Advancing knowledge of the plant nuclear periphery and its application for crop science

In this review, we explore recent advances in knowledge of the structure and dynamics of the plant nuclear envelope. As a paradigm, we focused our attention on the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, a structurally conserved bridging complex comprising SUN domain proteins in the inner nuclear membrane and KASH domain proteins in the outer nuclear membrane. Studies have revealed that this bridging complex has multiple functions with structural roles in positioning the nucleus within the cell, conveying signals across the membrane and organizing chromatin in the 3D nuclear space with impact on gene transcription. We also provide an up-to-date survey in nuclear dynamics research achieved so far in the model plant Arabidopsis thaliana that highlights its potential impact on several key plant functions such as growth, seed maturation and germination, reproduction and response to biotic and abiotic stress. Finally, we bring evidences that most of the constituents of the LINC Complex and associated components are, with some specificities, conserved in monocot and dicot crop species and are displaying very similar functions to those described for Arabidopsis. This leads us to suggest that a better knowledge of this system and a better account of its potential applications will in the future enhance the resilience and productivity of crop plants.

Chain reaction: LINC complexes and nuclear positioning

Nuclear positioning plays an essential role in defining cell architecture and behaviour in both development and disease, and nuclear location frequently adjusts according to internal and external cues. For instance, during periods of migration in many cell types, the nucleus may be actively repositioned behind the microtubule-organising centre. Nuclear movement, for the most part, is dependent upon coupling of the cytoskeleton to the nuclear periphery. This is accomplished largely through SUN and KASH domain proteins, which together assemble to form LINC (linker of the nucleoskeleton and cytoskeleton) complexes spanning the nuclear envelope. SUN proteins of the inner nuclear membrane provide a connection to nuclear structures while acting as a tether for outer nuclear membrane KASH proteins. The latter contain binding sites for diverse cytoskeletal components. Recent publications highlight new aspects of LINC complex regulation revealing that the interplay between SUN and KASH partners can strongly influence how the nucleus functionally engages with different branches of the cytoskeleton.

Exploring the evolution of the proteins of the plant nuclear envelope

In this study, we explore the plasticity during evolution of proteins of the higher plant nuclear envelope (NE) from the most ancestral plant species to advanced angiosperms. The higher plant NE contains a functional Linker of Nucleoskeleton and Cytoskeleton (LINC) complex based on conserved Sad1-Unc84 (SUN) domain proteins and plant specific Klarsicht/Anc1/Syne homology (KASH) domain proteins. Recent evidence suggests the presence of a plant lamina underneath the inner membrane and various coiled-coil proteins have been hypothesized to be associated with it including Crowded Nuclei (CRWN; also termed LINC and NMCP), Nuclear Envelope Associated Protein (NEAP) protein families as well as the CRWN binding protein KAKU4. SUN domain proteins appear throughout with a key role for mid-SUN proteins suggested. Evolution of KASH domain proteins has resulted in increasing complexity, with some appearing in all species considered, while other KASH proteins are progressively gained during evolution. Failure to identify CRWN homologs in unicellular organisms included in the study and their presence in plants leads us to speculate that convergent evolution may have occurred in the formation of the lamina with each kingdom having new proteins such as the Lamin B receptor (LBR) and Lamin-Emerin-Man1 (LEM) domain proteins (animals) or NEAPs and KAKU4 (plants). Our data support a model in which increasing complexity at the nuclear envelope occurred through the plant lineage and suggest a key role for mid-SUN proteins as an early and essential component of the nuclear envelope.

Characterization of two distinct subfamilies of SUN-domain proteins in Arabidopsis and their interactions with the novel KASH-domain protein AtTIK

SUN-domain proteins belong to a gene family including classical Cter-SUN and mid-SUN subfamilies differentiated by the position of the SUN domain within the protein. Although present in animal and plant species, mid-SUN proteins have so far remained poorly described. Here, we used a combination of genetics, yeast two-hybrid and in planta transient expression methods to better characterize the SUN family in Arabidopsis thaliana. First, we validated the mid-SUN protein subfamily as a monophyletic group conserved from yeast to plant. Arabidopsis Cter-SUN (AtSUN1 and AtSUN2) and mid-SUN (AtSUN3 and AtSUN4) proteins expressed as fluorescent protein fusions are membrane-associated and localize to the nuclear envelope (NE) and endoplasmic reticulum. However, only the Cter-SUN subfamily is enriched at the NE. We investigated interactions in and between members of the two subfamilies and identified the coiled-coil domain as necessary for mediating interactions. The functional significance of the mid-SUN subfamily was further confirmed in mutant plants as essential for early seed development and involved in nuclear morphology. Finally, we demonstrated that both subfamilies interact with the KASH domain of AtWIP1 and identified a new root-specific KASH-domain protein, AtTIK. AtTIK localizes to the NE and affects nuclear morphology. Our study indicates that Arabidopsis Cter-SUN and mid-SUN proteins are involved in a complex protein network at the nuclear membranes, reminiscent of the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex found in other kingdoms.

Temporal and tissue-specific disruption of LINC complexes in vivo

Migration and anchorage of nuclei within developing and adult tissues rely on Linkers of the Nucleoskeleton to the Cytoskeleton (LINC complexes). These macromolecular assemblies span the nuclear envelope and physically couple chromatin and nuclear lamina to cytoplasmic cytoskeletal networks. LINC complexes assemble within the perinuclear space through direct interactions between the respective evolutionary-conserved SUN and KASH domains of Sun proteins, which reside within the inner nuclear membrane, and Nesprins, which reside within the outer nuclear membrane. Here, we describe and validate a dominant-negative transgenic strategy allowing for the disruption of endogenous SUN/KASH interactions through the inducible expression of a recombinant KASH domain. Our approach, which is based on the Cre/Lox system, allows for the targeted disruption of LINC complexes in a wide array of mouse tissues or specific cell types thereof and bypasses the perinatal lethality and potential cell nonautonomous effects of current mouse models based on germline inactivation of genes encoding Sun proteins and Nesprins. For these reasons, this mouse model provides a useful tool to evaluate the physiological relevance of LINC complexes integrity during development and homeostasis in a wide array of mammalian tissues.

Developmental regulation of linkers of the nucleoskeleton to the cytoskeleton during mouse postnatal retinogenesis

Sun proteins and Nesprins are two families of proteins whose direct interactions across the nuclear envelope provide for the core of Linkers of the Nucleoskeleton to the Cytoskeleton (LINC complexes) that physically connect the nucleus interior to cytoskeletal networks. Whereas LINC complexes play essential roles in nuclear migration anchorage and underlie normal CNS development, the developmental regulation of their composition remains largely unknown. In this study, we examined the spatiotemporal expression of lamins, Sun proteins and Nesprins during postnatal mouse retinal development. Whereas retinal precursor cells mostly express B-type lamins, Sun1, and high molecular weight isoforms of Nesprins, post-mitotic retinal cells are characterized by a drastic downregulation of the latter, the expression of A-type lamins, and the strong induction of a specific isoform of Nesprin1 late in retinal development. Importantly, our results emphasize different spatiotemporal expression for Nesprin1 and Nesprin2 and further suggest an important role for KASH-less isoforms of Nesprin1 in the CNS. In conclusion, the transition from retinal precursor cells undergoing interkinetic nuclear migration to post-mitotic retinal cells undergoing nuclear translocation and/or anchorage is accompanied by a profound remodeling of LINC complexes composition. This remodeling may reflect different requirements of nuclear dynamics at different stages of CNS development.

LINCing complex functions at the nuclear envelope: what the molecular architecture of the LINC complex can reveal about its function

Linker of nucleoskeleton and cytoskeleton (LINC) complexes span the double membrane of the nuclear envelope (NE) and physically connect nuclear structures to cytoskeletal elements. LINC complexes are envisioned as force transducers in the NE, which facilitate processes like nuclear anchorage and migration, or chromosome movements. The complexes are built from members of two evolutionary conserved families of transmembrane (TM) proteins, the SUN (Sad1/UNC-84) domain proteins in the inner nuclear membrane (INM) and the KASH (Klarsicht/ANC-1/SYNE homology) domain proteins in the outer nuclear membrane (ONM). In the lumen of the NE, the SUN and KASH domains engage in an intimate assembly to jointly form a NE bridge. Detailed insights into the molecular architecture and atomic structure of LINC complexes have recently revealed the molecular basis of nucleo-cytoskeletal coupling. They bear important implications for LINC complex function and suggest new potential and as yet unexplored roles, which the complexes may play in the cell.