A zebrafish unc-45-related gene expressed during muscle development

Unconventional Myosins from Caenorhabditis elegans as a Probe to Study Human Orthologues

Unconventional myosins are a superfamily of actin-based motor proteins that perform a number of roles in fundamental cellular processes, including (but not limited to) intracellular trafficking, cell motility, endocytosis, exocytosis and cytokinesis. 40 myosins genes have been identified in humans, which belong to different 12 classes based on their domain structure and organisation. These genes are widely expressed in different tissues, and mutations leading to loss of function are associated with a wide variety of pathologies while over-expression often results in cancer. Caenorhabditis elegans (C. elegans) is a small, free-living, non-parasitic nematode. ~38% of the genome of C. elegans has predicted orthologues in the human genome, making it a valuable tool to study the function of human counterparts and human diseases. To date, 8 unconventional myosin genes have been identified in the nematode, from 6 different classes with high homology to human paralogues. The hum-1 and hum-5 (heavy chain of an unconventional myosin) genes encode myosin of class I, hum-2 of class V, hum-3 and hum-8 of class VI, hum-6 of class VII and hum-7 of class IX. The hum-4 gene encodes a high molecular mass myosin (307 kDa) that is one of the most highly divergent myosins and is a member of class XII. Mutations in many of the human orthologues are lethal, indicating their essential properties. However, a functional characterisation for many of these genes in C. elegans has not yet been performed. This article reviews the current knowledge of unconventional myosin genes in C. elegans and explores the potential use of the nematode to study the function and regulation of myosin motors to provide valuable insights into their role in diseases.

UNC-45A breaks the microtubule lattice independently of its effects on non-muscle myosin II

In invertebrates, UNC-45 regulates myosin stability and functions. Vertebrates have two distinct isoforms of the protein: UNC-45B, expressed in muscle cells only, and UNC-45A, expressed in all cells and implicated in regulating both non-muscle myosin II (NMII)- and microtubule (MT)-associated functions. Here, we show that, in vitro and in human and rat cells, UNC-45A binds to the MT lattice, leading to MT bending, breakage and depolymerization. Furthermore, we show that UNC-45A destabilizes MTs independent of its C-terminal NMII-binding domain and even in the presence of the NMII inhibitor blebbistatin. These findings identified UNC-45A as a novel type of MT-severing protein with a dual non-mutually exclusive role in regulating NMII activity and MT stability. Because many human diseases, from cancer to neurodegenerative diseases, are caused by or associated with deregulation of MT stability, our findings have profound implications in the biology of MTs, as well as the biology of human diseases and possible therapeutic implications for their treatment.This article has an associated First Person interview with the joint first authors of the paper.

The myosin chaperone UNC45B is involved in lens development and autosomal dominant juvenile cataract

Genome-wide linkage analysis, followed by targeted deep sequencing, in a Danish multigeneration family with juvenile cataract revealed a region of chromosome 17 co-segregating with the disease trait. Affected individuals were heterozygous for two potentially protein-disrupting alleles in this region, in ACACA and UNC45B. As alterations of the UNC45B protein have been shown to affect eye development in model organisms, effort was focused on the heterozygous UNC45B missense mutation. UNC45B encodes a myosin-specific chaperone that, together with the general heat shock protein HSP90, is involved in myosin assembly. The mutation changes p.Arg805 to Trp in the UCS domain, an amino acid that is highly conserved from yeast to human. UNC45B is strongly expressed in the heart and skeletal muscle tissue, but here we show expression in human embryo eye and zebrafish lens. The zebrafish mutant steif, carrying an unc45b nonsense mutation, has smaller eyes than wild-type embryos and shows accumulation of nuclei in the lens. Injection of RNA encoding the human wild-type UNC45B protein into the steif homozygous embryo reduced the nuclei accumulation and injection of human mutant UNC45B cDNA in wild-type embryos resulted in development of a phenotype similar to the steif mutant. The p.Arg805Trp alteration in the mammalian UNC45B gene suggests that developmental cataract may be caused by a defect in non-muscle myosin assembly during maturation of the lens fiber cells.

The UNC-45 myosin chaperone: from worms to flies to vertebrates

UNC-45 (uncoordinated mutant number 45) is a UCS (UNC-45, CRO1, She4p) domain protein that is critical for myosin stability and function. It likely aides in folding myosin during cellular differentiation and maintenance, and protects myosin from denaturation during stress. Invertebrates have a single unc-45 gene that is expressed in both muscle and nonmuscle tissues. Vertebrates possess one gene expressed in striated muscle (unc-45b) and another that is more generally expressed (unc-45a). Structurally, UNC-45 is composed of a series of α-helices connected by loops. It has an N-terminal tetratricopeptide repeat domain that binds to Hsp90 and a central domain composed of armadillo repeats. Its C-terminal UCS domain, which is also comprised of helical armadillo repeats, interacts with myosin. In this chapter, we present biochemical, structural, and genetic analyses of UNC-45 in Caenorhabditis elegans, Drosophila melanogaster, and various vertebrates. Further, we provide insights into UNC-45 functions, its potential mechanism of action, and its roles in human disease.

503unc, a small and muscle-specific zebrafish promoter

The muscle-specific UNC-45b assists in the folding of sarcomeric myosin. Analysis of the zebrafish unc-45b upstream region revealed that unc-45b promoter fragments reliably drive GFP expression after germline transmission. The muscle-specific 503-bp minimal promoter 503unc was identified to drive gene expression in the zebrafish musculature. In transgenic Tg(-503unc:GFP) zebrafish, GFP fluorescence was detected in the adaxial cells, their slow fiber descendants, and the fast muscle. At later stages, robust GFP fluorescence is eminent in the cardiac, cranial, fin, and trunk muscle, thereby recapitulating the unc-45b expression pattern. We propose that the 503unc promoter is a small and muscle-specific promoter that drives robust gene expression throughout the zebrafish musculature, making it a valuable tool for the exploration of zebrafish muscle.

Dual function of the UNC-45b chaperone with myosin and GATA4 in cardiac development

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.

At the Start of the Sarcomere: A Previously Unrecognized Role for Myosin Chaperones and Associated Proteins during Early Myofibrillogenesis

The development of striated muscle in vertebrates requires the assembly of contractile myofibrils, consisting of highly ordered bundles of protein filaments. Myofibril formation occurs by the stepwise addition of complex proteins, a process that is mediated by a variety of molecular chaperones and quality control factors. Most notably, myosin of the thick filament requires specialized chaperone activity during late myofibrillogenesis, including that of Hsp90 and its cofactor, Unc45b. Unc45b has been proposed to act exclusively as an adaptor molecule, stabilizing interactions between Hsp90 and myosin; however, recent discoveries in zebrafish and C. elegans suggest the possibility of an earlier role for Unc45b during myofibrillogenesis. This role may involve functional control of nonmuscle myosins during the earliest stages of myogenesis, when premyofibril scaffolds are first formed from dynamic cytoskeletal actin. This paper will outline several lines of evidence that converge to build a model for Unc45b activity during early myofibrillogenesis.

Germain A, Badders N, Taylor JP, Rosen A, Spinette S. A novel conserved isoform of the ubiquitin ligase UFD2a/UBE4B is expressed exclusively in mature striated muscle cells

Yeast Ufd2p was the first identified E4 multiubiquitin chain assembly factor. Its vertebrate homologues later referred to as UFD2a, UBE4B or E4B were also shown to have E3 ubiquitin ligase activity. UFD2a function in the brain has been well established in vivo, and in vitro studies have shown that its activity is essential for proper condensation and segregation of chromosomes during mitosis. Here we show that 2 alternative splice forms of UFD2a, UFD2a-7 and -7/7a, are expressed sequentially during myoblast differentiation of C2C12 cell cultures and during cardiotoxin-induced regeneration of skeletal muscle in mice. UFD2a-7 contains an alternate exon 7, and UFD2a-7/7a, the larger of the 2 isoforms, contains an additional novel exon 7a. Analysis of protein or mRNA expression in mice and zebrafish revealed that a similar pattern of isoform switching occurs during developmental myogenesis of cardiac and skeletal muscle. In vertebrates (humans, rodents, zebrafish), UFD2a-7/7a is expressed only in mature striated muscle. This unique tissue specificity is further validated by the conserved presence of 2 muscle-specific splicing regulatory motifs located in the 3' introns of exons 7 and 7a. UFD2a interacts with VCP/p97, an AAA-type ATPase implicated in processes whose functions appear to be regulated, in part, through their interaction with one or more of 15 previously identified cofactors. UFD2a-7/7a did not interact with VCP/p97 in yeast 2-hybrid experiments, which may allow the ATPase to bind cofactors that facilitate its muscle-specific functions. We conclude that the regulated expression of these UFD2a isoforms most likely imparts divergent functions that are important for myogenisis.

Hsp90 in non-mammalian metazoan model systems

The molecular chaperone Hsp90 has been discovered in the heat-shock response of the fruit fly more than 30years ago. Today, it is becoming clear that Hsp90 is in the middle of a regulatory system, participating in the modulation of many essential client proteins and signaling pathways. Exerting these activities, Hsp90 works together with about a dozen of cochaperones. Due to their organismal simplicity and the possibility to influence their genetics on a large scale, many studies have addressed the function of Hsp90 in several multicellular model systems. Defined pathways involving Hsp90 client proteins have been identified in the metazoan model systems of Caenorhabditis elegans, Drosophila melanogaster and the zebrafish Danio rerio. Here, we summarize the functions of Hsp90 during muscle maintenance, development of phenotypic traits and the involvement of Hsp90 in stress responses, all of which were largely uncovered using the model organisms covered in this review. These findings highlight the many specific and general actions of the Hsp90 chaperone machinery. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Changes in oxygen and carbon dioxide environment alter gene expression of cowpea bruchids

Hermetic storage is a widely adopted technique for preventing stored grain from being damaged by storage insect pests. In the air-tight container, insects consume oxygen through metabolism while concomitantly raising carbon dioxide concentrations through respiration. Previous studies on the impact of hypoxia and hypercapnia on feeding behavior of cowpea bruchids have shown that feeding activity gradually decreases in proportion to the changing gas concentrations and virtually ceases at approximately 3-6% (v/v) oxygen and 15-18% carbon dioxide. Further, a number of bruchid larvae are able to recover their feeding activity after days of low oxygen and high carbon dioxide, although extended exposure tends to reduce survival. In the current study, to gain insight into the molecular mechanism underpinning the hypoxia-coping response, we profiled transcriptomic responses to hypoxia/hypercapnia (3% oxygen, 17% carbon dioxide for 4 and 24h) using cDNA microarrays, followed by quantitative RT-PCR verification of selected gene expression changes. A total of 1046 hypoxia-responsive cDNAs were sequenced; these clustered into 765 contigs, of which 645 were singletons. Many (392) did not show homology with known genes, or had homology only with genes of unknown function in a BLAST search. The identified differentially-regulated sequences encoded proteins presumptively involved in nutrient transport and metabolism, cellular signaling and structure, development, and stress responses. Gene expression profiles suggested that insects compensate for lack of oxygen by coordinately reducing energy demand, shifting to anaerobic metabolism, and strengthening cellular structure and muscular contraction.

Knockdown and overexpression of Unc-45b result in defective myofibril organization in skeletal muscles of zebrafish embryos

Background

Unc-45 is a myosin chaperone and a Hsp90 co-chaperone that plays a key role in muscle development. Genetic and biochemical studies in C. elegans have demonstrated that Unc-45 facilitates the process of myosin folding and assembly in body wall muscles. Loss or overexpression of Unc-45 in C. elegans results in defective myofibril organization. In the zebrafish Danio rerio, unc-45b, a homolog of C. elegans unc-45, is expressed in both skeletal and cardiac muscles. Earlier studies indicate that mutation or knockdown of unc-45b expression in zebrafish results in a phenotype characterized by a loss of both thick and thin filament organization in skeletal and cardiac muscle. The effects of unc-45b knockdown on other sarcomeric structures and the phenotype of Unc-45b overexpression, however, are poorly understood in vertebrates.

Results

Both knockdown and overexpression provide useful tools to study gene function during animal development. Using such methods, we characterized the role of Unc-45b in myofibril assembly of skeletal muscle in Danio rerio. We showed that, in addition to thick and thin filament defects, knockdown of unc-45b expression disrupted sarcomere organization in M-lines and Z-lines of skeletal muscles in zebrafish embryos. Western blotting analysis showed that myosin protein levels were significantly decreased in unc-45b knockdown embryos. Similarly, embryos overexpressing Unc-45b also exhibited severely disorganized myosin thick filaments. Disruption of thick filament organization by Unc-45b overexpression depends on the C-terminal UCS domain in Unc-45b required for interaction with myosin. Deletion of the C-terminal UCS domain abolished the disruptive activity of Unc-45b in myosin thick filament organization. In contrast, deletion of the N-terminal TPR domain required for binding with Hsp90α had no effect.

Conclusion

Collectively, these studies indicate that the expression levels of Unc-45b must be precisely regulated to ensure normal myofibril organization. Loss or overexpression of Unc-45b leads to defective myofibril organization.

Lack of Apobec2-related proteins causes a dystrophic muscle phenotype in zebrafish embryos

Apo2 proteins interact with chaperone Unc45b (but not Hsp90) and are required for correct zebrafish skeletal musculature and heart function.

The chaperones Unc45b and Hsp90a are essential for folding of myosin in organisms ranging from worms to humans. We show here that zebrafish Unc45b, but not Hsp90a, binds to the putative cytidine deaminase Apobec2 (Apo2) in an interaction that requires the Unc45/Cro1p/She4p-related (UCS) and central domains of Unc45b. Morpholino oligonucleotide-mediated knockdown of the two related proteins Apo2a and Apo2b causes a dystrophic phenotype in the zebrafish skeletal musculature and impairs heart function. These phenotypic traits are shared with mutants of unc45b, but not with hsp90a mutants. Apo2a and -2b act nonredundantly and bind to each other in vitro, which suggests a heteromeric functional complex. Our results demonstrate that Unc45b and Apo2 proteins act in a Hsp90a-independent pathway that is required for integrity of the myosepta and myofiber attachment. Because the only known function of Unc45b is that of a chaperone, Apo2 proteins may be clients of Unc45b but other yet unidentified processes cannot be excluded.

Drosophila UNC-45 prevents heat-induced aggregation of skeletal muscle myosin and facilitates refolding of citrate synthase

UNC-45 belongs to the UCS (UNC-45, CRO1, She4p) domain protein family, whose members interact with various classes of myosin. Here we provide structural and biochemical evidence that Escherichia coli-expressed Drosophila UNC-45 (DUNC-45) maintains the integrity of several substrates during heat-induced stress in vitro. DUNC-45 displays chaperone function in suppressing aggregation of the muscle myosin heavy meromyosin fragment, the myosin S-1 motor domain, alpha-lactalbumin and citrate synthase. Biochemical evidence is supported by electron microscopy, which reveals the first structural evidence that DUNC-45 prevents inter- or intra-molecular aggregates of skeletal muscle heavy meromyosin caused by elevated temperatures. We also demonstrate for the first time that UNC-45 is able to refold a denatured substrate, urea-unfolded citrate synthase. Overall, this in vitro study provides insight into the fate of muscle myosin under stress conditions and suggests that UNC-45 protects and maintains the contractile machinery during in vivo stress.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

Myosin assembly, maintenance and degradation in muscle: Role of the chaperone UNC-45 in myosin thick filament dynamics

Myofibrillogenesis in striated muscle cells requires a precise ordered pathway to assemble different proteins into a linear array of sarcomeres. The sarcomere relies on interdigitated thick and thin filaments to ensure muscle contraction, as well as properly folded and catalytically active myosin head. Achieving this organization requires a series of protein folding and assembly steps. The folding of the myosin head domain requires chaperone activity to attain its functional conformation. Folded or unfolded myosin can spontaneously assemble into short myosin filaments, but further assembly requires the short and incomplete myosin filaments to assemble into the developing thick filament. These longer filaments are then incorporated into the developing sarcomere of the muscle. Both myosin folding and assembly require factors to coordinate the formation of the thick filament in the sarcomere and these factors include chaperone molecules. Myosin folding and sarcomeric assembly requires association of classical chaperones as well as folding cofactors such as UNC-45. Recent research has suggested that UNC-45 is required beyond initial myosin head folding and may be directly or indirectly involved in different stages of myosin thick filament assembly, maintenance and degradation.

Loss of unc45a precipitates arteriovenous shunting in the aortic arches

Aortic arch malformations are common congenital disorders that are frequently of unknown etiology. To gain insight into the factors that guide branchial aortic arch development, we examined the process by which these vessels assemble in wild type zebrafish embryos and in kurzschluss(tr12) (kus(tr12)) mutants. In wild type embryos, each branchial aortic arch first appears as an island of angioblasts in the lateral pharyngeal mesoderm, then elaborates by angiogenesis to connect to the lateral dorsal aorta and ventral aorta. In kus(tr12) mutants, angioblast formation and initial sprouting are normal, but aortic arches 5 and 6 fail to form a lumenized connection to the lateral dorsal aorta. Blood enters these blind-ending vessels from the ventral aorta, distending the arteries and precipitating fusion with an adjacent vein. This arteriovenous malformation (AVM), which shunts nearly all blood directly back to the heart, is not exclusively genetically programmed, as its formation correlates with blood flow and aortic arch enlargement. By positional cloning, we have identified a nonsense mutation in unc45a in kus(tr12) mutants. Our results are the first to ascribe a role for Unc45a, a putative myosin chaperone, in vertebrate development, and identify a novel mechanism by which an AVM can form.

Heat-shock protein 90alpha1 is required for organized myofibril assembly in skeletal muscles of zebrafish embryos

Heat-shock protein 90alpha (Hsp90alpha) is a member of the molecular chaperone family involved in protein folding and assembly. The role of Hsp90alpha in the developmental process, however, remains unclear. Here we report that zebrafish contains two Hsp90alpha genes, Hsp90alpha1, and Hsp90alpha2. Hsp90alpha1 is specifically expressed in developing somites and skeletal muscles of zebrafish embryos. We have demonstrated that Hsp90alpha1 is essential for myofibril organization in skeletal muscles of zebrafish embryos. Knockdown of Hsp90alpha1 resulted in paralyzed zebrafish embryos with poorly organized myofibrils in skeletal muscles. In contrast, knockdown of Hsp90alpha2 had no effect on muscle contraction and myofibril organization. The filament defects could be rescued in a cell autonomous manner by an ectopic expression of Hsp90alpha1. Biochemical analyses revealed that knockdown of Hsp90alpha1 resulted in significant myosin degradation and up-regulation of unc-45b gene expression. These results indicate that Hsp90alpha1 plays an important role in muscle development, likely through facilitating myosin folding and assembly into organized myofibril filaments.

UNC-45 is required for NMY-2 contractile function in early embryonic polarity establishment and germline cellularization in C. elegans

The Caenorhabditis elegans UNC-45 protein is required for proper body wall muscle assembly and acts as a molecular co-chaperone for type II myosins. In contrast to other body wall muscle components, UNC-45 is also abundant in the germline and embryo. We show that maternally provided UNC-45 acts with non-muscle myosin II (NMY-2) during embryonic polarity establishment, cytokinesis and germline cellularization. In embryos depleted for UNC-45, myosin contractility is eliminated resulting in embryonic defects in polar body extrusion, cytokinesis and establishment of polarity. Despite a lack of contractility in an unc-45(RNAi) embryo, NMY-2::GFP localizes to the cortex and accumulates at the presumptive cytokinetic furrow indicating that UNC-45 is not required for cortical localization. UNC-45 and NMY-2 are also required for fertility since the lack of either component results in complete sterility due to failed initiation of the cellularization furrows that separate syncytial nuclei into germ cells. In the absence of UNC-45, the actomyosin cytoskeleton does not contract despite non-functional myosin still directly binding actin. UNC-45 has been previously suggested to be required for the folding of the myosin head, and our results refine this hypothesis suggesting that UNC-45 is not required to fold or maintain the actin binding domain but is still required for myosin function.

The UCS factor Steif/Unc-45b interacts with the heat shock protein Hsp90a during myofibrillogenesis

Contraction of muscles is mediated by highly organized arrays of myosin motor proteins. We report here the characterization of a mutation of a UCS gene named steif/unc-45b that is required for the formation of ordered myofibrils in both the skeletal and cardiac muscles of zebrafish. We show that Steif/Unc-45b interacts with the chaperone Hsp90a in vitro. The two genes are co-expressed in the skeletal musculature and knockdown of Hsp90a leads to impaired myofibril formation in the same manner as lack of Steif/Unc-45b activity. Transcripts of both genes are up-regulated in steif mutants suggesting co-regulation of the two genes. Our data indicate a requirement of Steif/unc-45b and Hsp90a for the assembly of the contractile apparatus in the vertebrate skeletal musculature.

The myosin co-chaperone UNC-45 is required for skeletal and cardiac muscle function in zebrafish

The assembly of myosin into higher order structures is dependent upon accessory factors that are often tissue-specific. UNC-45 acts as such a molecular chaperone for myosin in the nematode Caenorhabditis elegans, in both muscle and non-muscle contexts. Although vertebrates contain homologues of UNC-45, their requirement for muscle function has not been assayed. We identified a zebrafish gene, unc45b, similar to a mammalian unc-45 homologue, expressed exclusively in striated muscle tissue, including the somites, heart and craniofacial muscle. Morpholino-oligonucleotide-mediated knockdown of unc45b results in paralysis and cardiac dysfunction. This paralysis is correlated with a loss of myosin filaments in the sarcomeres of the trunk muscle. Morphants lack circulation, heart looping and display severe cardiac and yolk-sac edema and also demonstrate ventral displacement of several jaw cartilages. Overall, this confirms a role for unc45b in zebrafish motility consistent with a function in myosin thick filament assembly and stability and uncovers novel roles for this gene in the function and morphogenesis of the developing heart and jaw. These results suggest that Unc45b acts as a chaperone that aids in the folding of myosin isoforms required for skeletal, cranial and cardiac muscle contraction.

Sid4: A secreted vertebrate immunoglobulin protein with roles in zebrafish embryogenesis

The small members of the immunoglobulin superfamily (IGSF) are a molecularly diverse group of proteins composed solely of immunoglobulin domains. They may be secreted or tethered to the cell mebrane via GPI linkages and are proposed to have important functions in vivo. However, very few small IGSFs have been functionally characterized. During an ongoing in situ hybridization analysis of expressed sequence tags in zebrafish we identified secreted immunoglobulin domain 4 (sid4), a gene encoding a soluble vertebrate protein composed solely of four immunoglobulin domains. Throughout development, sid4 is expressed in regions of the embryo undergoing active cell division and migration. Functional analysis using morpholino antisense oligonucleotides demonstrates that timing of gene expression is normal in morphants, but these embryos are smaller and exhibit defects in epiboly and patterning of axial and prechordal mesoderm. Analyses of chordin, pax2, krox20, and dlx2 expression in morphants demonstrate that early brain patterning is normal but later organization of hindbrain neurons and development of cranial neural crest are perturbed. Levels of apoptosis in morphants were normal prior to 90% epiboly, but were elevated after 10 h post-fertilization (hpf). Apoptosis does not account for early patterning defects of axial mesoderm, but likely contributes to overall reduction in embryo size. Phylogenetic analysis demonstrates that Sid4 is strikingly similar to the fibronectin binding Ig domains of Perlecan/HSPG2. Overall, our data demonstrate a fundamental role for sid4, possibly as a co-factor in extracellular matrix (ECM) interactions, in processes underlying tissue patterning and organogenesis in a vertebrate.

UCS protein Rng3p activates actin filament gliding by fission yeast myosin-II

We purified native Myo2p/Cdc4p/Rlc1p (Myo2), the myosin-II motor required for cytokinesis by Schizosaccharomyces pombe. The Myo2p heavy chain associates with two light chains, Cdc4p and Rlc1p. Although crude Myo2 supported gliding motility of actin filaments in vitro, purified Myo2 lacked this activity in spite of retaining full Ca-ATPase activity and partial actin-activated Mg-ATPase activity. Unc45-/Cro1p-/She4p-related (UCS) protein Rng3p restored the full motility and actin-activated Mg-ATPase activity of purified Myo2. The COOH-terminal UCS domain of Rng3p alone restored motility to pure Myo2. Thus, Rng3p contributes directly to the motility activity of native Myo2. Consistent with a role in Myo2 activation, Rng3p colocalizes with Myo2p in the cytokinetic contractile ring. The absence of Rlc1p or mutations in the Myo2p head or Rng3p compromise the in vitro motility of Myo2 and explain the defects in cytokinesis associated with some of these mutations. In contrast, Myo2 with certain temperature-sensitive forms of Cdc4p has normal motility, so these mutations compromise other functions of Cdc4p required for cytokinesis.

Regulation of the Myosin-Directed Chaperone UNC-45 by a Novel E3/E4-Multiubiquitylation Complex in C. elegans

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.

UCS proteins: managing the myosin motor

Recent studies indicate that myosin molecular motors interact inside cells with proteins containing a conserved 'UCS' domain. This appears to ensure proper folding of myosin heads so that they can perform their ATP-dependent actin-based motor functions.

The UCS domain protein She4p binds to myosin motor domains and is essential for class I and class V myosin function

BACKGROUND

Myosins are motor proteins involved in processes like cell motility, vesicle transport, or cytokinesis. In a variety of organisms, a novel group of proteins forming the UCS (UNC-45/CRO1/SHE4) domain-containing family are essential for proper myosin function. The Saccharomyces cerevisae UCS domain protein She4p is involved in two myosin-requiring events, endocytosis and mRNA localization.

RESULTS

In contrast to UCS domain proteins from other organisms that interact with class II myosins, we demonstrate that She4p associates with yeast class I and class V myosins. She4p binds to motor domains of class V myosin Myo4p and class I myosin Myo5p, and this binding depends on She4p's UCS domain. In vivo, She4p is essential for the function and localization of Myo3p, Myo4p, and Myo5p (but not of Myo2p) and for colocalization of class I myosins with cortical actin patches. In vitro, She4p stimulates binding of Myo5p to filamentous actin. Wild-type She4p, but not a mutant lacking the UCS domain, accumulates in a cap-like structure at the bud tip. This localization requires Myo2p and actin, suggesting a Myo2-dependent mechanism by which She4p is targeted to the bud cap. Localization of She4p is essential for proper positioning and myosin-actin association of cortical Myo5p.

CONCLUSIONS

Our results suggest that She4p is a novel myosin motor domain binding protein and operates as a localized regulator of myosin function of class I and likely class V myosins.