Plant KASH proteins SINE1 and SINE2 have synergistic and antagonistic interactions with actin-branching and actin-bundling factors

Linker of nucleoskeleton and cytoskeleton (LINC) complexes consist of outer nuclear membrane KASH proteins, interacting in the nuclear envelope lumen with inner nuclear membrane SUN proteins and connecting the nucleus and cytoskeleton. The paralogous Arabidopsis KASH proteins SINE1 and SINE2 function during stomatal dynamics induced by light-dark transitions and abscisic acid (ABA), which requires F-actin reorganization. SINE2 influences actin depolymerization and SINE1 actin repolymerization. The actin-related protein 2/3 (ARP2/3) complex, an actin nucleator, and the plant actin-bundling and -stabilizing factor SCAB1 are involved in stomatal aperture control. Here, we have tested the genetic interaction of SINE1 and SINE2 with SCAB1 and the ARP2/3 complex. We show that SINE1 and the ARP2/3 complex function in the same pathway during ABA-induced stomatal closure, while SINE2 and the ARP2/3 complex play opposing roles. The actin repolymerization defect observed in sine1-1 is partially rescued in scab1-2 sine1-1, while SINE2 is epistatic to SCAB1. In addition, SINE1 and ARP2/3 act synergistically in lateral root development. The absence of SINE2 renders trichome development independent of the ARP2/3 complex. Together, these data reveal complex and differential interactions of the two KASH proteins with the actin-remodeling apparatus and add evidence to the proposed differential role of SINE1 and SINE2 in actin dynamics.

A Nesprin-4/kinesin-1 cargo model for nuclear positioning in cochlear outer hair cells

Nuclear positioning is important for the functionality of many cell types and is mediated by interactions of cytoskeletal elements and nucleoskeleton proteins. Nesprin proteins, part of the linker of nucleoskeleton and cytoskeleton (LINC) complex, have been shown to participate in nuclear positioning in multiple cell types. Outer hair cells (OHCs) in the inner ear are specialized sensory epithelial cells that utilize somatic electromotility to amplify auditory signals in the cochlea. Recently, Nesprin-4 (encoded by Syne4) was shown to play a crucial role in nuclear positioning in OHCs. Syne4 deficiency in humans and mice leads to mislocalization of the OHC nuclei and cell death resulting in deafness. However, it is unknown how Nesprin-4 mediates the position of the nucleus, and which other molecular components are involved in this process. Here, we show that the interaction of Nesprin-4 and the microtubule motor kinesin-1 is mediated by a conserved 4 amino-acid motif. Using in vivo AAV gene delivery, we show that this interaction is critical for nuclear positioning and hearing in mice. Nuclear mislocalization and cell death of OHCs coincide with the onset of hearing and electromotility and are solely restricted to outer, but not inner, hair cells. Likewise, the C. elegans functional homolog of Nesprin-4, UNC-83, uses a similar motif to mediate interactions between migrating nuclei and kinesin-1. Overall, our results suggest that OHCs require unique cellular machinery for proper nuclear positioning at the onset of electromotility. This machinery relies on the interaction between Nesprin-4 and kinesin-1 motors supporting a microtubule cargo model for nuclear positioning.

SUN2 Modulates the Propagation of HSV-1

Herpesviruses assemble new viral particles in the nucleus. These nucleocapsids bud through the inner nuclear membrane to produce enveloped viral particles in the perinuclear space before fusing with the outer nuclear membrane to reach the cytoplasm. This unusual route is necessary since viral capsids are too large to pass through nuclear pores. However, the transient perinuclear nucleocapsids (250 nm in diameter) are also larger than the width of the perinuclear space (30 to 50 nm). Interestingly, linker of the nucleoskeleton and cytoskeleton (LINC) components SUN and KASH connect the inner and outer nuclear membranes and regulate their spacing. Previous work by others on the related pseudorabies virus and human cytomegalovirus showed that they functionally interact with SUN proteins. To clarify the role of SUN proteins, we explored their impact on herpes simplex virus 1 (HSV-1), another herpesvirus. Using dominant negative SUN mutants and RNA interference, we show that HSV-1 propagation is dependent on the LINC complex. In contrast to pseudorabies virus, SUN2 disruption by either approach led to increased HSV-1 extracellular viral yields. This SUN2 dependency may be linked to its greater impact on perinuclear spacing in infected cells compared to SUN1. Finally, the virus itself seems to modulate perinuclear spacing. IMPORTANCE The large size of herpesviruses prevents them from travelling across the nuclear pores, and they instead egress across the two nuclear membranes, generating short-lived enveloped perinuclear virions. This poses a challenge as the perinuclear space is smaller than the virions. This implies the separation (unzipping) of the two nuclear membranes to accommodate the viral particles. The LINC complex bridges the two nuclear membranes and is an important regulator of perinuclear spacing. Work by others hint at its functional implication during pseudorabies virus and cytomegalovirus propagation. The present study probes the importance for HSV-1 of the SUN proteins, the LINC components found in the inner nuclear membrane. Using dominant negative constructs and RNA interference (RNAi), the data reveal that SUN2 exhibits antiviral propriety toward HSV-1, as disrupting the protein leads to increased viral yields. This is in contrast with that reported for pseudorabies and suggests that differences among herpesviruses may, once again, prevail.

The assembly of a noncanonical LINC complex in Saccharomyces cerevisiae

The linker of nucleoskeleton and cytoskeleton (LINC) complex is a protein complex across the nuclear envelope and has maintained its general assembly mode throughout evolution. SUN and KASH proteins, which are the major components of LINC complex, interact with each other in the nuclear lumen to transmit forces across the nuclear envelope and have diverse functions. However, research of LINC complex in budding yeast has been limited due to the lack of identification of a canonical KASH protein and a cytoskeleton factor. Here, we review recent findings that addressed these puzzles in budding yeast. We highlight the distinct assembly model of the telomere-associated LINC complex in budding yeast, which could be beneficial for identifying LINC variants in other eukaryotes.

LINCking the Nuclear Envelope to Sperm Architecture

Nuclear architecture undergoes an extensive remodeling during spermatogenesis, especially at levels of spermatocytes (SPC) and spermatids (SPT). Interestingly, typical events of spermiogenesis, such as nuclear elongation, acrosome biogenesis, and flagellum formation, need a functional cooperation between proteins of the nuclear envelope and acroplaxome/manchette structures. In addition, nuclear envelope plays a key role in chromosome distribution. In this scenario, special attention has been focused on the LINC (linker of nucleoskeleton and cytoskeleton) complex, a nuclear envelope-bridge structure involved in the connection of the nucleoskeleton to the cytoskeleton, governing mechanotransduction. It includes two integral proteins: KASH- and SUN-domain proteins, on the outer (ONM) and inner (INM) nuclear membrane, respectively. The LINC complex is involved in several functions fundamental to the correct development of sperm cells such as head formation and head to tail connection, and, therefore, it seems to be important in determining male fertility. This review provides a global overview of the main LINC complex components, with a special attention to their subcellular localization in sperm cells, their roles in the regulation of sperm morphological maturation, and, lastly, LINC complex alterations associated to male infertility.

The Role of the LINC Complex in Sperm Development and Function

The LINC (LInker of Nucleoskeleton and Cytoskeleton) complex is localized within the nuclear envelope and consists of SUN (Sad1/UNc84 homology domain-containing) proteins located in the inner nuclear membrane and KASH (Klarsicht/Anc1/Syne1 homology domain-containing) proteins located in the outer nuclear membrane, hence linking nuclear with cytoplasmic structures. While the nucleoplasm-facing side acts as a key player for correct pairing of homolog chromosomes and rapid chromosome movements during meiosis, the cytoplasm-facing side plays a pivotal role for sperm head development and proper acrosome formation during spermiogenesis. A further complex present in spermatozoa is involved in head-to-tail coupling. An intact LINC complex is crucial for the production of fertile sperm, as mutations in genes encoding for complex proteins are known to be associated with male subfertility in both mice and men. The present review provides a comprehensive overview on our current knowledge of LINC complex subtypes present in germ cells and its central role for male reproduction. Future studies on distinct LINC complex components are an absolute requirement to improve the diagnosis of idiopathic male factor infertility and the outcome of assisted reproduction.

Arabidopsis KASH Proteins SINE1 and SINE2 Are Involved in Microtubule Reorganization During ABA-Induced Stomatal Closure

Abscisic acid (ABA) induces stomatal closure by utilizing complex signaling mechanisms, allowing for sessile plants to respond rapidly to ever-changing environmental conditions. ABA regulates the activity of plasma membrane ion channels and calcium-dependent protein kinases, Ca2+ oscillations, and reactive oxygen species (ROS) concentrations. Throughout ABA-induced stomatal closure, the cytoskeleton undergoes dramatic changes that appear important for efficient closure. However, the precise role of this cytoskeletal reorganization in stomatal closure and the nature of its regulation are unknown. We have recently shown that the plant KASH proteins SINE1 and SINE2 are connected to actin organization during ABA-induced stomatal closure but their role in microtubule (MT) organization remains to be investigated. We show here that depolymerizing MTs using oryzalin can restore ABA-induced stomatal closure deficits in sine1-1 and sine2-1 mutants. GFP-MAP4-visualized MT organization is compromised in sine1-1 and sine2-1 mutants during ABA-induced stomatal closure. Loss of SINE1 or SINE2 results in loss of radially organized MT patterning in open guard cells, aberrant MT organization during stomatal closure, and an overall decrease in the number of MT filaments or bundles. Thus, SINE1 and SINE2 are necessary for establishing MT patterning and mediating changes in MT rearrangement, which is required for ABA-induced stomatal closure.

MLKS2 is an ARM domain and F-actin-associated KASH protein that functions in stomatal complex development and meiotic chromosome segregation

The linker of nucleoskeleton and cytoskeleton (LINC) complex is an essential multi-protein structure spanning the eukaryotic nuclear envelope. The LINC complex functions to maintain nuclear architecture, positioning, and mobility, along with specialized functions in meiotic prophase and chromosome segregation. Members of the LINC complex were recently identified in maize, an important scientific and agricultural grass species. Here we characterized Maize LINC KASH AtSINE-like2, MLKS2, which encodes a highly conserved SINE-group plant KASH protein with characteristic N-terminal armadillo repeats (ARM). Using a heterologous expression system, we showed that actively expressed GFP-MLKS2 is targeted to the nuclear periphery and colocalizes with F-actin and the endoplasmic reticulum, but not microtubules in the cell cortex. Expression of GFP-MLKS2, but not GFP-MLKS2ΔARM, resulted in nuclear anchoring. Genetic analysis of transposon-insertion mutations, mlks2-1 and mlks2-2, showed that the mutant phenotypes were pleiotropic, affecting root hair nuclear morphology, stomatal complex development, multiple aspects of meiosis, and pollen viability. In male meiosis, the mutants showed defects for bouquet-stage telomere clustering, nuclear repositioning, perinuclear actin accumulation, dispersal of late prophase bivalents, and meiotic chromosome segregation. These findings support a model in which the nucleus is connected to cytoskeletal F-actin through the ARM-domain, predicted alpha solenoid structure of MLKS2. Functional conservation of MLKS2 was demonstrated through genetic rescue of the misshapen nuclear phenotype of an Arabidopsis (triple-WIP) KASH mutant. This study establishes a role for the SINE-type KASH proteins in affecting the dynamic nuclear phenomena required for normal plant growth and fertility.

Abbreviations: FRAP: Fluorescence recovery after photobleaching; DPI: Days post infiltration; OD: Optical density; MLKS2: Maize LINC KASH AtSINE-like2; LINC: Linker of nucleoskeleton and cytoskeleton; NE: Nuclear envelope; INM: Inner nuclear membrane; ONM: Outer nuclear membrane

Identification and characterization of genes encoding the nuclear envelope LINC complex in the monocot species Zea mays

The linker of nucleoskeleton to cytoskeleton (LINC) complex is an essential multi-protein structure spanning the nuclear envelope. It connects the cytoplasm to the nucleoplasm, functions to maintain nuclear shape and architecture and regulates chromosome dynamics during cell division. Knowledge of LINC complex composition and function in the plant kingdom is primarily limited to Arabidopsis, but critically missing from the evolutionarily distant monocots, which include grasses, the most important agronomic crops worldwide. To fill this knowledge gap, we identified and characterized 22 maize genes, including a new grass-specific KASH gene family. By using bioinformatic, biochemical and cell biological approaches, we provide evidence that representative KASH candidates localize to the nuclear periphery and interact with Zea mays (Zm)SUN2 in vivo FRAP experiments using domain deletion constructs verified that this SUN-KASH interaction was dependent on the SUN but not the coiled-coil domain of ZmSUN2. A summary working model is proposed for the entire maize LINC complex encoded by conserved and divergent gene families. These findings expand our knowledge of the plant nuclear envelope in a model grass species, with implications for both basic and applied cellular research.This article has an associated First Person interview with the first author of the paper.

Specialized Positioning of Myonuclei Near Cell-Cell Junctions

Skeletal muscles are large cells with multiple nuclei that are precisely positioned. The importance of the correct nuclear position is highlighted by the correlation between mispositioned nuclei and muscle disease (Spiro et al., 1966; Gueneau et al., 2009). Myonuclei are generally considered to be equivalent and therefore how far nuclei are from their nearest neighbor is the primary measurement of nuclear positioning. However, skeletal muscles have two specialized cell-cell contacts, the neuromuscular (NMJ) and the myotendinous junction (MTJ). Using these cell-cell contacts as reference points, we have determined that there are at least two distinct populations of myonuclei whose position is uniquely regulated. The post-synaptic myonuclei (PSMs) near the NMJ, and the myonuclei near the myotendinous junction myonuclei (MJMs) have different spacing requirements compared to other myonuclei. The correct positioning of pairs of PSMs depends on the specific action of dynein and kinesin. Positions of the PSMs and MJMs relative to the junctions that define them depend on the KASH-domain protein, Klar. We also found that MJMs are positioned close to the MTJ as a consequence of muscle stretching. Our study defines for the first time that nuclei in skeletal muscles are not all equally positioned, and that subsets of distinct myonuclei have specialized rules that dictate their spacing.

The KASH-containing isoform of Nesprin1 giant associates with ciliary rootlets of ependymal cells

Biallelic nonsense mutations of SYNE1 underlie a variable array of cerebellar and non-cerebellar pathologies of unknown molecular etiology. SYNE1 encodes multiple isoforms of Nesprin1 that associate with the nuclear envelope, with large cerebellar synapses and with ciliary rootlets of photoreceptors. Using two novel mouse models, we determined the expression pattern of Nesprin1 isoforms in the cerebellum whose integrity and functions are invariably affected by SYNE1 mutations. We further show that a giant isoform of Nesprin1 associates with the ciliary rootlets of ependymal cells that line brain ventricles and establish that this giant ciliary isoform of Nesprin1 harbors a KASH domain. Whereas cerebellar phenotypes are not recapitulated in Nes1g^STOP/STOP mice, these mice display a significant increase of ventricular volume. Together, these data fuel novel hypotheses about the molecular pathogenesis of SYNE1 mutations and support that KASH proteins may localize beyond the nuclear envelope in vivo.

Investigating LINC Complex Protein Homo-oligomerization in the Nuclear Envelopes of Living Cells Using Fluorescence Fluctuation Spectroscopy

Linkers of nucleoskeleton and cytoskeleton (LINC) complexes are conserved nuclear envelope (NE) spanning molecular bridges which mechanically integrate the nucleus with the cytoskeleton and mediate force transmission into the nucleoplasm. Despite their critical roles in fundamental cellular processes such as meiotic chromosome and nuclear positioning, the mechanism of LINC complex assembly in cells remains unclear. To begin to address this deficit, we recently developed z-scan fluorescence fluctuation spectroscopy (FFS) and brightness analysis as a method for quantifying the oligomeric states of fluorescent protein-tagged NE proteins including nesprins and SUN proteins. Since the homo-oligomerization of SUN2 is critical for its ability to interact with nesprins within the perinuclear space, the knowledge obtained through quantitative brightness experiments reveals important insights into the in vivo mechanisms of LINC complex assembly. Here we describe the procedure we use to determine the brightness of proteins in the NE of living cells. In addition to the measurement procedure, we discuss the instrumentation requirements and present the results of applying this procedure to measure the brightness of nesprin-2 and SUN2.

Genetic Analysis of Nuclear Migration and Anchorage to Study LINC Complexes During Development of Caenorhabditis elegans

Studying nuclear positioning in developing tissues of the model nematode Caenorhabditis elegans greatly contributed to the discovery of SUN and KASH proteins and the formation of the LINC model. Such studies continue to make important contributions into both how LINC complexes are regulated and how defects in LINC components disrupt normal development. The methods described explain how to observe and quantify the following: nuclear migration in embryonic dorsal hypodermal cells, nuclear migration through constricted spaces in larval P cells, nuclear positioning in the embryonic intestinal primordia, and nuclear anchorage in syncytial hypodermal cells. These methods will allow others to employ nuclear positioning in C. elegans as a model to further explore LINC complex regulation and function.

Centrifugal Displacement of Nuclei Reveals Multiple LINC Complex Mechanisms for Homeostatic Nuclear Positioning

Nuclear movement is critical for developmental events, cell polarity, and migration and is usually mediated by linker of nucleoskeleton and cytoskeleton (LINC) complexes connecting the nucleus to cytoskeletal elements. Compared to active nuclear movement, relatively little is known about homeostatic positioning of nuclei, including whether it is an active process. To explore homeostatic nuclear positioning, we developed a method to displace nuclei in adherent cells using centrifugal force. Nuclei displaced by centrifugation rapidly recentered by mechanisms that depended on cell context. In cell monolayers with wounds oriented orthogonal to the force, nuclei were displaced toward the front and back of the cells on the two sides of the wound. Nuclei recentered from both positions, but at different rates and with different cytoskeletal linkage mechanisms. Rearward recentering was actomyosin, nesprin-2G, and SUN2 dependent, whereas forward recentering was microtubule, dynein, nesprin-2G, and SUN1 dependent. Nesprin-2G engaged actin through its N terminus and microtubules through a novel dynein interacting site near its C terminus. Both activities were necessary to maintain nuclear position in uncentrifuged cells. Thus, even when not moving, nuclei are actively maintained in position by engaging the cytoskeleton through the LINC complex.

Implications for Diverse Functions of the LINC Complexes Based on the Structure

The linker of nucleoskeleton and cytoskeleton (LINC) complex is composed of the outer and inner nuclear membrane protein families Klarsicht, Anc-1, and Syne homology (KASH), and Sad1 and UNC-84 (SUN) homology domain proteins. Increasing evidence has pointed to diverse functions of the LINC complex, such as in nuclear migration, nuclear integrity, chromosome movement and pairing during meiosis, and mechanotransduction to the genome. In metazoan cells, the nuclear envelope possesses the nuclear lamina, which is a thin meshwork of intermediate filaments known as A-type and B-type lamins and lamin binding proteins. Both of lamins physically interact with the inner nuclear membrane spanning SUN proteins. The nuclear lamina has also been implicated in various functions, including maintenance of nuclear integrity, mechanotransduction, cellular signalling, and heterochromatin dynamics. Thus, it is clear that the LINC complex and nuclear lamins perform diverse but related functions. However, it is unknown whether the LINC complex-lamins interactions are involved in these diverse functions, and their regulation mechanism has thus far been elusive. Recent structural analysis suggested a dynamic nature of the LINC complex component, thus providing an explanation for LINC complex organization. This review, elaborating on the integration of crystallographic and biochemical data, helps to integrate this research to gain a better understanding of the diverse functions of the LINC complex.

Nuclei migrate through constricted spaces using microtubule motors and actin networks in C. elegans hypodermal cells

Cellular migrations through constricted spaces are a crucial aspect of many developmental and disease processes including hematopoiesis, inflammation and metastasis. A limiting factor in these events is nuclear deformation. Here, we establish an in vivo model in which nuclei can be visualized while moving through constrictions and use it to elucidate mechanisms for nuclear migration. C. elegans hypodermal P-cell larval nuclei traverse a narrow space that is about 5% their width. This constriction is blocked by fibrous organelles, structures that pass through P cells to connect the muscles to cuticle. Fibrous organelles are removed just prior to nuclear migration, when nuclei and lamins undergo extreme morphological changes to squeeze through the space. Both actin and microtubule networks are organized to mediate nuclear migration. The LINC complex, consisting of the SUN protein UNC-84 and the KASH protein UNC-83, recruits dynein and kinesin-1 to the nuclear surface. Both motors function in P-cell nuclear migration, but dynein, functioning through UNC-83, plays a more central role as nuclei migrate towards minus ends of polarized microtubule networks. Thus, the nucleoskeleton and cytoskeleton are coordinated to move nuclei through constricted spaces.

SUN anchors pollen WIP-WIT complexes at the vegetative nuclear envelope and is necessary for pollen tube targeting and fertility

LINC (linker of nucleoskeleton and cytoskeleton) complexes play an essential role in nuclear migration by connecting the nucleus to the cytoskeleton and/or motor proteins. Plant LINC complexes have recently been identified in Arabidopsis thaliana, with the inner nuclear membrane SUN and outer nuclear membrane WIP proteins comprising the first identified complex. A recent study identified a nuclear movement defect in Arabidopsis pollen vegetative nuclei linked to the outer nuclear envelope WIP and WIT proteins. However, the role that SUN proteins may play in pollen nuclear migration has yet to be addressed. To explore this question, a SUN2 lumenal domain that was targeted to the ER specifically in pollen was over-expressed. It is shown that the ER-targeted SUN2 lumenal domain was able to displace WIP and WIT proteins from the pollen vegetative nuclear envelope. Expression of this dominant-negative transgene led to impaired VN mobility, impaired pollen tube guidance, and defective pollen tube reception. The observed pollen defects are similar to phenotypes observed in a wip1-1 wip2-1 wip3-1 wit1-1 wit2-1 mutant. It is also shown that these defects were dependent on the KASH-binding function of the SUN2 lumenal domain. These data support a model where LINC complexes formed by SUN, WIP, and WIT at the VNE are responsible for VN migration and suggest an important function of SUN, WIP, and WIT in pollen tube guidance and reception.

Drosophila KASH-domain protein Klarsicht regulates microtubule stability and integrin receptor localization during collective cell migration

During collective migration of the Drosophila embryonic salivary gland, cells rearrange to form a tube of a distinct shape and size. Here, we report a novel role for the Drosophila Klarsicht-Anc-Syne Homology (KASH) domain protein Klarsicht (Klar) in the regulation of microtubule (MT) stability and integrin receptor localization during salivary gland migration. In wild-type salivary glands, MTs became progressively stabilized as gland migration progressed. In embryos specifically lacking the KASH domain containing isoforms of Klar, salivary gland cells failed to rearrange and migrate, and these defects were accompanied by decreased MT stability and altered integrin receptor localization. In muscles and photoreceptors, KASH isoforms of Klar work together with Klaroid (Koi), a SUN domain protein, to position nuclei; however, loss of Koi had no effect on salivary gland migration, suggesting that Klar controls gland migration through novel interactors. The disrupted cell rearrangement and integrin localization observed in klar mutants could be mimicked by overexpressing Spastin (Spas), a MT severing protein, in otherwise wild-type salivary glands. In turn, promoting MT stability by reducing spas gene dosage in klar mutant embryos rescued the integrin localization, cell rearrangement and gland migration defects. Klar genetically interacts with the Rho1 small GTPase in salivary gland migration and is required for the subcellular localization of Rho1. We also show that Klar binds tubulin directly in vitro. Our studies provide the first evidence that a KASH-domain protein regulates the MT cytoskeleton and integrin localization during collective cell migration.

A variant of Nesprin1 giant devoid of KASH domain underlies the molecular etiology of autosomal recessive cerebellar ataxia type I

Nonsense mutations across the whole coding sequence of Syne1/Nesprin1 have been linked to autosomal recessive cerebellar ataxia Type I (ARCA1). However, nothing is known about the molecular etiology of this late-onset debilitating pathology. In this work, we report that Nesprin1 giant is specifically expressed in CNS tissues. We also identified a CNS-specific splicing event that leads to the abundant expression of a KASH-LESS variant of Nesprin1 giant (KLNes1g) in the cerebellum. KLNes1g displayed a noncanonical localization at glomeruli of cerebellar mossy fibers whereas Nesprin2 exclusively decorated the nuclear envelope of all cerebellar neurons. In immunogold electron microscopy, KLNes1g colocalized both with synaptic vesicles within mossy fibers and with dendritic membranes of cerebellar granule neurons. We further identified vesicle- and membrane-associated proteins in KLNes1g immunoprecipitates. Together, our results suggest that the loss of function of KLNes1g resulting from Nesprin1 nonsense mutations underlies the molecular etiology of ARCA1.

Plant nuclear shape is independently determined by the SUN-WIP-WIT2-myosin XI-i complex and CRWN1

Nuclei undergo dynamic shape changes during plant development, but the mechanism is unclear. In Arabidopsis, Sad1/UNC-84 (SUN) proteins, WPP domain-interacting proteins (WIPs), WPP domain-interacting tail-anchored proteins (WITs), myosin XI-i, and CROWDED NUCLEI 1 (CRWN1) have been shown to be essential for nuclear elongation in various epidermal cell types. It has been proposed that WITs serve as adaptors linking myosin XI-i to the SUN-WIP complex at the nuclear envelope (NE). Recently, an interaction between Arabidopsis SUN1 and SUN2 proteins and CRWN1, a plant analog of lamins, has been reported. Therefore, the CRWN1-SUN-WIP-WIT-myosin XI-i interaction may form a linker of the nucleoskeleton to the cytoskeleton complex. In this study, we investigate this proposed mechanism in detail for nuclei of Arabidopsis root hairs and trichomes. We show that WIT2, but not WIT1, plays an essential role in nuclear shape determination by recruiting myosin XI-i to the SUN-WIP NE bridges. Compared with SUN2, SUN1 plays a predominant role in nuclear shape. The NE localization of SUN1, SUN2, WIP1, and a truncated WIT2 does not depend on CRWN1. While crwn1 mutant nuclei are smooth, the nuclei of sun or wit mutants are invaginated, similar to the reported myosin XI-i mutant phenotype. Together, this indicates that the roles of the respective WIT and SUN paralogs have diverged in trichomes and root hairs, and that the SUN-WIP-WIT2-myosin XI-i complex and CRWN1 independently determine elongated nuclear shape. This supports a model of nuclei being shaped both by cytoplasmic forces transferred to the NE and by nucleoplasmic filaments formed under the NE.

The plant nuclear envelope as a multifunctional platform LINCed by SUN and KASH

The nuclear envelope (NE) is a double membrane system enclosing the genome of eukaryotes. Besides nuclear pore proteins, which form channels at the NE, nuclear membranes are populated by a collection of NE proteins that perform various cellular functions. However, in contrast to well-conserved nuclear pore proteins, known NE proteins share little homology between opisthokonts and plants. Recent studies on NE protein complexes formed by Sad1/UNC-84 (SUN) and Klarsicht/ANC-1/Syne-1 Homology (KASH) proteins have advanced our understanding of plant NE proteins and revealed their function in anchoring other proteins at the NE, nuclear shape determination, nuclear positioning, anti-pathogen defence, root development, and meiotic chromosome organization. In this review, we discuss the current understanding of plant SUN, KASH, and other related NE proteins, and compare their function with the opisthokont counterparts.

Nuclei in motion: movement and positioning of plant nuclei in development, signaling, symbiosis, and disease

While textbook figures imply nuclei as resting spheres at the center of idealized cells, this picture fits few real situations. Plant nuclei come in many shapes and sizes, and can be actively transported within the cell. In several contexts, this nuclear movement is tightly coupled to a developmental program, the response to an abiotic signal, or a cellular reprogramming during either mutualistic or parasitic plant-microbe interactions. While many such phenomena have been observed and carefully described, the underlying molecular mechanism and the functional significance of the nuclear movement are typically unknown. Here, we survey recent as well as older literature to provide a concise starting point for applying contemporary molecular, genetic and biochemical approaches to this fascinating, yet poorly understood phenomenon.

How plants LINC the SUN to KASH

Linkers of the nucleoskeleton to the cytoskeleton (LINC) complexes formed by SUN and KASH proteins are conserved eukaryotic protein complexes that bridge the nuclear envelope (NE) via protein-protein interactions in the NE lumen. Revealed by opisthokont studies, LINC complexes are key players in multiple cellular processes, such as nuclear and chromosomal positioning and nuclear shape determination, which in turn influence the generation of gametes and several aspects of development. Although comparable processes have long been known in plants, the first plant nuclear envelope bridging complexes were only recently identified. WPP domain-interacting proteins at the outer NE have little homology to known opisthokont KASH proteins, but form complexes with SUN proteins at the inner NE that have plant-specific properties and functions. In this review, we will address the importance of LINC complex-regulated processes, describe the plant NE bridging complexes and compare them to opisthokont LINC complexes.