Characterization of two isoforms of the skeletal muscle LIM protein 1, SLIM1. Localization of SLIM1 at focal adhesions and the isoform slimmer in the nucleus of myoblasts and cytoplasm of myotubes suggests distinct roles in the cytoskeleton and in nuclear-cytoplasmic communication.

We have cloned and characterized a novel isoform of the skeletal muscle LIM protein 1 (SLIM1), designated SLIMMER. SLIM1 contains an N-terminal single zinc finger followed by four LIM domains. SLIMMER is identical to SLIM1 over the first three LIM domains but contains a novel C-terminal 96 amino acids with three potential bipartite nuclear localization signals, a putative nuclear export sequence, and 27 amino acids identical to the RBP-J binding region of KyoT2, a murine isoform of SLIM1. SLIM1 localized to the cytosol of Sol8 myoblasts and myotubes. SLIMMER was detected in the nucleus of myoblasts and, following differentiation into myotubes, was exclusively cytosolic. Recombinant green fluorescent protein-SLIM1 localized to the cytoplasm and associated with focal adhesions and actin filaments in COS-7 cells, while green fluorescent protein-SLIMMER was predominantly nuclear. SLIMMER truncation mutants revealed that the first nuclear localization signal mediates nuclear localization. The addition of the proposed nuclear export sequence decreased the level of exclusively nuclear expression and increased cytosolic SLIMMER expression in COS-7 cells. The leucine-rich nuclear export signal was required for the export of SLIMMER from the nucleus of myoblasts to the cytoplasm of myotubes. Collectively, these results suggest distinct roles for SLIM1 and SLIMMER in focal adhesions and nuclear-cytoplasmic communication.     

The LIM proteins FHL1 and FHL3 are expressed differently in skeletal muscle

We have determined the complete mRNA sequence of FHL3 (formerly SLIM2). We have confirmed that it is a member of the family of LIM proteins that share a similar secondary protein structure, renamed as Four-and-a-Half-LIM domain (or FHL) proteins in accordance with this structure. The "half-LIM" domain is a single zinc finger domain that may represent a subfamily of LIM domains and defines this particular family of LIM proteins. The distribution of FHL mRNA expression within a variety of murine tissues is complex. Both FHL1 and FHL3 were expressed in a number of skeletal muscles while FHL2 was expressed at high levels in cardiac muscle. Localisation of FHL3 to human chromosome 1 placed this gene in the proximity of, but not overlapping with, alleles associated with muscle diseases. FHL1 and FHL3 mRNAs were reciprocally expressed in the murine C2C12 skeletal muscle cell line and this suggested that the pattern of expression was linked to key events in myogenesis.     

Skeletal muscle LIM protein 1 (SLIM1/FHL1) induces alpha 5 beta 1-integrin-dependent myocyte elongation

Skeletal muscle LIM protein 1 (SLIM1/FHL1) contains four and a half LIM domains and is highly expressed in skeletal and cardiac muscle. Elevated SLIM1 mRNA expression has been associated with postnatal skeletal muscle growth and stretch-induced muscle hypertrophy in mice. Conversely, SLIM1 mRNA levels decrease during muscle atrophy. Together, these observations suggest a link between skeletal muscle growth and increased SLIM1 expression. However, the precise function of SLIM1 in skeletal muscle, specifically the role of SLIM1 during skeletal muscle differentiation, is not known. This study investigated the effect of increased SLIM1 expression during skeletal muscle differentiation. Western blot analysis showed an initial decrease followed by an increase in SLIM1 expression during differentiation. Overexpression of SLIM1 in Sol8 or C₂C₁₂ skeletal muscle cell lines, at levels observed during hypertrophy, induced distinct effects in differentiating myocytes and undifferentiated reserve cells, which were distinguished by differential staining for two markers of differentiation, MyoD and myogenin. In differentiating skeletal myocytes, SLIM1 overexpression induced hyperelongation, which, by either plating cells on poly-L-lysine or using a series of peptide blockade experiments, was shown to be specifically dependent on ligand binding to the ₅₁-integrin, whereas in reserve cells, SLIM1 overexpression induced the formation of multiple cytoplasmic protrusions (branching), which was also integrin mediated. These results suggest that SLIM1 may play an important role during the early stages of skeletal muscle differentiation, specifically in ₅₁-integrin-mediated signaling pathways. myoblast; proteins and differentiation     

Skeletal muscle LIM protein 1 regulates integrin-mediated myoblast adhesion,spreading, and migration

The skeletal muscle LIM protein 1 (SLIM1) is highly expressed in skeletal and cardiac muscle, and itsexpression is downregulated significantly in dilated humancardiomyopathy. However, the function of SLIM1 is unknown. In thisstudy, we investigated the intracellular localization of SLIM1.Endogenous and recombinant SLIM1 localized to the nucleus, stressfibers, and focal adhesions in skeletal myoblasts plated onfibronectin, collagen, or laminin. However, after inhibition ofintegrin signaling either by plating on poly-L-lysine or bysoluble RGD peptide, SLIM1 localized diffusely in the cytosol, withdecreased nuclear expression. Disruption of the actin cytoskeleton bycytochalasin D did not inhibit nuclear localization of SLIM1 inintegrin-activated cells. Green fluorescent protein-tagged SLIM1shuttled in the nucleus of untransfected NIH 3T3 cells, in aheterokaryon fusion assay. Overexpression of SLIM1 in Sol8 myoblastsinhibited cell adhesion and promoted cell spreading and migration.These studies show SLIM1 localizes in an integrin-dependent manner tothe nucleus and focal adhesions where it functions downstream ofintegrin activation to promote cell spreading and migration.

     

Interaction of hCLIM1, an enigma family protein, with alpha-actinin 2

Enigma proteins are proteins that possess a PDZ domain at the amino terminal and one to three LIM domains at the carboxyl terminal. They are cytoplasmic proteins that are involved with the cytoskeleton and signal transduction pathway. By virtue of the two protein interacting domains, they are capable of protein-protein interactions. Here we report a study on a human Enigma protein hCLIM1, in particular. Our study describes the interaction of the human 36 kDa carboxyl terminal LIM domain protein (hCLIM1), the human homologue of CLP36 in rat, with alpha-actinin 2, the skeletal muscle isoform of alpha-actinin. hCLIM1 protein was shown to interact with alpha-actinin 2 by yeast two-hybrid screening and immunochemical analyses. Yeast two-hybrid analyses also demonstrated that the LIM domain of hCLIM1 binds to the EF-hand region of alpha-actinin 2, defining a new mode of LIM domain interactions. Immunofluorescent study demonstrates that hCLIM1 colocalizes with alpha-actinin at the Z-disks in human myocardium. Taken together, our experimental results suggest that hCLIM1is a novel cytoskeletal protein and may act as an adapter that brings other proteins to the cytoskeleton.     

The rhombotin gene family encode related LIM-domain proteins whose differing expression suggests multiple roles in mouse development.

The rhombotin (RBTN1 or Ttg-1) gene was first identified at a chromosome translocation in a T-cell acute leukaemia and later used to isolate two related genes (RBTN2 or Ttg-2 and RBTN3). Complete characterization of these genes in man and mouse shows that all three encode cysteine-rich proteins with typical LIM domains. RBTN1 and RBTN3-derived proteins have 98% identity in the LIM domains but are located on separate chromosomes in man and in mouse while RBTN1 and RBTN2, both located on human chromosome 11p but are on separate chromosomes in mouse, are only 48% identical in this part of the protein. The exon organization of RBTN1 and RBTN3 genes are similar, both having an intron, absent from the RBTN2 gene, in the LIM2-encoding region. The remarkable similarity between rbtn-1 and rbtn-3 proteins is parallelled in their expression patterns in mouse development, since both genes show high expression in restricted areas of the brain, but little lymphoid expression. rbtn-2 expression, however, is more ubiquitous. This gene shows a low level of thymus expression but high expression in fetal liver, adult spleen and B-cell lines, consistent with a role in B-cell development. These results suggest multiple cellular targets for the action of these proteins during development.     

Up-regulation of skeletal muscle LIM protein 1 gene by 25-hydroxycholesterol may mediate morphological changes of rat aortic smooth muscle cells

Changes in the expression level of the skeletal muscle LIM protein 1 (SLIM1) in cultured A10 cells were monitored in response to 25-hydroxycholesterol (25-HC), an oxidized form of cholesterol present in the oxidized low-density lipoproteins. The level of SLIM1 mRNA was elevated in a time- and concentration-dependent manner by treatment of 25-HC. Expressions of smooth muscle (SM) alpha-actin and calponin-1 (CNN-1), early markers for SMC differentiation, were also increased by the 25-HC treatments. Expressions of all three genes (SLIM1, SM alpha-actin and CNN-1) were simultaneously elevated in the cells treated with 9-cis retinoic acid (RA). On the other hand, the SLIM1 expression induced by the 25-HC or 9-cis RA (as well as SM alpha-actin and CNN-1) was decreased by the treatment of 15d-PGJ2. Since the 25-HC, 9-cis RA and 15d-PGJ2 were ligands for the LXR, RXRalpha and PPARgamma respectively, there might be a functional positive cross-talk between LXR and RXRalpha pathways and a negative cross-talk between PPARgamma and LXR and/or RXRalpha pathways in the regulation of SLIM1 expression. The cells stably transfected with the expressional vector for SLIM1 also showed an elevation in the levels of SM alpha-actin and CNN-1. In addition, an over-production of SLIM1 in the cells resulted in a change in the cell-shape into a spindle-like form, which is identical to that observed after a prolonged treatment of the cells with cholesterol.     

ZRP-1, a zyxin-related protein, interacts with the second PDZ domain of the cytosolic protein tyrosine phosphatase hPTP1E.

Protein-protein interactions play an important role in the specificity of cellular signaling cascades. By using the yeast two-hybrid system, a specific interaction was identified between the second PDZ domain of the cytosolic protein tyrosine phosphatase hPTP1E and a novel protein, which was termed ZRP-1 to indicate its sequence similarity to the Zyxin protein family. The mRNA encoding this protein is distributed widely in human tissues and contains an open reading frame of 1428 base pairs, predicting a polypeptide of 476 amino acid residues. The deduced protein displays a proline-rich amino-terminal region and three double zinc finger LIM domains at its carboxyl terminus. The specific interaction of this novel protein with the second PDZ domain of hPTP1E was demonstrated both in vitro, using bacterially expressed proteins, and in vivo, by co-immunoprecipitation studies. Deletion analysis indicated that an intact carboxyl terminus is required for its interaction with the second PDZ domain of hPTP1E in the yeast two-hybrid system and suggested that other sequences, including the LIM domains, also participate in the interaction. The genomic organization of the ZRP-1 coding sequence is identical to that of the lipoma preferred partner gene, another Zyxin-related protein, suggesting that the two genes have evolved from a recent gene duplication event.     

一个新的家蚕LIMonly蛋白基因的序列分析和原核表达

LIM蛋白是一类含有LIM结构域的真核蛋白家族,它可介导细胞分化和发育,转录调控和激活、参与细胞骨架形成等多种重要的生物调节过程。从家蚕蛹cDNA文库获得一个新的编码LIM only蛋白的基因,我们将其命名为BmLIMO（BombyxmoriLIMonly protein）。该基因ORF长度为561 bp,编码长度为186个氨基酸残基的蛋白质,预测分子量为21.6 kDa,等电点为9.14。对BmLIMO基因进行了生物信息学方面的分析,并用DNA重组技术将Bm-LIMO片段克隆到原核表达载体pET-28α中,在大肠杆菌BL21（DE3）中成功表达了可溶性BmLIMO蛋白,这为进一步研究该蛋白功能奠定了基础。     

Solution structure of the chicken cysteine-rich protein, CRP1, a double-LIM protein implicated in muscle differentiation

The mechanism by which the contractile machinery of muscle is assembled and maintained is not well-understood. Members of the cysteine-rich protein (CRP) family have been implicated in these processes. Three vertebrate CRPs (CRP1-3) that exhibit developmentally regulated muscle-specific expression have been identified. All three proteins are associated with the actin cytoskeleton, and one has been shown to be required for striated muscle structure and function. The vertebrate CRPs identified to date display a similar molecular architecture; each protein is comprised of two tandemly arrayed LIM domains, protein-binding motifs found in a number of proteins with roles in cell differentiation. Each LIM domain coordinates two Zn(II) ions that are bound independently in CCHC (C=Cys, H=His) and CCCC modules. Here we describe the solution structure of chicken CRP1 determined by homonuclear and 1H-15N heteronuclear magnetic resonance spectroscopy. Comparison of the structures of the two LIM domains of CRP1 reveals a high degree of similarity in their tertiary folds. In addition, the two component LIM domains represent two completely independent folding units and exhibit no apparent interactions with each other. The structural independence and spatial separation of the two LIM domains of CRP1 are compatible with an adapter or linker role for the protein.