Nuclear migration and positioning within cells are critical for many developmental processes and are governed by the cytoskeletal network. Although mechanisms of nuclear-cytoskeletal attachment are unclear, growing evidence links a novel family of nuclear envelope (NE) proteins that share a conserved C-terminal SUN (Sad1/UNC-84 homology) domain. Analysis of Caenorhabditis elegans mutants has implicated UNC-84 in actin-mediated nuclear positioning by regulating NE anchoring of a giant actin-binding protein, ANC-1. Here, we report the identification of SUN1 as a lamin A-binding protein in a yeast two-hybrid screen. We demonstrate that SUN1 is an integral membrane protein located at the inner nuclear membrane. While the N-terminal domain of SUN1 is responsible for detergent-resistant association with the nuclear lamina and lamin A binding, lamin A/C expression is not required for SUN1 NE localization. Furthermore, SUN1 does not interact with type B lamins, suggesting that NE localization is ensured by binding to an additional nuclear component(s), most likely chromatin. Importantly, we find that the luminal C-terminal domain of SUN1 interacts with the mammalian ANC-1 homologs nesprins 1 and 2 via their conserved KASH domain. Our data provide evidence of a physical nuclear-cytoskeletal connection that is likely to be a key mechanism in nuclearcytoplasmic communication and regulation of nuclear position.The nuclear envelope (NE) is a double-membrane structure that separates chromatin from the cytoplasm, thereby allowing regulation of DNA replication and gene expression in eukaryotic cells. Nuclear pore complexes span the double membrane and regulate the passage of molecules between the cytoplasm and the nucleus (16). The outer nuclear membrane (ONM) is contiguous with, and biochemically similar to, the endoplasmic reticulum (ER). In contrast, the inner nuclear membrane (INM) contains a unique set of integral membrane proteins. Both nuclear pore complexes and INM proteins are anchored by association with the nuclear lamina, a network of lamin intermediate filaments that underlies the INM. The lamina, together with the associated INM proteins, provides structural support for the NE and sites for attachment of chromatin to the nuclear periphery (reviewed in reference 11).Most mammalian cells express two classes of lamin protein, types A and B (reviewed in reference 26). A-type lamins, the major isoforms of which are lamins A and C, are alternative splice products of the LMNA gene (8, 23). B-type lamins are mainly composed of lamins B1 and B2, which are encoded by separate genes (LMNB1 and LMNB2, respectively). A-and B-type lamins differ in their patterns of expression. While type B lamins are found in all nucleated somatic cells, type A lamins are absent in early embryos, their expression correlating with terminal differentiation (33). This has led to the suggestion that A-type lamins, although not essential for individual cell survival, are involved in determining differentiation patterns, possibly through effects on c...

The lipodystrophies are a group of disorders characterized by the absence or reduction of subcutaneous adipose tissue. Partial lipodystrophy (PLD; MIM 151660) is an inherited condition in which a regional (trunk and limbs) loss of fat occurs during the peri-pubertal phase. Additionally, variable degrees of resistance to insulin action, together with a hyperlipidaemic state, may occur and simulate the metabolic features commonly associated with predisposition to atherosclerotic disease. The PLD locus has been mapped to chromosome 1q with no evidence of genetic heterogeneity. We, and others, have refined the location to a 5.3-cM interval between markers D1S305 and D1S1600 (refs 5, 6). Through a positional cloning approach we have identified five different missense mutations in LMNA among ten kindreds and three individuals with PLD. The protein product of LMNA is lamin A/C, which is a component of the nuclear envelope. Heterozygous mutations in LMNA have recently been identified in kindreds with the variant form of muscular dystrophy (MD) known as autosomal dominant Emery-Dreifuss MD (EDMD-AD; ref. 7) and dilated cardiomyopathy and conduction-system disease (CMD1A). As LMNA is ubiquitously expressed, the finding of site-specific amino acid substitutions in PLD, EDMD-AD and CMD1A reveals distinct functional domains of the lamin A/C protein required for the maintenance and integrity of different cell types.

The nuclear envelope (NE) LINC complex, in mammals comprised of SUN domain and nesprin proteins, provides a direct connection between the nuclear lamina and the cytoskeleton, which contributes to nuclear positioning and cellular rigidity. SUN1 and SUN2 interact with lamin A, but lamin A is only required for NE localization of SUN2, and it remains unclear how SUN1 is anchored. Here, we identify emerin and short nesprin-2 isoforms as novel nucleoplasmic binding partners of SUN1/2. These have overlapping binding sites distinct from the lamin A binding site. However, we demonstrate that tight association of SUN1 with the nuclear lamina depends upon a short motif within residues 209–228, a region that does not interact significantly with known SUN1 binding partners. Moreover, SUN1 localizes correctly in cells lacking emerin. Importantly then, the major determinant of SUN1 NE localization has yet to be identified. We further find that a subset of lamin A mutations, associated with laminopathies Emery-Dreifuss muscular dystrophy (EDMD) and Hutchinson-Gilford progeria syndrome (HGPS), disrupt lamin A interaction with SUN1 and SUN2. Despite this, NE localization of SUN1 and SUN2 is not impaired in cell lines from either class of patients. Intriguingly, SUN1 expression at the NE is instead enhanced in a significant proportion of HGPS but not EDMD cells and strongly correlates with pre-lamin A accumulation due to preferential interaction of SUN1 with pre-lamin A. We propose that these different perturbations in lamin A-SUN protein interactions may underlie the opposing effects of EDMD and HGPS mutations on nuclear and cellular mechanics.

The gene encoding nuclear lamins A and C is mutated in at least three inherited disorders. Two of these, Emery-Dreifuss muscular dystrophy (EDMD-AD) and a form of dilated cardiomyopathy (CMD1A), involve muscle defects, and the other, familial partial lipodystrophy (FPLD), involves loss of subcutaneous adipose tissue. Mutations causing FPLD, in contrast to those causing muscle disorders, are tightly clustered within the C-terminal domain of lamin A/C. We investigated the expression and subcellular localization of FPLD lamin A mutants and found no abnormalities. We therefore set out to identify proteins interacting with the C-terminal domain of lamin A by screening a mouse 3T3-L1 adipocyte library in a yeast two-hybrid interaction screen. Using this approach, the adipocyte differentiation factor, sterol response element binding protein 1 (SREBP1) was identified as a novel lamin A interactor. In vitro glutathione S-transferase pull-down and in vivo co-immunoprecipitation studies confirmed an interaction between lamin A and both SREBP1a and 1c. A binding site for lamin A was identified in the N-terminal transcription factor domain of SREBP1, between residues 227 and 487. The binding of lamin A to SREBP1 was noticeably reduced by FPLD mutations. Interestingly, one EDMD-AD mutation also interfered with the interaction between lamin A and SREBP1. Whilst the physiological relevance of this interaction has yet to be elucidated, these data raise the intriguing possibility that fat loss seen in laminopathies may be caused, at least in part, by reduced binding of the adipocyte differentiation factor SREBP1 to lamin A.

SummaryThe nucleus is the main microtubule-organizing center (MTOC) in muscle cells due to the accumulation of centrosomal proteins and microtubule (MT) nucleation activity at the nuclear envelope (NE) [1, 2, 3, 4]. The relocalization of centrosomal proteins, including Pericentrin, Pcm1, and γ-tubulin, depends on Nesprin-1, an outer nuclear membrane (ONM) protein that connects the nucleus to the cytoskeleton via its N-terminal region [5, 6, 7]. Nesprins are also involved in the recruitment of kinesin to the NE and play a role in nuclear positioning in skeletal muscle cells [8, 9, 10, 11, 12]. However, a function for MT nucleation from the NE in nuclear positioning has not been established. Using the proximity-dependent biotin identification (BioID) method [13, 14], we found several centrosomal proteins, including Akap450, Pcm1, and Pericentrin, whose association with Nesprin-1α is increased in differentiated myotubes. We show that Nesprin-1α recruits Akap450 to the NE independently of kinesin and that Akap450, but not other centrosomal proteins, is required for MT nucleation from the NE. Furthermore, we demonstrate that this mechanism is disrupted in congenital muscular dystrophy patient myotubes carrying a nonsense mutation within the SYNE1 gene (23560 G>T) encoding Nesprin-1 [15, 16]. Finally, using computer simulation and cell culture systems, we provide evidence for a role of MT nucleation from the NE on nuclear spreading in myotubes. Our data thus reveal a novel function for Nesprin-1α/Nesprin-1 in nuclear positioning through recruitment of Akap450-mediated MT nucleation activity to the NE.