![]() ![]() ![]() elegans, the Drosophila SUN protein Klaroid is present in almost every cell type, whereas SUN4/Spag4 appears to be strictly confined to the male germ line. elegans SUN proteins show no overlapping activity, indicating that their respective LINC complexes occupy distinct functions. elegans, UNC-84 is expressed in most cells, whereas SUN-1/Matefin expression is restricted to germ cells. The expression of the individual SUN proteins depends on the cell type, suggesting cell type-specific adaption of LINC complexes to meet distinct cellular and physiological requirements. While single cell organisms apparently carry only one SUN domain protein, nematodes and flies contain two genes for SUN proteins, and the mammalian genome encodes at least five distinct members of the SUN protein family, Sun1-5. With increasing complexity of the organism, the number of SUN proteins also increases. The most recognizable feature of SUN proteins is a stretch of ~175 amino acids, usually at the very C terminus, termed ‘SUN domain’ based on the homology between Sad1 from Schizosaccharomyces pombe and UN C-84 from Caenorhabditis elegans. At the N terminus they contain a variable nucleoplasmic region, followed by a transmembrane helix connecting into a predicted coiled-coil segment localized to the PNS. SUN proteins are type II membrane proteins conserved across all eukaryotes and typically found in the INM. The SUN-KASH interaction complex has recently been solved by X-ray crystallography, providing a rich basis for detailed studies on LINC function. The center of LINC complexes is the interaction of INM-resident SUN ( Sad1 and UNC-84) proteins with ONM-resident KASH ( Klarsicht, ANC-1 and SYNE/Nesprin-1 and -2 Homology) proteins within the PNS. Physical interactions across the nuclear envelope are mediated via LInkers of Nucleoskeleton and Cytoskeleton (LINC) complexes. Underscoring the general importance of the nucleoskeleton and its link to the cytoskeleton, a growing number of diverse genetic disorders, including neurological, muscular, and premature aging have been linked to mutations in its constituents. Third, these nucleocytoplasmic linkages can also be used to determine the position of specific nuclear structures, like the ends of paired chromosomes during meiosis. Second, physical connections across the NE are attractive candidates for mediating mechanotransduction, a very fast signaling mechanism that results in transcription programs triggered by extracellular stimuli. First, the position of the nucleus within a cell needs to be maintained in many cell types, notably in neurons and muscle cells, suggesting that nuclei require a mechanism to be pulled into their desired place. Mechanical coupling of the nucleus to the cytoskeleton can potentially serve many purposes. Since that initial discovery, interest in the subject has grown rapidly. That the nucleus is also mechanically tethered to its environment has only been uncovered more recently. Mechanical communication has long been recognized for cells interacting with their surrounding. Macromolecular trafficking between the nucleus and the cytoplasm occurs mainly through NPCs, although recent findings suggest a vesicular transport mechanism across the PNS akin to nuclear egress by herpes viruses may be used for exceptionally large nuclear export cargo. ONM and INM are fused at circular openings, occupied by nuclear pore complexes (NPCs). The NE is an extension of the endoplasmic reticulum (ER) and consists of an outer nuclear membrane (ONM) and an inner nuclear membrane (INM), evenly separated by the perinuclear space (PNS) of ~50 nm width. This way, transcription and translation are spatially separated in eukaryotes, enabling sophisticated regulatory mechanisms for gene expression. The nuclear envelope (NE) is a double-lipid bilayer that separates the nucleus from the cytoplasm. ![]()
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