of dynein and myosin VI motor proteins in endocytosis

Endocytosis is indispensable for multiple cellular processes, including signalling, cell adhesion, migration, as well as the turnover of plasma membrane lipids and proteins. The dynamic interplay and regulation of different endocytic entry routes requires multiple cytoskeletal elements, especially motor proteins that bind to membranes and transport vesicles along the actin and microtubule cytoskeletons. Dynein and kinesin motor proteins transport vesicles along microtubules, whereas myosins drive vesicles along actin filaments. Here, we present a brief overview of multiple endocytic pathways and our current understanding of the involvement of these motor proteins in the regulation of the different cellular entry routes. We particularly focus on structural and mechanistic details of the retrograde motor proteins dynein and myosin VI (also known as MYO6), along with their adaptors, which have important roles in the early events of endocytosis. We conclude by highlighting the key challenges in elucidating the involvement of motor proteins in endocytosis and intracellular membrane trafficking.

Unique Tubulin-Based Structures in the Zoonotic Apicomplexan Parasite Cryptosporidium parvum

Cryptosporidium parasites are known to be highly divergent from other apicomplexan species at evolutionary and biological levels. Here we provide evidence showing that the zoonotic Cryptosporidium parvum also differs from other apicomplexans, such as Toxoplasma gondii, by possessing only two tubulin-based filamentous structures, rather than an array of subpellicular microtubules. Using an affinity-purified polyclonal antibody against C. parvum β-tubulin (CpTubB), we observed a long and a short microtubule that are rigid and stable in the sporozoites and restructured during the intracellular parasite development. In asexual development (merogony), the two restructuring microtubules are present in pairs (one pair per nucleus or merozoites). In sexual developmental stages, tubulin-based structures are detectable only in microgametes, but undetectable in macrogametes. These observations indicate that C. parvum parasites use unique microtubule structures that differ from other apicomplexans as part of their cytoskeletal elements.

LMBD1 protein participates in cell mitosis by regulating microtubule assembly

LMBD1 was previously demonstrated to regulate the endocytosis of insulin receptor on the cell surface and to mediate the export of cobalamin from the lysosomes to the cytosol, but little is known about its function in mitosis. In this study, interactome analysis data indicate that LMBD1 is involved in cytoskeleton regulation. Both immunoprecipitation and GST pulldown assays demonstrated the association of LMBD1 with tubulin. Immunofluorescence staining also showed the colocalization of LMBD1 with microtubule in both interphase and mitotic cells. LMBD1 specifically accelerates microtubule assembly dynamics in vitro and antagonizes the microtubule-disruptive effect of vinblastine. In addition, LMBRD1-knockdown impairs mitotic spindle formation, inhibits tubulin polymerization, and diminishes the mitosis-associated tubulin acetylation. The reduced acetylation can be reversed by ectopic expression of LMBD1 protein. These results suggest that LMBD1 protein stabilizes microtubule intermediates. Furthermore, embryonic fibroblasts derived from Lmbrd1 heterozygous knockout mice showed abnormality in microtubule formation, mitosis, and cell growth. Taken together, LMBD1 plays a pivotal role in regulating microtubule assembly that is essential for the process of cell mitosis.

STRIPAK regulates Slik localization to control mitotic morphogenesis and epithelial integrity

Proteins of the ezrin, radixin, and moesin (ERM) family control cell and tissue morphogenesis. We previously reported that moesin, the only ERM in Drosophila, controls mitotic morphogenesis and epithelial integrity. We also found that the Pp1-87B phosphatase dephosphorylates moesin, counteracting its activation by the Ste20-like kinase Slik. To understand how this signaling pathway is itself regulated, we conducted a genome-wide RNAi screen, looking for new regulators of moesin activity. We identified that Slik is a new member of the striatin-interacting phosphatase and kinase complex (STRIPAK). We discovered that the phosphatase activity of STRIPAK reduces Slik phosphorylation to promote its cortical association and proper activation of moesin. Consistent with this finding, inhibition of STRIPAK phosphatase activity causes cell morphology defects in mitosis and impairs epithelial tissue integrity. Our results implicate the Slik–STRIPAK complex in the control of multiple morphogenetic processes.

TACC3 Regulates Microtubule Plus-End Dynamics and Cargo Transport in Interphase Cells

End-binding proteins (EBs) are widely viewed as master regulators of microtubule dynamics and function. Here, we show that while EB1 mediates the dynamic microtubule capture of herpes simplex virus type 1 (HSV-1) in fibroblasts, in neuronal cells, infection occurs independently of EBs through stable microtubules. Prompted by this, we find that transforming acid coiled-coil protein 3 (TACC3), widely studied in mitotic spindle formation, regulates the cytoplasmic localization of the microtubule polymerizing factor chTOG and influences microtubule plus-end dynamics during interphase to control infection in distinct cell types. Furthermore, perturbing TACC3 function in neuronal cells resulted in the formation of disorganized stable, detyrosinated microtubule networks and changes in cellular morphology, as well as impaired trafficking of both HSV-1 and transferrin. These trafficking defects in TACC3-depleted cells were reversed by the depletion of kinesin-1 heavy chains. As such, TACC3 is a critical regulator of interphase microtubule dynamics and stability that influences kinesin-1-based cargo trafficking.

While EB proteins are widely studied as master regulators of microtubule plus-end dynamics, Furey et al. report EB-independent regulation of microtubule arrays and cargo trafficking by the transforming acid coiled-coil-containing protein, TACC3. By controlling the formation of detyrosinated stable microtubule networks, TACC3 influences kinesin-1-based sorting of both host and pathogenic cargoes.

Cytoskeletal disorganization underlies PABPN1-mediated myogenic disability

Muscle wasting and atrophy are regulated by multiple molecular processes, including mRNA processing. Reduced levels of the polyadenylation binding protein nucleus 1 (PABPN1), a multifactorial regulator of mRNA processing, cause muscle atrophy. A proteomic study in muscles with reduced PABPN1 levels suggested dysregulation of sarcomeric and cytoskeletal proteins. Here we investigated the hypothesis that reduced PABPN1 levels lead to an aberrant organization of the cytoskeleton. MURC, a plasma membrane-associated protein, was found to be more abundant in muscles with reduced PABPN1 levels, and it was found to be expressed at regions showing regeneration. A polarized cytoskeletal organization is typical for muscle cells, but muscle cells with reduced PABPN1 levels (named as shPAB) were characterized by a disorganized cytoskeleton that lacked polarization. Moreover, cell mechanical features and myogenic differentiation were significantly reduced in shPAB cells. Importantly, restoring cytoskeletal stability, by actin overexpression, was beneficial for myogenesis, expression of sarcomeric proteins and proper localization of MURC in shPAB cell cultures and in shPAB muscle bundle. We suggest that poor cytoskeletal mechanical features are caused by altered expression levels of cytoskeletal proteins and contribute to muscle wasting and atrophy.

Microtubule disruption changes endothelial cell mechanics and adhesion

The interest in studying the mechanical and adhesive properties of cells has increased in recent years. The cytoskeleton is known to play a key role in cell mechanics. However, the role of the microtubules in shaping cell mechanics is not yet well understood. We have employed Atomic Force Microscopy (AFM) together with confocal fluorescence microscopy to determine the role of microtubules in cytomechanics of Human Umbilical Vein Endothelial Cells (HUVECs). Additionally, the time variation of the adhesion between tip and cell surface was studied. The disruption of microtubules by exposing the cells to two colchicine concentrations was monitored as a function of time. Already, after 30 min of incubation the cells stiffened, their relaxation times increased (lower fluidity) and the adhesion between tip and cell decreased. This was accompanied by cytoskeletal rearrangements, a reduction in cell area and changes in cell shape. Over the whole experimental time, different behavior for the two used concentrations was found while for the control the values remained stable. This study underlines the role of microtubules in shaping endothelial cell mechanics.

Disassembly of Dipeptide Single Crystals Can Transform the Lipid Membrane into a Network

Coupling between cytoskeleton and membranes is critical to cell movement as well as organelle formation. Here, we demonstrate that self-assembled single crystals of a dipeptide, diphenylalanine (FF), can interact with liposomes to form cytoskeleton-like structures. Under a physiological condition, disassembly of FF crystals deforms and translocates supported lipid membrane. The system exhibits similar dynamic characteristics to the endoplasmic reticulum (ER) network in cells. This bottom-up system thus indicates that external matter can participate in the deformation of liposomes, and disassembly of the nanostructures enables a system with distinct dynamic behaviors.

Small Heat Shock Protein αB-Crystallin Controls Shape and Adhesion of Glioma and Myoblast Cells in the Absence of Stress

Cell shape and adhesion and their proper controls are fundamental for all biological systems. Mesenchymal cells migrate at an average rate of 6 to 60 μm/hr, depending on the extracellular matrix environment and cell signaling. Myotubes, fully differentiated muscle cells, are specialized for power-generation and therefore lose motility. Cell spreading and stabilities of focal adhesion are regulated by the critical protein vinculin from immature myoblast to mature costamere of differentiated myotubes where myofibril Z-band linked to sarcolemma. The Z-band is constituted from microtubules, intermediate filaments, cell adhesion molecules and other adapter proteins that communicate with the outer environment. Mesenchymal cells, including myoblast cells, convert actomyosin contraction forces to tension through mechano-responsive adhesion assembly complexes as Z-band equivalents. There is growing evidence that microtubule dynamics are involved in the generation of contractile forces; however, the roles of microtubules in cell adhesion dynamics are not well determined. Here, we show for the first time that αB-crystallin, a molecular chaperon for tubulin/microtubules, is involved in cell shape determination. Moreover, knockdown of this molecule caused myoblasts and glioma cells to lose their ability for adhesion as they tended to behave like migratory cells. Surprisingly, αB-crystallin knockdown in both C6 glial cells and L6 myoblast permitted cells to migrate more rapidly (2.7 times faster for C6 and 1.3 times faster for L6 cells) than dermal fibroblast. On the other hand, overexpression of αB-crystallin in cells led to an immortal phenotype because of persistent adhesion. Position of matured focal adhesion as visualized by vinculin immuno-staining, stress fiber direction, length, and density were clearly αB-crystallin dependent. These results indicate that the small HSP αB-crystallin has important roles for cell adhesion, and thus microtubule dynamics are necessary for persistent adhesion.

Human Sirtuin 2 Localization, Transient Interactions, and Impact on the Proteome Point to Its Role in Intracellular Trafficking

Human sirtuin 2 (SIRT2) is an NAD⁺-dependent deacetylase that primarily functions in the cytoplasm, where it can regulate α-tubulin acetylation levels. SIRT2 is linked to cancer progression, neurodegeneration, and infection with bacteria or viruses. However, the current knowledge about its interactions and the means through which it exerts its functions has remained limited. Here, we aimed to gain a better understanding of its cellular functions by characterizing SIRT2 subcellular localization, the identity and relative stability of its protein interactions, and its impact on the proteome of primary human fibroblasts. To assess the relative stability of SIRT2 interactions, we used immunoaffinity purification in conjunction with both label-free and metabolic labeling quantitative mass spectrometry. In addition to the expected associations with cytoskeleton proteins, including its known substrate TUBA1A, our results reveal that SIRT2 specifically interacts with proteins functioning in membrane trafficking, secretory processes, and transcriptional regulation. By quantifying their relative stability, we found most interactions to be transient, indicating a dynamic SIRT2 environment. We discover that SIRT2 localizes to the ER-Golgi intermediate compartment (ERGIC), and that this recruitment requires an intact ER-Golgi trafficking pathway. Further expanding these findings, we used microscopy and interaction assays to establish the interaction and coregulation of SIRT2 with liprin-β1 scaffolding protein (PPFiBP1), a protein with roles in focal adhesions disassembly. As SIRT2 functions may be accomplished via interactions, enzymatic activity, and transcriptional regulation, we next assessed the impact of SIRT2 levels on the cellular proteome. SIRT2 knockdown led to changes in the levels of proteins functioning in membrane trafficking, including some of its interaction partners. Altogether, our study expands the knowledge of SIRT2 cytoplasmic functions to define a previously unrecognized involvement in intracellular trafficking pathways, which may contribute to its roles in cellular homeostasis and human diseases.

Nanoelectromechanical Chip (NELMEC) Combination of Nanoelectronics and Microfluidics to Diagnose Epithelial and Mesenchymal Circulating Tumor Cells from Leukocytes

An integrated nano-electromechanical chip (NELMEC) has been developed for the label-free distinguishing of both epithelial and mesenchymal circulating tumor cells (ECTCs and MCTCs, respectively) from white blood cells (WBCs). This nanoelectronic microfluidic chip fabricated by silicon micromachining can trap large single cells (>12 µm) at the opening of the analysis microchannel arrays. The nature of the captured cells is detected using silicon nanograss (SiNG) electrodes patterned at the entrance of the channels. There is an observable difference between the membrane capacitance of the ECTCs and MCTCs and that of WBCs (measured using SiNG electrodes), which is the key indication for our diagnosis. The NELMEC chip not only solves the problem of the size overlap between CTCs and WBCs but also detects MCTCs without the need for any markers or tagging processes, which has been an important problem in previously reported CTC detection systems. The great conductivity of the gold-coated SiNG nanocontacts as well as their safe penetration into the membrane of captured cells, facilitate a precise and direct signal extraction to distinguish the type of captured cell. The results achieved from epithelial (MCF-7) and mesenchymal (MDA-MB231) breast cancer cells circulated in unprocessed blood suggest the significant applications for these diagnostic abilities of NELMEC.

TMF/ARA160 Governs the Dynamic Spatial Orientation of the Golgi Apparatus during Sperm Development

TMF/ARA160 is known to be a TATA element Modulatory Factor (TMF). It was initially identified as a DNA-binding factor and a coactivator of the Androgen receptor. It was also characterized as a Golgi-associated protein, which is essential for acrosome formation during functional sperm development. However, the molecular roles of TMF in this intricate process have not been revealed. Here, we show that during spermiogenesis, TMF undergoes a dynamic change of localization throughout the Golgi apparatus. Specifically, TMF translocates from the cis-Golgi to the trans-Golgi network and to the emerging vesicles surface, as the round spermatids develop. Notably, lack of TMF led to an abnormal spatial orientation of the Golgi and to the deviation of the trans-Golgi surface away from the nucleus of the developing round spermatids. Concomitantly, pro-acrosomal vesicles derived from the TMF-/- Golgi lacked targeting properties and did not tether to the spermatid nuclear membrane thereby failing to form the acrosome anchoring scaffold, the acroplaxome, around the cell-nucleus. Absence of TMF also perturbed the positioning of microtubules, which normally lie in proximity to the Golgi and are important for maintaining Golgi spatial orientation and dynamics and for chromatoid body formation, which is impaired in TMF-/- spermatids. In-silico evaluation combined with molecular and electron microscopic analyses revealed the presence of a microtubule interacting domain (MIT) in TMF, and confirmed the association of TMF with microtubules in spermatogenic cells. Furthermore, the MIT domain in TMF, along with microtubules integrity, are required for stable association of TMF with the Golgi apparatus. Collectively, we show here for the first time that a Golgi and microtubules associated protein is crucial for maintaining proper Golgi orientation during a cell developmental process.

Cytoplasmic dynein and its regulatory proteins in Golgi pathology in nervous system disorders

The Golgi apparatus is a dynamic organelle involved in processing and sorting of lipids and proteins. In neurons, the Golgi apparatus is important for the development of axons and dendrites and maintenance of their highly complex polarized morphology. The motor protein complex cytoplasmic dynein has an important role in Golgi apparatus positioning and function. Together, with dynactin and other regulatory factors it drives microtubule minus-end directed motility of Golgi membranes. Inhibition of dynein results in fragmentation and dispersion of the Golgi ribbon in the neuronal cell body, resembling the Golgi abnormalities observed in some neurodegenerative disorders, in particular motor neuron diseases. Mutations in dynein and its regulatory factors, including the dynactin subunit p150Glued, BICD2 and Lis-1, are associated with several human nervous system disorders, including cortical malformation and motor neuropathy. Here we review the role of dynein and its regulatory factors in Golgi function and positioning, and the potential role of dynein malfunction in causing Golgi apparatus abnormalities in nervous system disorders.

Connecting the cytoskeleton to the endoplasmic reticulum and Golgi

A tendency in cell biology is to divide and conquer. For example, decades of painstaking work have led to an understanding of endoplasmic reticulum (ER) and Golgi structure, dynamics, and transport. In parallel, cytoskeletal researchers have revealed a fantastic diversity of structure and cellular function in both actin and microtubules. Increasingly, these areas overlap, necessitating an understanding of both organelle and cytoskeletal biology. This review addresses connections between the actin/microtubule cytoskeletons and organelles in animal cells, focusing on three key areas: ER structure and function; ER-to-Golgi transport; and Golgi structure and function. Making these connections has been challenging for several reasons: the small sizes and dynamic characteristics of some components; the fact that organelle-specific cytoskeletal elements can easily be obscured by more abundant cytoskeletal structures; and the difficulties in imaging membranes and cytoskeleton simultaneously, especially at the ultrastructural level. One major concept is that the cytoskeleton is frequently used to generate force for membrane movement, with two potential consequences: translocation of the organelle, or deformation of the organelle membrane. While initially discussing issues common to metazoan cells in general, we subsequently highlight specific features of neurons, since these highly polarized cells present unique challenges for organellar distribution and dynamics.

Microtubule motors power plasma membrane tubulation in clathrin-independent endocytosis

How the plasma membrane is bent to accommodate clathrin-independent endocytosis remains uncertain. Recent studies suggest Shiga and cholera toxin induce membrane curvature required for their uptake into clathrin-independent carriers by binding and cross-linking multiple copies of their glycosphingolipid receptors on the plasma membrane. But it remains unclear if toxin-induced sphingolipid crosslinking provides sufficient mechanical force for deforming the plasma membrane, or if host cell factors also contribute to this process. To test this, we imaged the uptake of cholera toxin B-subunit into surface-derived tubular invaginations. We found that cholera toxin mutants that bind to only one glycosphingolipid receptor accumulated in tubules, and that toxin binding was entirely dispensable for membrane tubulations to form. Unexpectedly, the driving force for tubule extension was supplied by the combination of microtubules, dynein and dynactin, thus defining a novel mechanism for generating membrane curvature during clathrin-independent endocytosis.

The role of signalling and the cytoskeleton during Vaccinia Virus egress

Viruses are obligate intracellular parasites that are critically dependent on their hosts to replicate and generate new progeny. To achieve this goal, viruses have evolved numerous elegant strategies to subvert and utilise the different cellular machineries and processes of their unwilling hosts. Moreover, they often accomplish this feat with a surprisingly limited number of proteins. Among the different systems of the cell, the cytoskeleton is often one of the first to be hijacked as it provides a convenient transport system for viruses to reach their site of replication with relative ease. At the latter stages of their replication cycle, the cytoskeleton also provides an efficient means for newly assembled viral progeny to reach the plasma membrane and leave the infected cell. In this review we discuss how Vaccinia virus takes advantage of the microtubule and actin cytoskeletons of its host to promote the spread of infection into neighboring cells. In particular, we highlight how analysis of actin-based motility of Vaccinia has provided unprecedented insights into how a phosphotyrosine-based signalling network is assembled and functions to stimulate Arp2/3 complex-dependent actin polymerization. We also suggest that the formin FHOD1 promotes actin-based motility of the virus by capping the fast growing ends of actin filaments rather than directly promoting filament assembly. We have come a long way since 1976, when electron micrographs of vaccinia-infected cells implicated the actin cytoskeleton in promoting viral spread. Nevertheless, there are still many unanswered questions concerning the role of signalling and the host cytoskeleton in promoting viral spread and pathogenesis.

FHIP and FTS proteins are critical for dynein-mediated transport of early endosomes in Aspergillus

The minus end-directed microtubule motor cytoplasmic dynein transports various cellular cargoes, including early endosomes, but how dynein binds to its cargo remains unclear. Recently fungal Hook homologues were found to link dynein to early endosomes for their transport. Here we identified FhipA in Aspergillus nidulans as a key player for HookA (A. nidulans Hook) function via a genome-wide screen for mutants defective in early-endosome distribution. The human homologue of FhipA, FHIP, is a protein in the previously discovered FTS/Hook/FHIP (FHF) complex, which contains, besides FHIP and Hook proteins, Fused Toes (FTS). Although this complex was not previously shown to be involved in dynein-mediated transport, we show here that loss of either FhipA or FtsA (A. nidulans FTS homologue) disrupts HookA-early endosome association and inhibits early endosome movement. Both FhipA and FtsA associate with early endosomes, and interestingly, while FtsA-early endosome association requires FhipA and HookA, FhipA-early endosome association is independent of HookA and FtsA. Thus FhipA is more directly linked to early endosomes than HookA and FtsA. However, in the absence of HookA or FtsA, FhipA protein level is significantly reduced. Our results indicate that all three proteins in the FtsA/HookA/FhipA complex are important for dynein-mediated early endosome movement.

HookA is a novel dynein-early endosome linker critical for cargo movement in vivo

HookA is a novel linker protein that binds to endosomes and to dynein–dynactin and promotes dynein–early endosome interaction in Aspergillus.

Cytoplasmic dynein transports membranous cargoes along microtubules, but the mechanism of dynein–cargo interaction is unclear. From a genetic screen, we identified a homologue of human Hook proteins, HookA, as a factor required for dynein-mediated early endosome movement in the filamentous fungus Aspergillus nidulans. HookA contains a putative N-terminal microtubule-binding domain followed by coiled-coil domains and a C-terminal cargo-binding domain, an organization reminiscent of cytoplasmic linker proteins. HookA–early endosome interaction occurs independently of dynein–early endosome interaction and requires the C-terminal domain. Importantly, HookA interacts with dynein and dynactin independently of HookA–early endosome interaction but dependent on the N-terminal part of HookA. Both dynein and the p25 subunit of dynactin are required for the interaction between HookA and dynein–dynactin, and loss of HookA significantly weakens dynein–early endosome interaction, causing a virtually complete absence of early endosome movement. Thus, HookA is a novel linker important for dynein–early endosome interaction in vivo.

Opposing microtubule motors control motility, morphology and cargo segregation during ER-to-Golgi transport

We recently demonstrated that dynein and kinesin motors drive multiple aspects of endosomal function in mammalian cells. These functions include driving motility, maintaining morphology (notably through providing longitudinal tension to support vesicle fission), and driving cargo sorting. Microtubule motors drive bidirectional motility during traffic between the endoplasmic reticulum (ER) and Golgi. Here, we have examined the role of microtubule motors in transport carrier motility, morphology, and domain organization during ER-to-Golgi transport. We show that, consistent with our findings for endosomal dynamics, microtubule motor function during ER-to-Golgi transport of secretory cargo is required for motility, morphology, and cargo sorting within vesicular tubular carriers en route to the Golgi. Our data are consistent with previous findings that defined roles for dynein-1, kinesin-1 (KIF5B) and kinesin-2 in this trafficking step. Our high resolution tracking data identify some intriguing aspects. Depletion of kinesin-1 reduces the number of motile structures seen, which is in line with other findings relating to the role of kinesin-1 in ER export. However, those transport carriers that were produced had a much greater run length suggesting that this motor can act as a brake on anterograde motility. Kinesin-2 depletion did not significantly reduce the number of motile transport carriers but did cause a similar increase in run length. These data suggest that kinesins act as negative regulators of ER-to-Golgi transport. Depletion of dynein not only reduced the number of motile carriers formed but also caused tubulation of carriers similar to that seen for sorting nexin-coated early endosomes. Our data indicated that the previously observed anterograde–retrograde polarity of transport carriers in transit to the Golgi from the ER is maintained by microtubule motor function.

Actin in action: imaging approaches to study cytoskeleton structure and function

The cytoskeleton plays several fundamental roles in the cell, including organizing the spatial arrangement of subcellular organelles, regulating cell dynamics and motility, providing a platform for interaction with neighboring cells, and ultimately defining overall cell shape. Fluorescence imaging has proved to be vital in furthering our understanding of the cytoskeleton, and is now a mainstay technique used widely by cell biologists. In this review we provide an introduction to various imaging modalities used to study focal adhesions and the actin cytoskeleton, and using specific examples we highlight a number of recent studies in animal cells that have advanced our knowledge of cytoskeletal behavior.

Organization of the ER-Golgi interface for membrane traffic control

Coat protein complex I (COPI) and COPII are required for bidirectional membrane trafficking between the endoplasmic reticulum (ER) and the Golgi. While these core coat machineries and other transport factors are highly conserved across species, high-resolution imaging studies indicate that the organization of the ER-Golgi interface is varied in eukaryotic cells. Regulation of COPII assembly, in some cases to manage distinct cellular cargo, is emerging as one important component in determining this structure. Comparison of the ER-Golgi interface across different systems, particularly mammalian and plant cells, reveals fundamental elements and distinct organization of this interface. A better understanding of how these interfaces are regulated to meet varying cellular secretory demands should provide key insights into the mechanisms that control efficient trafficking of proteins and lipids through the secretory pathway.

Golgi as an MTOC: making microtubules for its own good

In cells, microtubules (MTs) are nucleated at MT-organizing centers (MTOCs). The centrosome-based MTOCs organize radial MT arrays, which are often not optimal for polarized trafficking. A recently discovered subset of non-centrosomal MTs nucleated at the Golgi has proven to be indispensable for the Golgi organization, post-Golgi trafficking and cell polarity. Here, we summarize the history of this discovery, known molecular prerequisites of MT nucleation at the Golgi and unique functions of Golgi-derived MTs.

Cytoskeleton-dependent endomembrane organization in plant cells: an emerging role for microtubules

Movement of secretory organelles is a fascinating yet largely mysterious feature of eukaryotic cells. Microtubule-based endomembrane and organelle motility utilizing the motor proteins dynein and kinesin is commonplace in animal cells. In contrast, it has been long accepted that intracellular motility in plant cells is predominantly driven by myosin motors dragging organelles and endomembrane-bounded cargo along actin filament bundles. Consistent with this, defects in the acto-myosin cytoskeleton compromise plant growth and development. Recent findings, however, challenge the actin-centric view of the motility of critical secretory organelles and distribution of associated protein machinery. In this review, we provide an overview of the current knowledge on actin-mediated organelle movement within the secretory pathway of plant cells, and report on recent and exciting findings that support a critical role of microtubules in plant cell development, in fine-tuning the positioning of Golgi stacks, as well as their involvement in cellulose synthesis and auxin polar transport. These emerging aspects of the biology of microtubules highlight adaptations of an ancestral machinery that plants have specifically evolved to support the functioning of the acto-myosin cytoskeleton, and mark new trends in our global appreciation of the complexity of organelle movement within the plant secretory pathway.