Hsp90 protein in fission yeast Swo1p and UCS protein Rng3p facilitate myosin II assembly and function

Beyond Chaperoning: UCS Proteins Emerge as Regulators of Myosin-Mediated Cellular Processes

The UCS (UNC-45/CRO1/She4p) family of proteins has emerged as chaperones specific for the folding, assembly, and function of myosin. UCS proteins participate in various myosin-dependent cellular processes including myofibril organization and muscle functions, cell differentiation, striated muscle development, cytokinesis, and endocytosis. Mutations in the genes that code for UCS proteins cause serious defects in myosin-dependent cellular processes. UCS proteins that contain an N-terminal tetratricopeptide repeat (TPR) domain are called UNC-45. Vertebrates usually possess two variants of UNC-45, the ubiquitous general-cell UNC-45 (UNC-45A) and the striated muscle UNC-45 (UNC-45B), which is exclusively expressed in skeletal and cardiac muscles. Except for the TPR domain in UNC-45, UCS proteins comprise of several irregular armadillo (ARM) repeats that are organized into a central domain, a neck region, and the canonical C-terminal UCS domain that functions as the chaperoning module. With or without TPR, UCS proteins form linear oligomers that serve as scaffolds that mediate myosin folding, organization into myofibrils, repair, and motility. This chapter reviews emerging functions of these proteins with a focus on UNC-45 as a dedicated chaperone for folding, assembly, and function of myosin at protein and potentially gene levels. Recent experimental evidences strongly support UNC-45 as an absolute regulator of myosin, with each domain of the chaperone playing different but complementary roles during the folding, assembly, and function of myosin, as well as recruiting Hsp90 as a co-chaperone to optimize key steps. It is becoming increasingly clear that UNC-45 also regulates the transcription of several genes involved in myosin-dependent cellular processes.

UCS Chaperone Folding of the Myosin Head: A Function That Evolved before Animals and Fungi Diverged from a Common Ancestor More than a Billion Years Ago

The folding of the myosin head often requires a UCS (Unc45, Cro1, She4) domain-containing chaperone. Worms, flies, and fungi have just a single UCS protein. Vertebrates have two; one (Unc45A) which functions primarily in non-muscle cells and another (Unc45B) that is essential for establishing and maintaining the contractile apparatus of cardiac and skeletal muscles. The domain structure of these proteins suggests that the UCS function evolved before animals and fungi diverged from a common ancestor more than a billion years ago. UCS proteins of metazoans and apicomplexan parasites possess a tetratricopeptide repeat (TPR), a domain for direct binding of the Hsp70/Hsp90 chaperones. This, however, is absent in the UCS proteins of fungi and largely nonessential for the UCS protein function in Caenorhabditis elegans and zebrafish. The latter part of this review focusses on the TPR-deficient UCS proteins of fungi. While these are reasonably well studied in yeasts, there is little precise information as to how they might engage in interactions with the Hsp70/Hsp90 chaperones or might assist in myosin operations during the hyphal growth of filamentous fungi.

The molecular chaperone Hsp90 regulates heterochromatin assembly through stabilizing multiple complexes in fission yeast

In the fission yeast Schizosaccharomyces pombe, both RNAi machinery and RNAi-independent factors mediate transcriptional and posttranscriptional silencing and heterochromatin formation. Here, we show that the silencing of reporter genes at major native heterochromatic loci (centromeres, telomeres, mating-type locus and rDNA regions) and an artificially induced heterochromatin locus is alleviated in a fission yeast hsp90 mutant, hsp90-G84C Also, H3K9me2 enrichment at heterochromatin regions, especially at the mating-type locus and subtelomeres, is compromised, suggesting heterochromatin assembly defects. We further discovered that Hsp90 is required for stabilization or assembly of the RNA-induced transcriptional silencing (RITS) and Argonaute siRNA chaperone (ARC) RNAi effector complexes, the RNAi-independent factor Fft3, the shelterin complex subunit Poz1 and the Snf2/HDAC-containing repressor complex (SHREC). Our ChIP data suggest that Hsp90 regulates the efficient recruitment of the methyltransferase/ubiquitin ligase complex CLRC by shelterin to chromosome ends and targeting of the SHREC and Fft3 to mating type locus and/or rDNA region. Finally, our genetic analyses demonstrated that increased heterochromatin spreading restores silencing at subtelomeres in the hsp90-G84C mutant. Thus, this work uncovers a conserved factor critical for promoting RNAi-dependent and -independent heterochromatin assembly and gene silencing through stabilizing multiple effectors and effector complexes.

Mechanism of Long-Range Chromosome Motion Triggered by Gene Activation

Movement of chromosome sites within interphase cells is critical for numerous pathways including RNA transcription and genome organization. Yet, a mechanism for reorganizing chromatin in response to these events had not been reported. Here, we delineate a molecular chaperone-dependent pathway for relocating activated gene loci in yeast. Our presented data support a model in which a two-authentication system mobilizes a gene promoter through a dynamic network of polymeric nuclear actin. Transcription factor-dependent nucleation of a myosin motor propels the gene locus through the actin matrix, and fidelity of the actin association was ensured by ARP-containing chromatin remodelers. Motor activity of nuclear myosin was dependent on the Hsp90 chaperone. Hsp90 further contributed by biasing the remodeler-actin interaction toward nucleosomes with the non-canonical histone H2A.Z, thereby focusing the pathway on select sites such as transcriptionally active genes. Together, the system provides a rapid and effective means to broadly yet selectively mobilize chromatin sites.

Tdp1 processes chromate-induced single-strand DNA breaks that collapse replication forks

Hexavalent chromium [Cr(VI)] damages DNA and causes cancer, but it is unclear which DNA damage responses (DDRs) most critically protect cells from chromate toxicity. Here, genome-wide quantitative functional profiling, DDR measurements and genetic interaction assays in Schizosaccharomyces pombe reveal a chromate toxicogenomic profile that closely resembles the cancer chemotherapeutic drug camptothecin (CPT), which traps Topoisomerase 1 (Top1)-DNA covalent complex (Top1cc) at the 3’ end of single-stand breaks (SSBs), resulting in replication fork collapse. ATR/Rad3-dependent checkpoints that detect stalled and collapsed replication forks are crucial in Cr(VI)-treated cells, as is Mus81-dependent sister chromatid recombination (SCR) that repairs single-ended double-strand breaks (seDSBs) at broken replication forks. Surprisingly, chromate resistance does not require base excision repair (BER) or interstrand crosslink (ICL) repair, nor does co-elimination of XPA-dependent nucleotide excision repair (NER) and Rad18-mediated post-replication repair (PRR) confer chromate sensitivity in fission yeast. However, co-elimination of Tdp1 tyrosyl-DNA phosphodiesterase and Rad16-Swi10 (XPF-ERCC1) NER endonuclease synergistically enhances chromate toxicity in top1Δ cells. Pnk1 polynucleotide kinase phosphatase (PNKP), which restores 3’-hydroxyl ends to SSBs processed by Tdp1, is also critical for chromate resistance. Loss of Tdp1 ameliorates pnk1Δ chromate sensitivity while enhancing the requirement for Mus81. Thus, Tdp1 and PNKP, which prevent neurodegeneration in humans, repair an important class of Cr-induced SSBs that collapse replication forks.

Hexavalent chromium is a carcinogen that is found at toxic waste sites and in some groundwater supplies. Cellular metabolism converts chromium into DNA-damaging chromate, but it is unclear which types of chromate-DNA lesions are most dangerous, and which cellular mechanisms most critically prevent chromium toxicity. This study uses whole-genome profiling to identify DNA repair pathways that are crucial for chromate resistance in fission yeast. The resulting ‘toxicogenomic’ profile of chromate closely matches camptothecin, a natural product representing a class of chemotherapeutic drugs that cause replication fork collapse by poisoning Topoisomerase 1 (Top1), which relaxes supercoiled DNA by creating and resealing single-strand breaks (SSBs). Genetic interaction analyses uncover important roles for Tdp1 tyrosyl-DNA phosphodiesterase and Pnk1 polynucleotide 5’-kinase 3’-phosphatase (PNKP), which repair camptothecin-induced SSBs and prevent neurological disease in humans. However, chromium toxicity does not involve Top1. As Tdp1 and Pnk1 repair SSBs with 3’-blocked termini, these data suggest that Top1-independent 3’-blocked SSBs contribute to the carcinogenic and mutagenic properties of chromium.

UCS protein function is partially restored in the Saccharomyces cerevisiae she4 mutant with expression of the human UNC45-GC, but not UNC45-SM

A dedicated UNC45, Cro1, She4 (UCS) domain-containing protein assists in the Hsp90-mediated folding of the myosin head. Only weak sequence conservation exists between the single UCS protein of simple eukaryotes (She4 in budding yeast) and the two UCS proteins of higher organisms (the general cell and striated muscle UNC45s; UNC45-GC and UNC45-SM, respectively). In vertebrates, UNC45-GC facilitates cytoskeletal functions, whereas the 55% identical UNC45-SM assists assembly of the contractile apparatus of cardiac and skeletal muscles. A Saccharomyces cerevisiae she4Δ mutant, totally lacking any UCS protein, was engineered to express as its sole Hsp90 either the Hsp90α or the Hsp90β isoforms of human cytosolic Hsp90. A transient induction of the human UNC45-GC, but not UNC45-SM, could rescue the defective endocytosis in these she4Δ cells at 39 °C, irrespective of whether they possessed Hsp90α or Hsp90β. UNC45-GC-mediated rescue of the localisation of a Myo5-green fluorescent protein (GFP) fusion to cortical patches at 39 °C was more efficient in the yeast containing Hsp90α, though this may relate to more efficient functioning of Hsp90α as compared to Hsp90β in these strains. Furthermore, inducible expression of UNC45-GC, but not UNC45-SM, could partially rescue survival at a more extreme temperature (45 °C) that normally causes she4Δ mutant yeast cells to lyse. The results indicate that UCS protein function has been most conserved-yeast to man-in the UNC45-GC, not UNC45-SM. This may reflect UNC45-GC being the vertebrate UCS protein that assists formation of the actomyosin complexes needed for cytokinesis, cell morphological change, and organelle trafficking-events also facilitated by the myosins in yeast.

Steric hindrance in the upper 50 kDa domain of the motor Myo2p leads to cytokinesis defects in fission yeast

Cytokinesis in many eukaryotes requires a contractile actomyosin ring that is placed at the division site. In fission yeast, which is an attractive organism for the study of cytokinesis, actomyosin ring assembly and contraction requires the myosin II heavy chain Myo2p. Although myo2-E1, a temperature-sensitive mutant defective in the upper 50 kDa domain of Myo2p, has been studied extensively, the molecular basis of the cytokinesis defect is not understood. Here, we isolate myo2-E1-Sup2, an intragenic suppressor that contains the original mutation in myo2-E1 (G345R) and a second mutation in the upper 50 kDa domain (Y297C). Unlike myo2-E1-Sup1, a previously characterized myo2-E1 suppressor, myo2-E1-Sup2 reverses actomyosin ring contraction defects in vitro and in vivo Structural analysis of available myosin motor domain conformations suggests that a steric clash in myo2-E1, which is caused by the replacement of a glycine with a bulky arginine, is relieved in myo2-E1-Sup2 by mutation of a tyrosine to a smaller cysteine. Our work provides insight into the function of the upper 50 kDa domain of Myo2p, informs a molecular basis for the cytokinesis defect in myo2-E1, and may be relevant to the understanding of certain cardiomyopathies.

Fission yeast myosin Myo2 is down-regulated in actin affinity by light chain phosphorylation

Studies in fission yeast Schizosaccharomyces pombe have provided the basis for the most advanced models of the dynamics of the cytokinetic contractile ring. Myo2, a class-II myosin, is the major source of tension in the contractile ring, but how Myo2 is anchored and regulated to produce force is poorly understood. To enable more detailed biochemical/biophysical studies, Myo2 was expressed in the baculovirus/Sf9 insect cell system with its two native light chains, Rlc1 and Cdc4. Milligram yields of soluble, unphosphorylated Myo2 were obtained that exhibited high actin-activated ATPase activity and in vitro actin filament motility. The fission yeast specific chaperone Rng3 was thus not required for expression or activity. In contrast to nonmuscle myosins from animal cells that require phosphorylation of the regulatory light chain for activation, phosphorylation of Rlc1 markedly reduced the affinity of Myo2 for actin. Another unusual feature of Myo2 was that, unlike class-II myosins, which generally form bipolar filamentous structures, Myo2 showed no inclination to self-assemble at approximately physiological salt concentrations, as analyzed by sedimentation velocity ultracentrifugation. This lack of assembly supports the hypothesis that clusters of Myo2 depend on interactions at the cell cortex in structural units called nodes for force production during cytokinesis.

Mutation of the Ser18 phosphorylation site on the sole Saccharomyces cerevisiae UCS protein, She4, can compromise high-temperature survival

Folding of the myosin head often requires the joint actions of Hsp90 and a dedicated UNC45, Cro1, She4 (UCS) domain-containing cochaperone protein. Relatively weak sequence conservation exists between the single UCS protein of simple eukaryotes (She4 in budding yeast) and the two UCS proteins of higher organisms (the general cell and smooth muscle UNC45s; UNC45-GC and UNC45-SM respectively). In vertebrates, UNC45-GC facilitates cytoskeletal function whereas the 55% identical UNC45-SM assists in the assembly of the contractile apparatus of cardiac and skeletal muscles. UNC45-SM, unlike UNC45-GC, shares with yeast She4 an IDSL sequence motif known to be a site of in vivo serine phosphorylation in yeast. Investigating this further, we found that both a non-phosphorylatable (S18A) and a phosphomimetic (S18E) mutant form of She4 could rescue the type 1 myosin localisation and endocytosis defects of the yeast she4Δ mutant at 39 °C. Nevertheless, at higher temperature (45 °C), only She4 (S18A), not She4(S18E), could substantially rescue the cell lysis defect of she4Δ mutant cells. In the yeast two-hybrid system, the non-phosphorylatable S18A and S251A mutant forms of She4 and UNC45-SM still displayed the stress-enhanced in vivo interaction with Hsp90 seen with the wild-type She4 and UNC45-SM. Such high-temperature enforcement to interaction was though lost with the phosphomimetic mutant forms (She4(S18E) and UNC45-SM (S251E)), an indication that phosphorylation might suppress these increases in She4/Hsp90 and UNC45-SM/Hsp90 interaction with stress.

UCS proteins: chaperones for myosin and co-chaperones for Hsp90

The UCS (UNC-45/CRO1/She4p) family of proteins has emerged as chaperones that are specific for the folding, assembly and function of myosin. These proteins participate in various important myosin-dependent cellular processes that include myofibril organization and muscle functions, cell differentiation, cardiac and skeletal muscle development, cytokinesis and endocytosis. Mutations in the genes that code for UCS proteins cause serious defects in these actomyosin-based processes. Homologs of UCS proteins can be broadly divided into (1) animal UCS proteins, generally known as UNC-45 proteins, which contain an N-terminal tetratricopeptide repeat (TPR) domain in addition to the canonical UCS domain, and (2) fungal UCS proteins, which lack the TPR domain. Structurally, except for TPR domain, both sub-classes of UCS proteins comprise of several irregular armadillo (ARM) repeats that are divided into two-domain architecture: a combined central-neck domain and a C-terminal UCS domain. Structural analyses suggest that UNC-45 proteins form elongated oligomers that serve as scaffolds to recruit Hsp90 and/or Hsp70 to form a multi-protein chaperoning complex that assists myosin heads to fold and simultaneously organize them into myofibrils. Similarly, fungal UCS proteins may dimerize to promote folding of non-muscle myosins as well as determine their step size along actin filaments. These findings confirm UCS proteins as a new class of myosin-specific chaperones and co-chaperones for Hsp90. This chapter reviews the implications of the outcome of studies on these proteins in cellular processes such as muscle formation, and disease states such as myopathies and cancer.

A Toxoplasma gondii class XIV myosin, expressed in Sf9 cells with a parasite co-chaperone, requires two light chains for fast motility

Many diverse myosin classes can be expressed using the baculovirus/Sf9 insect cell expression system, whereas others have been recalcitrant. We hypothesized that most myosins utilize Sf9 cell chaperones, but others require an organism-specific co-chaperone. TgMyoA, a class XIVa myosin from the parasite Toxoplasma gondii, is required for the parasite to efficiently move and invade host cells. The T. gondii genome contains one UCS family myosin co-chaperone (TgUNC). TgMyoA expressed in Sf9 cells was soluble and functional only if the heavy and light chain(s) were co-expressed with TgUNC. The tetratricopeptide repeat domain of TgUNC was not essential to obtain functional myosin, implying that there are other mechanisms to recruit Hsp90. Purified TgMyoA heavy chain complexed with its regulatory light chain (TgMLC1) moved actin in a motility assay at a speed of ∼1.5 μm/s. When a putative essential light chain (TgELC1) was also bound, TgMyoA moved actin at more than twice that speed (∼3.4 μm/s). This result implies that two light chains bind to and stabilize the lever arm, the domain that amplifies small motions at the active site into the larger motions that propel actin at fast speeds. Our results show that the TgMyoA domain structure is more similar to other myosins than previously appreciated and provide a molecular explanation for how it moves actin at fast speeds. The ability to express milligram quantities of a class XIV myosin in a heterologous system paves the way for detailed structure-function analysis of TgMyoA and identification of small molecule inhibitors.

Proteome-wide search for PP2A substrates in fission yeast

PP2A (protein phosphatase 2A) is a major phosphatase in eukaryotic cells that plays an essential role in many processes. PP2A mutations in Schizosaccharomyces pombe result in defects of cell cycle control, cytokinesis and morphogenesis. Which PP2A substrates are responsible for these changes is not known. In this work, we searched for PP2A substrates in S. pombe using two approaches, 2D-DIGE analysis of PP2A complex mutants and identification of PP2A interacting proteins. In both cases, we used MS to identify proteins of interest. In the DIGE experiment, we compared proteomes of wild-type S. pombe, deletion of pta2, the phosphoactivator of the PP2A catalytic subunit, and pab1-4, a mutant of B-type PP2A regulatory subunit. A total of 1742 protein spots were reproducibly resolved by 2D-DIGE and 51 spots demonstrated significant changes between PP2A mutants and the wild-type control. MS analysis of these spots identified 27 proteins that include key regulators of glycerol synthesis, carbon metabolism, amino acid biosyntesis, vitamin production, and protein folding. Importantly, we independently identified a subset of these proteins as PP2A binding partners by affinity precipitation, suggesting they may be direct targets of PP2A. We have validated our approach by demonstrating that phosphorylation of Gpd1, a key enzyme in glycerol biogenesis, is regulated by PP2A and that ability of cells to respond to osmotic stress by synthesizing glycerol is compromised in the PP2A mutants. Our work contributes to a better understanding of PP2A function and identifies potential PP2A substrates.

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.

UCS protein Rng3p is essential for myosin-II motor activity during cytokinesis in fission yeast

UCS proteins have been proposed to operate as co-chaperones that work with Hsp90 in the de novo folding of myosin motors. The fission yeast UCS protein Rng3p is essential for actomyosin ring assembly and cytokinesis. Here we investigated the role of Rng3p in fission yeast myosin-II (Myo2p) motor activity. Myo2p isolated from an arrested rng3-65 mutant was capable of binding actin, yet lacked stability and activity based on its expression levels and inactivity in ATPase and actin filament gliding assays. Myo2p isolated from a myo2-E1 mutant (a mutant hyper-sensitive to perturbation of Rng3p function) showed similar behavior in the same assays and exhibited an altered motor conformation based on limited proteolysis experiments. We propose that Rng3p is not required for the folding of motors per se, but instead works to ensure the activity of intrinsically unstable myosin-II motors. Rng3p is specific to conventional myosin-II and the actomyosin ring, and is not required for unconventional myosin motor function at other actin structures. However, artificial destabilization of myosin-I motors at endocytic actin patches (using a myo1-E1 mutant) led to recruitment of Rng3p to patches. Thus, while Rng3p is specific to myosin-II, UCS proteins are adaptable and can respond to changes in the stability of other myosin motors.

Nonmedially assembled F-actin cables incorporate into the actomyosin ring in fission yeast

In many eukaryotes, cytokinesis requires the assembly and constriction of an actomyosin-based contractile ring. Despite the central role of this ring in cytokinesis, the mechanism of F-actin assembly and accumulation in the ring is not fully understood. In this paper, we investigate the mechanism of F-actin assembly during cytokinesis in Schizosaccharomyces pombe using lifeact as a probe to monitor actin dynamics. Previous work has shown that F-actin in the actomyosin ring is assembled de novo at the division site. Surprisingly, we find that a significant fraction of F-actin in the ring was recruited from formin-Cdc12p nucleated long actin cables that were generated at multiple nonmedial locations and incorporated into the ring by a combination of myosin II and myosin V activities. Our results, together with findings in animal cells, suggest that de novo F-actin assembly at the division site and directed transport of F-actin cables assembled elsewhere can contribute to ring assembly.

Conditional inactivation of replication proteins in fission yeast using hormone-binding domains

The fission yeast Schizosaccharomyces pombe is a useful model for analysing DNA replication as genetic methods to allow conditional inactivation of relevant proteins can provide important information about S-phase execution. A number of strategies are available to allow regulation of protein level or activity but there are disadvantages specific to each method and this may have limitations for particular proteins or experiments. We have investigated the utility of the inducible hormone-binding domain (HBD) system, which has been described in other organisms but little used in fission yeast, for the creation of conditional-lethal replication mutants. In this method, proteins are tagged with HBD and can be regulated with β-estradiol. In this article, we describe the application of this method in fission yeast, specifically with regard to analysis of the function of GINS, an essential component of the eukaryotic replicative helicase, the CMG complex.

The myosin-binding UCS domain but not the Hsp90-binding TPR domain of the UNC-45 chaperone is essential for function in Caenorhabditis elegans

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.

Approaches for defining the Hsp90-dependent proteome

Hsp90 is the target of ongoing drug discovery studies seeking new compounds to treat cancer, neurodegenerative diseases, and protein folding disorders. To better understand Hsp90's roles in cellular pathologies and in normal cells, numerous studies have utilized proteomics assays and related high-throughput tools to characterize its physical and functional protein partnerships. This review surveys these studies, and summarizes the strengths and limitations of the individual attacks. We also include downloadable spreadsheets compiling all of the Hsp90-interacting proteins identified in more than 23 studies. These tools include cross-references among gene aliases, human homologues of yeast Hsp90-interacting proteins, hyperlinks to database entries, summaries of canonical pathways that are enriched in the Hsp90 interactome, and additional bioinformatic annotations. In addition to summarizing Hsp90 proteomics studies performed to date and the insights they have provided, we identify gaps in our current understanding of Hsp90-mediated proteostasis. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Differential turnover of myosin chaperone UNC-45A isoforms increases in metastatic human breast cancer

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.

Drosophila UNC-45 accumulates in embryonic blastoderm and in muscles, and is essential for muscle myosin stability

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.

Three's company: the fission yeast actin cytoskeleton

How the actin cytoskeleton assembles into different structures to drive diverse cellular processes is a fundamental cell biological question. In addition to orchestrating the appropriate combination of regulators and actin-binding proteins, different actin-based structures must insulate themselves from one another to maintain specificity within a crowded cytoplasm. Actin specification is particularly challenging in complex eukaryotes where a multitude of protein isoforms and actin structures operate within the same cell. Fission yeast Schizosaccharomyces pombe possesses a single actin isoform that functions in three distinct structures throughout the cell cycle. In this review we explore recent studies in fission yeast that help unravel how different actin structures operate in cells.

UNC-45/CRO1/She4p (UCS) protein forms elongated dimer and joins two myosin heads near their actin binding region

UNC-45/CRO1/She4p (UCS) proteins have variously been proposed to affect the folding, stability, and ATPase activity of myosins. They are the only proteins known to interact directly with the motor domain. To gain more insight into UCS function, we determined the atomic structure of the yeast UCS protein, She4p, at 2.9 Å resolution. We found that 16 helical repeats are organized into an L-shaped superhelix with an amphipathic N-terminal helix dangling off the short arm of the L-shaped molecule. In the crystal, She4p forms a 193-Å-long, zigzag-shaped dimer through three distinct and evolutionary conserved interfaces. We have identified She4p's C-terminal region as a ligand for a 27-residue-long epitope on the myosin motor domain. Remarkably, this region consists of two adjacent, but distinct, binding epitopes localized at the nucleotide-responsive cleft between the nucleotide- and actin-filament-binding sites. One epitope is situated inside the cleft, the other outside the cleft. After ATP hydrolysis and Pi ejection, the cleft narrows at its base from 20 to 12 Å thereby occluding the inside the cleft epitope, while leaving the adjacent, outside the cleft binding epitope accessible to UCS binding. Hence, one cycle of higher and lower binding affinity would accompany one ATP hydrolysis cycle and a single step in the walk on an actin filament rope. We propose that a UCS dimer links two myosins at their motor domains and thereby functions as one of the determinants for step size of myosin on actin filaments.

Genome-wide mapping of myosin protein-RNA networks suggests the existence of specialized protein production sites

Motor proteins can organize posttranscriptional processes by transporting ribonucleoprotein complexes to specific locations. To investigate a possible role of myosin proteins in gene expression control, I have identified mRNAs associated with five myosin heavy chains in the fission yeast Schizosaccharomyces pombe, by purifying the proteins and identifying bound transcripts using DNA microarrays. Each myosin coimmunoprecipitated with 5-13 different mRNAs (approximately 0.1-0.2% of all genes), including those encoding four different myosin heavy chains. Moreover, one of the myosins (Myo1) interacted with mRNAs encoding components of the cortical actin cytoskeleton. These interactions were not observed in control immunoprecipitates. A myosin-specific chaperone (Rng3) that interacts cotranslationally with myosin mRNAs was essential for the association between myosin proteins and transcripts but not between Myo1 and other mRNAs. Finally, proteins encoded by the Myo1-associated mRNAs immunoprecipitated each other's transcripts, but not myosin mRNAs. These interactions suggest the existence of two distinct myosin-containing ribonucleoprotein complexes: those containing myosin mRNAs and those associated with Myo1. They are distinguished by their mRNA composition, requirement for the Rng3 chaperone and the presence of nonmyosin cytoskeletal proteins. I propose that these complexes represent specialized sites for the production of myosin proteins and the assembly of cytoskeletal components, respectively.

GINS inactivation phenotypes reveal two pathways for chromatin association of replicative alpha and epsilon DNA polymerases in fission yeast

The tetrameric GINS complex, consisting of Sld5-Psf1-Psf2-Psf3, plays an essential role in the initiation and elongation steps of eukaryotic DNA replication, although its biochemical function is unclear. Here we investigate the function of GINS in fission yeast, using fusion of Psf1 and Psf2 subunits to a steroid hormone-binding domain (HBD) to make GINS function conditional on the presence of beta-estradiol. We show that inactivation of Psf1-HBD causes a tight but rapidly reversible DNA replication arrest phenotype. Inactivation of Psf2-HBD similarly blocks premeiotic DNA replication and leads to loss of nuclear localization of another GINS subunit, Psf3. Inactivation of GINS has distinct effects on the replication origin association and chromatin binding of two of the replicative DNA polymerases. Inactivation of Psf1 leads to loss of chromatin binding of DNA polymerase epsilon, and Cdc45 is similarly affected. In contrast, chromatin association of the catalytic subunit of DNA polymerase alpha is not affected by defective GINS function. We suggest that GINS functions in a pathway that involves Cdc45 and is necessary for DNA polymerase epsilon chromatin binding, but that a separate pathway sets up the chromatin association of DNA polymerase alpha.

Rng3, a member of the UCS family of myosin co-chaperones, associates with myosin heavy chains cotranslationally

The production of functional myosin heavy chains in many eukaryotic organisms requires the function of proteins containing UCS domains (UNC-45/CRO1/She4), which bind to the myosin head domain and stimulate its folding. UCS proteins are essential for myosin-related functions such as muscle formation, RNA localization and cytokinesis. Here, we show that the Schizosaccharomyces pombe UCS protein Rng3 associates with polysomes, suggesting that UCS proteins might assist myosin folding cotranslationally. To identify Rng3 cotranslational targets systematically, we purified Rng3-associated RNAs and used DNA microarrays to identify the transcripts. Rng3 copurified with only seven transcripts (around 0.1% of S. pombe genes), including all five messenger RNAs encoding myosin heavy chains. These results suggest that every myosin heavy chain in S. pombe is a cotranslational target of Rng3. Furthermore, our data suggest that microarray-based approaches allow the genome-wide identification of cotranslational chaperone targets, and thus pave the way for the dissection of translation-linked chaperone networks.

Heat shock protein 90 regulates development in Dictyostelium discoideum

Cytosolic heat shock protein 90 (Hsp90) has been implicated in diverse biological processes such as protein folding, cell cycle control, signal transduction, development, and morphological evolution. Model systems available for studying Hsp90 function either allow ease of manipulation for biochemical studies or facilitate a phenomenological study of its role in influencing phenotype. In this work, we have explored the use of the cellular slime mold Dictyostelium discoideum to examine cellular functions of Hsp90 in relation to its multicellular development. In addition to cloning, purification, biochemical characterization, and examination of its crystal structure, our studies, using a pharmacological inhibitor of Hsp90, demonstrate a role for the cytoplasmic isoform (HspD) in D. discoideum development. Inhibition of HspD function using geldanamycin (GA) resulted in delayed aggregation and arrest of D. discoideum development at the 'mound' stage. Crystal structure of the amino-terminal domain of HspD showed a binding pocket similar to that described for yeast Hsp90. Fluorescence spectroscopy, as well as GA-coupled beads affinity pulldown, confirmed a specific interaction between HspD and GA. The results presented here provide an important insight into the function of HspD in D. discoideum development and emphasize the potential of the cellular slime mold to serve as an effective model for studying the many roles of Hsp90 at cellular and organismal levels.

Schizosaccharomyces pombe Hsp90/Git10 is required for glucose/cAMP signaling

The fission yeast Schizosaccharomyces pombe senses environmental glucose through a cAMP-signaling pathway. Elevated cAMP levels activate protein kinase A (PKA) to inhibit transcription of genes involved in sexual development and gluconeogenesis, including the fbp1(+) gene, which encodes fructose-1,6-bisphosphatase. Glucose-mediated activation of PKA requires the function of nine glucose-insensitive transcription (git) genes, encoding adenylate cyclase, the PKA catalytic subunit, and seven "upstream" proteins required for glucose-triggered adenylate cyclase activation. We describe the cloning and characterization of the git10(+) gene, which is identical to swo1(+) and encodes the S. pombe Hsp90 chaperone protein. Glucose repression of fbp1(+) transcription is impaired by both git10(-) and swo1(-) mutant alleles of the hsp90(+) gene, as well as by chemical inhibition of Hsp90 activity and temperature stress to wild-type cells. Unlike the swo1(-) mutant alleles, the git10-201 allele supports cell growth at 37 degrees , while severely reducing glucose repression of an fbp1-lacZ reporter, suggesting a separation-of-function defect. Sequence analyses of three swo1(-) alleles and the one git10(-) allele indicate that swo1(-) mutations alter core functional domains of Hsp90, while the git10(-) mutation affects the Hsp90 central domain involved in client protein binding. These results suggest that Hsp90 plays a specific role in the S. pombe glucose/cAMP pathway.

Yeast UCS proteins promote actomyosin interactions and limit myosin turnover in cells

Two functions are proposed for the conserved family of UCS proteins: helping to fold myosin motor proteins and stimulating the motor function of folded myosins. We examined both functions in yeast. The fission yeast UCS protein (Rng3p) concentrates in nodes containing myosin-II (Myo2) and other proteins that condense into the cytokinetic contractile ring. Both the N-terminal (central) and C-terminal (UCS) domains of Rng3p can concentrate independently in contractile rings, but only full-length Rng3p supports contractile ring function in vivo. The presence of Rng3p in ATPase assays doubles the apparent affinity (K(ATPase)) of both native Myo2 and recombinant heads of Myo2 for actin filaments. Rng3p promotes gliding of actin filaments by full-length Myo2 molecules, but not Myo2 heads alone. Myo2 isolated from mutant strains defective for Rng3p function is soluble and supports actin filament gliding. In budding yeast the single UCS protein (She4p) acts on both myosin-I isoforms (Myo3p and Myo5p) and one of two myosin-V isoforms (Myo4p). Myo5p turns over approximately 10 times faster in she4Delta cells than wild-type cells, reducing the level of Myo5p in cells 10-fold and in cortical actin patches approximately 4-fold. Nevertheless, Myo5p isolated from she4Delta cells has wild-type ATPase and motility activities. Thus, a fraction of this yeast myosin can fold de novo in the absence of UCS proteins, but UCS proteins promote myosin stability and interactions with actin.

Protein quality control gets muscle into shape

The synthesis, assembly and organisation of structural and motor proteins during muscle formation requires temporal and spatial control directed by specialized chaperones. For example, alphaB-crystallin, GimC and TRiC facilitate the assembly of sarcomeric proteins such as desmin and actin. Recent studies have demonstrated that the chaperone family of UCS proteins (UNC-45-CRO1-She4p) is required for the proper function of myosin motors. Mutations in the myosin-directed chaperone unc-45, a founding member of this family, lead to disorganisation of striated muscle in Caenorhabditiselegans. In addition to the involvement of client-specific chaperones, myofibrillogenesis also involves ubiquitin-dependent degradation of regulatory muscle proteins. Here, we highlight the interplay between chaperone activity and protein degradation in respect to the formation and maintenance of muscle during physiological and pathological conditions.

Heat shock protein 90 inhibitors protect and restore pulmonary endothelial barrier function

Heat shock protein 90 (hsp90) inhibitors inactivate and/or degrade various client proteins, including many involved in inflammation. Increased vascular permeability is a hallmark of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Thus, we tested the hypothesis that hsp90 inhibitors may prevent and/or restore endothelial cell (EC) permeability after injury. Exposure of confluent bovine pulmonary arterial endothelial cell (BPAEC) monolayer to TGF-beta1, thrombin, bacterial lipopolysaccharide (LPS), or vascular endothelial growth factor (VEGF) increased BPAEC permeability, as revealed by decreased transendothelial electrical resistance (TER). Treatment of injured endothelium with hsp90 inhibitors completely restored TER of BPAEC. Similarly, preincubation of BPAEC with hsp90 inhibitors prevented the decline in TER induced by the exposure to thrombin, LPS, VEGF, or TGF-beta1. In addition, hsp90 inhibitors restored the EC barrier function after PMA or nocodazole-induced hyperpermeability. These effects of the hsp90 inhibitors were associated with the restoration of TGF-beta1- or nocodazole-induced decrease in VE-cadherin and beta-catenin expression at EC junctions. The protective effect of hsp90 inhibitors on TGF-beta1-induced hyperpermeability was critically dependent upon preservation of F-actin cytoskeleton and was associated with the inhibition of agonist-induced myosin light chain (MLC) and myosin phosphatase target subunit 1 (MYPT1) phosphorylation, F-actin stress fibers formation, microtubule disassembly, increase in hsp27 phosphorylation, and association of hsp90 with hsp27, but independent of p38MAPK activity. We conclude that hsp90 inhibitors exert barrier protective effects on BPAEC, at least in part, via inhibition of hsp27-mediated, agonist-induced cytoskeletal rearrangement, and therefore may have useful therapeutic value in ALI, ARDS, and other pulmonary inflammatory disease.

Unc45 activates Hsp90-dependent folding of the myosin motor domain

Myosin folding and assembly in striated muscle are mediated by the general chaperones Hsc70 and Hsp90 and involve a myosin-specific co-chaperone related to the Caenorhabditis elegans gene unc-45. Two unc-45 genes are found in vertebrates, a general cell isoform, unc45a, and a striated muscle-specific isoform, unc45b. We have investigated the role of both isoforms of mouse Unc45 in myosin folding using an in vitro synthesis and folding assay. A smooth muscle myosin motor domain (MD) fused to green fluorescent protein (GFP) (MD::GFP) was used as substrate, and folding was measured by native gel electrophoresis and functional assays. In the absence of Unc45, the MD::GFP chimera folds poorly. Addition of either Unc45a or Unc45b dramatically enhances the folding in a reaction that is dependent on Hsp90 ATPase activity. Unc45a is more effective than Unc45b with a higher apparent affinity and greater extent of folding. The Unc45-Hsp90 chaperone complex acts late in the folding pathway and promotes motor domain maturation after release from the ribosome. Unc45a behaves kinetically as an activator of the folding reaction by stimulating the rate of the Hsp90-dependent folding by >20-fold with an apparent K(act) of 33 nm. This analysis of vertebrate Unc45 isoforms clearly demonstrates a direct role for Unc45 in Hsp90-mediated myosin motor domain folding and highlights major differences between the isoforms in substrate specificity and mechanism.

Shuttling of the chaperones Unc45b and Hsp90a between the A band and the Z line of the myofibril

The formation of thick filaments in striated muscle involves the chaperones Hsp90a and Unc45. We show that Unc45b and Hsp90a, two zebrafish orthologues, colocalize with myosin during myofibrillogenesis and associate with the Z line when myofibril assembly is completed. In response to stress or damage to the myofiber, Unc45b and Hsp90a dissociate from the Z line and transiently associate with myosin. Although chaperone activity of Unc45b requires the full-length protein, only the central and Unc45-Cro1p-She4p domains are required to anchor it to the Z line, and multiple subdomains mediate association with nascent myosin. We propose that the Z line serves as a reservoir for chaperones, allowing a rapid mobilization in response to muscle damage. Our data are consistent with a differential affinity model as an explanation for the shuttling of the chaperones between the Z line and myosin.

The ATPase-dependent chaperoning activity of Hsp90a regulates thick filament formation and integration during skeletal muscle myofibrillogenesis

The mechanisms that regulate sarcomere assembly during myofibril formation are poorly understood. In this study, we characterise the zebrafish sloth(u45) mutant, in which the initial steps in sarcomere assembly take place, but thick filaments are absent and filamentous I-Z-I brushes fail to align or adopt correct spacing. The mutation only affects skeletal muscle and mutant embryos show no other obvious phenotypes. Surprisingly, we find that the phenotype is due to mutation in one copy of a tandemly duplicated hsp90a gene. The mutation disrupts the chaperoning function of Hsp90a through interference with ATPase activity. Despite being located only 2 kb from hsp90a, hsp90a2 has no obvious role in sarcomere assembly. Loss of Hsp90a function leads to the downregulation of genes encoding sarcomeric proteins and upregulation of hsp90a and several other genes encoding proteins that may act with Hsp90a during sarcomere assembly. Our studies reveal a surprisingly specific developmental role for a single Hsp90 gene in a regulatory pathway controlling late steps in sarcomere assembly.

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.

Myosin II co-chaperone general cell UNC-45 overexpression is associated with ovarian cancer, rapid proliferation, and motility

Both tumor cell proliferation and metastasis are dependent on myosin II. Because UNC-45 is required to chaperone the assembly of a functional myosin II motor, we examined the expression of the general cell (GC) UNC-45 isoform in ovarian tumors. Serous carcinoma expressed elevated levels of GC UNC-45 compared with normal ovarian surface epithelium and benign cystadenoma. High-stage exhibited greater GC UNC-45 expression than low-stage serous carcinoma. Similarly, GC UNC-45 transcripts and protein levels were higher in ovarian cell lines than in immortalized ovarian surface epithelial cells. Elevation of GC UNC-45 levels by ectopic expression enhanced the rate of ovarian cancer cell proliferation, whereas siRNA knockdown of GC UNC-45 suppressed proliferation without altering myosin II levels. GC UNC-45 and myosin II were diffuse within the cytoplasm of confluent interphase cells, but both accumulated together at the cleavage furrow during cytokinesis. GC UNC-45 and myosin II also trafficked to the leading edges of ovarian cancer cells induced to move in a scratch assay. Knockdown of GC UNC-45 reduced the spreading ability of ovarian cancer cells whereas it was enhanced by GC UNC-45 overexpression. In sum, these findings implicate elevated GC UNC-45 protein expression in ovarian carcinoma proliferation and metastasis.

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.