The UCS family of myosin chaperones

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.

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 UNC-45 chaperone mediates sarcomere assembly through myosin degradation in Caenorhabditis elegans

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.

The ubiquitin-selective chaperone CDC-48/p97 links myosin assembly to human myopathy

Protein degradation in eukaryotes often requires the ubiquitin-selective chaperone p97 for substrate recruitment and ubiquitin-chain assembly. However, the physiological relevance of p97, and its role in developmental processes, remain unclear. Here, we discover an unanticipated function for CDC-48/p97 in myosin assembly and myofibril organization, both in Caenorhabditis elegans and humans. The developmentally regulated assembly of a CDC-48-UFD-2-CHN-1 complex links turnover of the myosin-directed chaperone UNC-45 to functional muscle formation. Our data suggest a similarly conserved pathway regulating myosin assembly in humans. Remarkably, mutations in human p97, known to cause hereditary inclusion-body myopathy, abrogate UNC-45 degradation and result in severely disorganized myofibrils, detrimental towards sarcomeric function. These results identify a key role for CDC-48/p97 in the process of myofibre differentiation and maintenance, which is abolished during pathological conditions leading to protein aggregation and inclusion-body formation in human skeletal muscle.

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.

GCUNC-45 is a novel regulator for the progesterone receptor/hsp90 chaperoning pathway

The hsp90 chaperoning pathway is a multiprotein system that is required for the production or activation of many cell regulatory proteins, including the progesterone receptor (PR). We report here the identity of GCUNC-45 as a novel modulator of PR chaperoning by hsp90. GCUNC-45, previously implicated in the activities of myosins, can interact in vivo and in vitro with both PR-A and PR-B and with hsp90. Overexpression and knockdown experiments show GCUNC-45 to be a positive factor in promoting PR function in the cell. GCUNC-45 binds to the ATP-binding domain of hsp90 to prevent the activation of its ATPase activity by the cochaperone Aha1. This effect limits PR chaperoning by hsp90, but this can be reversed by FKBP52, a cochaperone that is thought to act later in the pathway. These findings reveal a new cochaperone binding site near the N terminus of hsp90, add insight on the role of FKBP52, and identify GCUNC-45 as a novel regulator of the PR signaling pathway.

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

The F-actin-based molecular motor myosin II is involved in a variety of cellular processes such as muscle contraction, cell motility, and cytokinesis. In recent years, a family of myosin II-specific cochaperones of the UCS family has been identified from work with yeasts, fungi, worms, and humans. Biochemical analyses have shown that a complex of Hsp90 and the Caenorhabditis elegans UCS domain protein UNC-45 prevent myosin head aggregation, thereby allowing it to assume a proper structure. Here we demonstrate that a temperature-sensitive mutant of the fission yeast Hsp90 (Swo1p), swo1-w1, is defective in actomyosin ring assembly at the restrictive temperature. Two alleles of swo1, swo1-w1 and swo1-26, showed synthetic lethality with a specific mutant allele of the fission yeast type II myosin head, myo2-E1, but not with two other mutant alleles of myo2 or with mutations affecting 14 other genes important for cytokinesis. swo1-w1 also showed a strong genetic interaction with rng3-65, a gene encoding a mutation in the fission yeast UCS domain protein Rng3p, which has previously been shown to be important for myosin II assembly. A similar deleterious effect was found when myo2-E1, swo1-w1, and rng3-65 were pharmacologically treated with geldanamycin to partially inhibit Hsp90 function. Interestingly, Swo1p-green fluorescent protein is detected at the improperly assembled actomyosin rings in myo2-E1 but not in a wild-type strain. Yeast two-hybrid and coimmunoprecipitation analyses verified interactions between Rng3p and the myosin head domain as well as interactions between Rng3p and Swo1p. Our analyses of Myo2p, Swo1p, and the UCS domain protein Rng3p establish that Swo1p and Rng3p collaborate in vivo to modulate myosin II function.

A two-hybrid screen of the yeast proteome for Hsp90 interactors uncovers a novel Hsp90 chaperone requirement in the activity of a stress-activated mitogen-activated protein kinase, Slt2p (Mpk1p)

The Hsp90 chaperone cycle catalyzes the final activation step of several important eukaryotic proteins (Hsp90 "clients"). Although largely a functional form of Hsp90, an Hsp90-Gal4p DNA binding domain fusion (Hsp90-BD) displays no strong interactions in the yeast two-hybrid system, consistent with a general transience of most Hsp90-client associations. Strong in vivo interactions are though detected when the E33A mutation is introduced into this bait, a mutation that should arrest Hsp90-client complexes at a stage where the client is stabilized, yet prevented from attaining its active form. This E33A mutation stabilized the two-hybrid interactions of the Hsp90-BD fusion with approximately 3% of the Saccharomyces cerevisiae proteome in a screen of the 6,000 yeast proteins expressed as fusions to the Gal4p activation domain (AD). Among the detected interactors were the two stress-activated mitogen-activated protein (MAP) kinases of yeast, Hog1p and Slt2p (Mpk1p). Column retention experiments using wild-type and mutant forms of Hsp90 and Slt2p MAP kinase, as well as quantitative measurements of the effects of stress on the two-hybrid interaction of mutant Hsp90-BD and AD-Slt2p fusions, revealed that Hsp90 binds exclusively to the dually Thr/Tyr-phosphorylated, stress-activated form of Slt2p [(Y-P,T-P)Slt2p] and also to the MAP kinase domain within this (Y-P,T-P)Slt2p. Phenotypic analysis of a yeast mutant that expresses a mutant Hsp90 (T22Ihsp82) revealed that Hsp90 function is essential for this (Y-P,T-P)Slt2p to activate one of its downstream targets, the Rlm1p transcription factor. The interaction between Hsp90 and (Y-P,T-P)Slt2p, characterized in this study, is probably essential in this Hsp90 facilitation of the Rlm1p activation by Slt2p.

Functional specificity of co-chaperone interactions with Hsp90 client proteins

A wide array of proteins in signal transduction pathways depend on Hsp90 and other chaperone components for functional maturation, regulation, and stability. Among these Hsp90 client proteins are steroid receptors, members from other classes of transcription factors, and representatives of both serine/threonine and tyrosine kinase families. Typically, dynamic complexes form on the client protein, and these consist of Hsp90- plus bound co-chaperones that often have enzymatic activities. In addition to its direct influence on client folding, Hsp90 locally concentrates co-chaperone activity within the client complex, and dynamic exchange of co-chaperones on Hsp90 facilitates sampling of co-chaperone activities that may, or may not, act on the client protein. We are just beginning to understand the nature of biochemical and molecular interactions between co-chaperone and Hsp90-bound client. This review focuses on the differential effects of Hsp90 co-chaperones toward client protein function and on the specificity that allows co-chaperones to discriminate between even closely related clients.

Investigating the protein-protein interactions of the yeast Hsp90 chaperone system by two-hybrid analysis: potential uses and limitations of this approach

The Hsp90 chaperone cycle involves sequential assembly of different Hsp90-containing multiprotein complexes, the accessory proteins ("cochaperones") that are associated with these complexes being exchanged as the cycle proceeds from its early to its late stages. To gain insight as to whether the 2-hybrid system could be used to probe the interactions of this Hsp90 system, yeast transformants were constructed that express the Gal4p deoxyribonucleic acid-binding domain (BD) fused to the 2 Hsp90 isoforms and the various Hsp90 system cochaperones of yeast. These "bait" fusions were then introduced by mating into other transformants expressing nearly all the 6000 proteins of yeast expressed as fusions to the Gal4p activation domain (AD). High throughput 2-hybrid screening revealed the ability of Hsp90 and Hsp90 system cochaperones to engage in stable interactions in vivo, both with each other and with the various other proteins of the yeast proteome. Consistent with the transience of most chaperone associations, interactions to Hsp90 itself were invariably weak and generally influenced by stress. Mutations within a Hsp90-BD bait fusion and an AD-Cdc37 "prey" fusion were used to provide in vivo confirmation of the in vitro data that shows that Cdc37p is interacting with the "relaxed" conformation of Hsp90 and also to provide indications that Cdc37p needs to be phosphorylated at its N-terminus for any appreciable interaction with Hsp90. A number of potentially novel cochaperone interactions were also identified, providing a framework for these to be analyzed further using other techniques.

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.

Maternal UNC-45 is involved in cytokinesis and colocalizes with non-muscle myosin in the early Caenorhabditis elegans embryo

The Caenorhabditis elegans UNC-45 protein contains tetratricopeptide repeats and a domain with similarity to fungal proteins, and it differentially colocalizes with myosin heavy chain B in the body wall muscles of adult worms. Although it is essential for normal myosin filament assembly in body wall muscle development, strong mutants show a previously unexplained maternal effect. We show here that the UNC-45 protein is maternally contributed and is present in all cells of the early embryo whereas zygotic UNC-45 expression is only detected in the developing muscle cells. Embryos produced from adults with reduced germline expression of UNC-45 exhibit cytokinesis defects suggesting that UNC-45 has a novel role in the early embryo in addition to muscle development. Yeast two-hybrid screens show that UNC-45 can directly interact with NMY-2, a non-muscle type II myosin, and UNC-45 and NMY-2 colocalize at cell boundaries in early embryos. Localization of UNC-45 at these boundaries is dependent upon the presence of NMY-2. Our results suggest that UNC-45 interacts with more than one type of myosin and functions in the embryo to regulate cytoplasmic myosin assembly and/or stability during cytokinesis.

Myosin assembly: the power of multiubiquitylation

Ubiquitylation provides a means of targeting substrate proteins for degradation by the proteasome. Novel findings in C. elegans (Hoppe et al., 2004, this issue of Cell) establish that two ubiquitin-ligases team up to multiubiquitylate the myosin chaperone UNC-45, suggesting a novel link between regulated protein degradation and myosin assembly.

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.

Chaperone-mediated folding and assembly of myosin in striated muscle

De novo folding and assembly of striated muscle myosin was analyzed by expressing a GFP-tagged embryonic myosin heavy chain (GFP-myosin) in post-mitotic C2C12 myocytes using replication defective adenoviruses. In the early stages of muscle differentiation, the GFP-myosin accumulates in bright globular foci and short filamentous structures that are later replaced by brightly fluorescent myofibrils. Time-lapse microscopy shows that the intermediates are dynamic and are present in elongating and fusing myocytes and in multinucleated myotubes. Immunostaining reveals the co-localization of the molecular chaperones Hsc70 and Hsp90 with the GFP-myosin in the intermediates, but not in the mature myofibrils. Uninfected cells have similar intermediates suggesting a common pathway for myosin maturation. Two conformation-sensitive antibodies that bind the unfolded motor domain and the coiled-coil conformation of the rod demonstrate that in the intermediates, the myosin rod is folded but the motor domain is not folded. Electron microscopy reveals that the intermediates contain loose filament bundles surrounded by a protein rich matrix. Geldanamycin, a specific inhibitor of Hsp90, reversibly blocks myofibril assembly and triggers accumulation of myosin folding intermediates. We conclude that multimeric complexes of nascent myosin filaments associated with Hsc70 and Hsp90 are intermediates in the folding and assembly pathway of muscle myosin.

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.

Two mammalian UNC-45 isoforms are related to distinct cytoskeletal and muscle-specific functions

Previous studies have shown that the UNC-45 protein of C. elegans is required for normal thick filament assembly, binds Hsp90 and the myosin head, and shows molecular chaperone activity. We report here that mice and humans each have two genes that are located on different chromosomes, encode distinct UNC-45-like protein isoforms, and are expressed either in multiple tissues or only in cardiac and skeletal muscles. Their expression is regulated during muscle differentiation in vitro, with the striated muscle isoform mRNA appearing during myoblast fusion. Antisense experiments in C2C12 skeletal myogenic cells demonstrate that decreasing the general cell isoform mRNA reduces proliferation and fusion, while decreasing the striated muscle isoform mRNA affects fusion and sarcomere organization. These results suggest that the general cell UNC-45 isoform may have primarily cytoskeletal functions and that the striated muscle UNC-45 isoform may be restricted to roles in muscle-specific differentiation.