[Show abstract][Hide abstract] ABSTRACT: Molecular chaperones are required for successful folding and assembly of sarcomeric myosin in skeletal and cardiac muscle. Here, we show that the chaperone UNC-45B inhibits the actin translocation function of myosin. Further, we show that Hsp90, another chaperone involved in sarcomere development, allows the myosin to resume actin translocation. These previously unknown activities may play a key role in sarcomere development, preventing untimely myosin powerstrokes from disrupting the precise alignment of the sarcomere until it has formed completely.
[Show abstract][Hide abstract] ABSTRACT: Duplications spanning nine genes at the genomic locus 16p13.1 predispose individuals to acute aortic dissections. The most
likely candidate gene in this region leading to the predisposition for dissection is MYH11, which encodes smooth muscle myosin heavy chain (SM-MHC). The effects of increased expression of MYH11 on smooth muscle cell (SMC) phenotypes were explored using mouse aortic SMCs with transgenic overexpression of one isoform
of SM-MHC. We found that these cells show increased expression of Myh11 and myosin filament-associated contractile genes at the message level when compared with control SMCs, but not at the protein
level due to increased protein degradation. Increased expression of Myh11 resulted in endoplasmic reticulum (ER) stress in SMCs, which led to a paradoxical decrease of protein levels through increased
autophagic degradation. An additional consequence of ER stress in SMCs was increased intracellular calcium ion concentration,
resulting in increased contractile signaling and contraction. The increased signals for contraction further promote transcription
of contractile genes, leading to a feedback loop of metabolic abnormalities in these SMCs. We suggest that overexpression
of MYH11 can lead to increased ER stress and autophagy, findings that may be globally implicated in disease processes associated with
[Show abstract][Hide abstract] ABSTRACT: Research on Caenorhabditis elegans has led to the discovery of the consequences of mutation in myosin, its associated proteins, and the extracellular matrix-membrane cytoskeleton complex. Key results include understanding thick filament structure and assembly, the regulation of sarcomeric protein turnover, and the organization of thick and thin filaments into ordered sarcomeres. These results are critical to studies of cardiovascular diseases such as the cardiomyopathies, congenital septal defects, aneurysms of the thoracic aorta, and cardiac remodeling in heart failure.
No preview · Article · Nov 2012 · Trends in cardiovascular medicine
[Show abstract][Hide abstract] ABSTRACT: Myosins are molecular motors that convert chemical energy into mechanical work. Allosterically coupling ATP-binding, hydrolysis, and binding/dissociation to actin filaments requires precise and coordinated structural changes that are achieved by the structurally complex myosin motor domain. UNC-45, a member of the UNC-45/Cro1/She4p family of proteins, acts as a chaperone for myosin and is essential for proper folding and assembly of myosin into muscle thick filaments in vivo. The molecular mechanisms by which UNC-45 interacts with myosin to promote proper folding of the myosin head domain are not known. We have devised a novel approach, to our knowledge, to analyze the interaction of UNC-45 with the myosin motor domain at the single molecule level using atomic force microscopy. By chemically coupling a titin I27 polyprotein to the motor domain of myosin, we introduced a mechanical reporter. In addition, the polyprotein provided a specific attachment point and an unambiguous mechanical fingerprint, facilitating our atomic force microscopy measurements. This approach enabled us to study UNC-45-motor domain interactions. After mechanical unfolding, the motor domain interfered with refolding of the otherwise robust I27 modules, presumably by recruiting them into a misfolded state. In the presence of UNC-45, I27 folding was restored. Our single molecule approach enables the study of UNC-45 chaperone interactions with myosin and their consequences for motor domain folding and misfolding in mechanistic detail.
Full-text · Article · May 2012 · Biophysical Journal
[Show abstract][Hide abstract] ABSTRACT: Cardiac development requires interplay between the regulation of gene expression and the assembly of functional sarcomeric proteins. We report that UNC-45b recessive loss-of-function mutations in C3H and C57BL/6 inbred mouse strains cause arrest of cardiac morphogenesis at the formation of right heart structures and failure of contractile function. Wild-type C3H and C57BL/6 embryos at the same stage, E9.5, form actively contracting right and left atria and ventricles. The known interactions of UNC-45b as a molecular chaperone are consistent with diminished accumulation of the sarcomeric myosins, but not their mRNAs, and the resulting decreased contraction of homozygous mutant embryonic hearts. The novel finding that GATA4 accumulation is similarly decreased at the protein but not mRNA levels is also consistent with the function of UNC-45b as a chaperone. The mRNAs of known downstream targets of GATA4 during secondary cardiac field development, the cardiogenic factors Hand1, Hand2 and Nkx-2.5, are also decreased, consistent with the reduced GATA4 protein accumulation. Direct binding studies show that the UNC-45b chaperone forms physical complexes with both the alpha and beta cardiac myosins and the cardiogenic transcription factor GATA4. Co-expression of UNC-45b with GATA4 led to enhanced transcription from GATA promoters in naïve cells. These novel results suggest that the heart-specific UNC-45b isoform functions as a molecular chaperone mediating contractile function of the sarcomere and gene expression in cardiac development.
Preview · Article · May 2012 · Journal of Cell Science
[Show abstract][Hide abstract] ABSTRACT: The nematode Caenorhabditis elegans has become established as a major experimental organism with applications to many biomedical research areas. The body wall muscle cells are a useful model for the study of human cardiomyocytes and their homologous structures and proteins. The ability to readily identify mutations affecting these proteins and structures in C elegans and to be able to rigorously characterize their genotypes and phenotypes at the cellular and molecular levels permits mechanistic studies of the responsible interactions relevant to the inherited human cardiomyopathies. Future work in C elegans muscle holds great promise in uncovering new mechanisms in the pathogenesis of these cardiac disorders.
No preview · Article · Oct 2011 · Circulation Research
[Show abstract][Hide abstract] ABSTRACT: The UNC-45 family of molecular chaperones is expressed in metazoan organisms from Caenorhabditis elegans to humans. The UNC-45 protein is essential in C. elegans for early body-wall muscle cell development and A-band assembly. We show that the myosin-binding UCS domain of UNC-45 alone is sufficient to rescue lethal unc-45 null mutants arrested in embryonic muscle development and temperature-sensitive loss-of-function unc-45 mutants defective in worm A-band assembly. Removal of the Hsp90-binding TPR domain of UNC-45 does not affect rescue. Similar results were obtained with overexpression of the same fragments in wild-type nematodes when assayed for diminution of myosin accumulation and assembly. Titration experiments show that, on a per molecule basis, UCS has greater activity in C. elegans muscle in vivo than full-length UNC-45 protein, suggesting that UNC-45 is inhibited by either the TPR domain or its interaction with the general chaperone Hsp90. In vitro experiments with purified recombinant C. elegans Hsp90 and UNC-45 proteins show that they compete for binding to C. elegans myosin. Our in vivo genetic and in vitro biochemical experiments are consistent with a novel inhibitory role for Hsp90 with respect to UNC-45 action.
No preview · Article · Sep 2011 · Journal of Cell Science
[Show abstract][Hide abstract] ABSTRACT: UNC-45A is a molecular chaperone targeted to non-muscle myosins and is essential for cell division. Here, we show that UNC-45A mRNA and protein expression was elevated in human breast carcinomas and cell lines derived from breast carcinoma metastases. Moreover, small hairpin RNA knockdowns of endogenously overexpressed UNC-45A in the most metastatic cell line led to significant decreases in the rates of cell proliferation and invasion, concomitant with reduction in the interaction of myosin II with actin filaments. Exploring the mechanism of these findings further, we found that UNC-45A is alternatively expressed at the mRNA and protein levels as two isoforms. The two isoforms differ only by a proline-rich 15-amino-acid sequence near the amino-terminus. In the increased expression with metastatic activity, the ratio of the isoform mRNAs remained constant, but the 929-amino-acid protein isoform showed increases up to about 3-fold in comparison to the 944-amino-acid isoform. The differential accumulation was explained by cellular labeling experiments that showed that the 944 isoform is degraded at a 5-fold greater rate than the 929 isoform and that this degradation required the ubiquitin-proteasome system.
No preview · Article · Jul 2011 · Journal of Molecular Biology
[Show abstract][Hide abstract] ABSTRACT: UNC-45 is a chaperone that facilitates folding of myosin motor domains. We have used Drosophila melanogaster to investigate the role of UNC-45 in muscle development and function. Drosophila UNC-45 (dUNC-45) is expressed at all developmental stages. It colocalizes with non-muscle myosin in embryonic blastoderm of 2-hour-old embryos. At 14 hours, it accumulates most strongly in embryonic striated muscles, similarly to muscle myosin. dUNC-45 localizes to the Z-discs of sarcomeres in third instar larval body-wall muscles. We produced a dunc-45 mutant in which zygotic expression is disrupted. This results in nearly undetectable dUNC-45 levels in maturing embryos as well as late embryonic lethality. Muscle myosin accumulation is robust in dunc-45 mutant embryos at 14 hours. However, myosin is dramatically decreased in the body-wall muscles of 22-hour-old mutant embryos. Furthermore, electron microscopy showed only a few thick filaments and irregular thick-thin filament lattice spacing. The lethality, defective protein accumulation, and ultrastructural abnormalities are rescued with a wild-type dunc-45 transgene, indicating that the mutant phenotypes arise from the dUNC-45 deficiency. Overall, our data indicate that dUNC-45 is important for myosin accumulation and muscle function. Furthermore, our results suggest that dUNC-45 acts post-translationally for proper myosin folding and maturation.
Preview · Article · Mar 2011 · Journal of Cell Science
[Show abstract][Hide abstract] ABSTRACT: UNC-45 is a prototype of the protein family characterized by the presence of the C-terminal UCS (UNC-45/CR01/She4p) domain.
These proteins function in various important actin- and myosin-dependent cellular processes that include myofibril organization
and muscle functions, cell differentiation, embryonic development, cytokinesis and endocytosis. Mutations in the genes that
code for UCS domain proteins cause serious defects in these actomyosin-based processes. Homologs of UCS domain proteins have
been identified in fungi, nematodes, insects, fish, amphibians, birds and mammals. In addition to the UCS domain, the animal
homologs (UNC-45) contain an N-terminal TPR domain and a conserved central region. UNC-45 has been shown to act as chaperone
to fold the heads of myosin heavy chain of various types. Apart from assisting myosin heads to fold correcdy, UNC-45 is known
to bind Hsp90 direcdy and several UCS protein complexes appear to be dependent on the Hsp90 chaperone machinery. These findings
suggest that UNC-45 and other proteins containing the UCS domain are a new class of Hsp90 co-chaperones.
[Show abstract][Hide abstract] ABSTRACT: Myosin motors are central to diverse cellular processes in eukaryotes. Homologues of the myosin chaperone UNC-45 have been implicated in the assembly and function of myosin-containing structures in organisms from fungi to humans. In muscle, the assembly of sarcomeric myosin is regulated to produce stable, uniform thick filaments. Loss-of-function mutations in Caenorhabditis elegans UNC-45 lead to decreased muscle myosin accumulation and defective thick filament assembly, resulting in paralyzed animals. We report that transgenic worms overexpressing UNC-45 also display defects in myosin assembly, with decreased myosin content and a mild paralysis phenotype. We find that the reduced myosin accumulation is the result of degradation through the ubiquitin/proteasome system. Partial proteasome inhibition is able to restore myosin protein and worm motility to nearly wild-type levels. These findings suggest a mechanism in which UNC-45-related proteins may contribute to the degradation of myosin in conditions such as heart failure and muscle wasting.
Full-text · Article · May 2007 · The Journal of Cell Biology
[Show abstract][Hide abstract] ABSTRACT: Focal adhesions are multiprotein assemblages that link cells to the extracellular matrix. The transmembrane protein, integrin, is a key component of these structures. In vertebrate muscle, focal adhesion-like structures called costameres attach myofibrils at the periphery of muscle cells to the cell membrane. In Caenorhabditis elegans muscle, all the myofibrils are attached to the cell membrane at both dense bodies (Z-disks) and M-lines. Clustered at the base of dense bodies and M-lines, and associated with the cytoplasmic tail of beta-integrin, is a complex of many proteins, including UNC-97 (vertebrate PINCH). Previously, we showed that UNC-97 interacts with UNC-98, a 37-kD protein, containing four C2H2 Zn fingers, that localizes to M-lines. We report that UNC-98 also interacts with the C-terminal portion of a myosin heavy chain. Multiple lines of evidence support a model in which UNC-98 links integrin-associated proteins to myosin in thick filaments at M-lines.
Full-text · Article · Jan 2007 · The Journal of Cell Biology
[Show abstract][Hide abstract] ABSTRACT: Myosins are a large family of actin-based motor proteins that are involved in a variety of cellular processes. Class II, or conventional, myosins are organized into a number of multi-component structures such as muscle thick filaments, non-muscle filaments and the actomyosin ring during cell division. A number of conditions must be met for the proper assembly and organization of myosin II-containing structures, including the correct stoichiometry of myosin and its associated proteins, and the conformation and regulation of the myosin molecule itself by molecular chaperones and protein kinases. In this review we discuss the use of model organisms in the genetic analysis of the assembly and organization of myosin-containing structures.
No preview · Article · Nov 2005 · Cellular and Molecular Life Sciences CMLS
[Show abstract][Hide abstract] ABSTRACT: The organization of the motor protein myosin into motile cellular structures requires precise temporal and spatial control. Caenorhabditis elegans UNC-45 facilitates this by functioning both as a chaperone and as a Hsp90 cochaperone for myosin during thick filament assembly. Consequently, mutations in C. elegans unc-45 result in paralyzed animals with severe myofibril disorganization in striated body wall muscles. Here, we report a new E3/E4 complex, formed by CHN-1, the C. elegans ortholog of CHIP (carboxyl terminus of Hsc70-interacting protein), and UFD-2, an enzyme known to have ubiquitin conjugating E4 activity in yeast, as necessary and sufficient to multiubiquitylate UNC-45 in vitro. The phenotype of unc-45 temperature-sensitive animals is partially suppressed by chn-1 loss of function, while UNC-45 overexpression in worms deficient for chn-1 results in severely disorganized muscle cells. These results identify CHN-1 and UFD-2 as a functional E3/E4 complex and UNC-45 as its physiologically relevant substrate.
[Show abstract][Hide abstract] ABSTRACT: Myotonic dystrophy protein kinase (DMPK) is the protein product of the human DM-1 locus on chromosome 19q13.1 and has been implicated in the cardiac and behavioral dysfunctions of the disorder. DMPK contains four distinct regions: a leucine-rich repeat (L), a serine-threonine protein kinase catalytic domain (PK), an alpha-helical coiled-coil region (H), and a putative transmembrane-spanning tail (T). Multiple protein kinases that participate in cytoskeletal and cell cycle functions share homology with DMPK in the PK and H regions. Here we show that the LPKH and PKH subfragments of DMPK formed dimers of 140000 molecular weight, whereas the LPK subfragment remained a monomer of 62000 apparent molecular weight. The H domain thus appeared to be required for dimerization of DMPK subfragments. Caspase 1 cleaved LPKH between the PK and H regions. After cleavage, LPKH dimers became LPK-like monomers, consistent with the H domain mediating dimerization. The V(max) and k(cat)/K(m) of LPKH with a synthetic peptide kinase substrate were over 10-fold greater than either LPK or caspase-cleaved LPKH. The K(m) of dimeric LPKH was over three-fold greater than those of the monomeric proteins. Dimerization appeared to significantly affect the catalytic efficiency and substrate binding of DMPK. These interactions are likely to be functionally significant in other members of the myotonic dystrophy family of protein kinases with extensive coiled-coil domains.
[Show abstract][Hide abstract] ABSTRACT: Serum response factor (SRF) is a phosphoprotein that regulates skeletal and cardiac alpha-actin gene transcription. Myotonic dystrophy protein kinase (DMPK), a muscle- and neuron-restricted kinase, enhanced SRF-mediated promoter activity of the skeletal and cardiac alpha-actin genes in C2C12 myoblasts as well as in nonmyogenic cells. DMPK phosphorylated SRF in vitro in the alphaI coil of the DNA-binding domain in the MADS box, a highly conserved region required for DNA binding, dimerization, and co-activator interaction in COS and CV1 cells. Threonine 159 in the MADS box alphaI coil was a specific phosphorylation target in vitro as well as in vivo of both DMPK and protein kinase C-alpha. Substitution of threonine 159 with the nonphosphorylatable residue alanine markedly diminished activation of the cardiac alpha-actin promoter in the presence of kinase, while its substitution with aspartic acid, to introduce a negative charge and mimic phosphorylation, restored activation completely. Phosphorylation of the MADS box may constitute a novel mechanism for regulation of SRF-dependent actin gene transcription.
[Show abstract][Hide abstract] ABSTRACT: Myotonic dystrophy (DM) is associated with an expanded triplet repeat in the 3'-untranslated region of the gene for myotonic dystrophy protein kinase (DMPK), which may reduce DMPK expression. It is unclear how reduced DMPK expression might contribute to the symptoms of DM because the normal function of DMPK is not yet understood. Thus we investigated the function of DMPK to gain insight into how reduced DMPK expression might lead to cognitive dysfunction in DM. We recently demonstrated a role for DMPK in modifying the cytoskeleton, and remodeling of the cytoskeleton is thought to be important for cognitive function. Therefore we hypothesized that DMPK might normally contribute to synaptic plasticity and cognitive function via an effect on actin cytoskeletal rearrangements. To test for involvement of DMPK in synaptic plasticity, we utilized the DMPK null mouse. This mouse showed no changes in baseline synaptic transmission in hippocampal area CA1, nor any changes in long-term synaptic potentiation (LTP) measured 3 h after induction. There was a significant decrease, however, in the decremental potentiation with a duration of 30-180 min that accompanies LTP. These results suggest a role for DMPK in synaptic plasticity that could be relevant to the cognitive dysfunction associated with DM.
Full-text · Article · Apr 2003 · Journal of Neurophysiology