Functions of the myosin ATP and actin binding sites are required for C. elegans thick filament assembly

Substantial rDNA copy number reductions alter timing of development and produce variable tissue-specific phenotypes in C. elegans

The genes that encode ribosomal RNAs are present in several hundred copies in most eukaryotes. These vast arrays of repetitive ribosomal DNA (rDNA) have been implicated not just in ribosome biogenesis, but also aging, cancer, genome stability, and global gene expression. rDNA copy number is highly variable among and within species; this variability is thought to associate with traits relevant to human health and disease. Here we investigate the phenotypic consequences of multicellular life at the lower bounds of rDNA copy number. We use the model Caenorhabditis elegans, which has previously been found to complete embryogenesis using only maternally provided ribosomes. We find that individuals with rDNA copy number reduced to ∼5% of wild type are capable of further development with variable penetrance. Such individuals are sterile and exhibit severe morphological defects, particularly in post-embryonically dividing tissues such as germline and vulva. Developmental completion and fertility are supported by an rDNA copy number ∼10% of wild type, with substantially delayed development. Worms with rDNA copy number reduced to ∼33% of wild type display a subtle developmental timing defect that was absent in worms with higher copy numbers. Our results support the hypothesis that rDNA requirements vary across tissues and indicate that the minimum rDNA copy number for fertile adulthood is substantially less than the lowest naturally observed total copy number. The phenotype of individuals with severely reduced rDNA copy number is highly variable in penetrance and presentation, highlighting the need for continued investigation into the biological consequences of rDNA copy number variation.

Mechanical instability generated by Myosin 19 contributes to mitochondria cristae architecture and OXPHOS

The folded mitochondria inner membrane-cristae is the structural foundation for oxidative phosphorylation (OXPHOS) and energy production. By mechanically simulating mitochondria morphogenesis, we speculate that efficient sculpting of the cristae is organelle non-autonomous. It has long been inferred that folding requires buckling in living systems. However, the tethering force for cristae formation and regulation has not been identified. Combining electron tomography, proteomics strategies, super resolution live cell imaging and mathematical modeling, we reveal that the mitochondria localized actin motor-myosin 19 (Myo19) is critical for maintaining cristae structure, by associating with the SAM-MICOS super complex. We discover that depletion of Myo19 or disruption of its motor activity leads to altered mitochondria membrane potential and decreased OXPHOS. We propose that Myo19 may act as a mechanical tether for effective ridging of the mitochondria cristae, thus sustaining the energy homeostasis essential for various cellular functions.

The structure of the mitochondrial inner membrane, or cristae, is important for functional oxidative phosphorylation and energy production. Here, the authors show that loss of myosin 19 impairs cristae structure as well as energy production, connecting motor activity to membrane potential.

Coordination of mitochondrial and cellular dynamics by the actin-based motor Myo19

Myosin XIX (Myo19) is an actin-based motor that competes with adaptors of microtubule-based motors for binding to the outer mitochondrial transmembrane proteins Miro1 and Miro2 (collectively Miro, also known as RhoT1 and RhoT2, respectively). Here, we investigate which mitochondrial and cellular processes depend on the coordination of Myo19 and microtubule-based motor activities. To this end, we created Myo19-deficient HEK293T cells. Mitochondria in these cells were not properly fragmented at mitosis and were partitioned asymmetrically to daughter cells. Respiratory functions of mitochondria were impaired and ROS generation was enhanced. On a cellular level, cell proliferation, cytokinesis and cell-matrix adhesion were negatively affected. On a molecular level, Myo19 regulates focal adhesions in interphase, and mitochondrial fusion and mitochondrially associated levels of fission protein Drp1 and adaptor proteins dynactin and TRAK1 at prometaphase. These alterations were due to a disturbed coordination of Myo19 and microtubule-based motor activities by Miro.

Local Myo9b RhoGAP activity regulates cell motility

To migrate, cells assume a polarized morphology, extending forward with a leading edge with their trailing edge retracting back toward the cell body. Both cell extension and retraction critically depend on the organization and dynamics of the actin cytoskeleton, and the small, monomeric GTPases Rac and Rho are important regulators of actin. Activation of Rac induces actin polymerization and cell extension, whereas activation of Rho enhances acto-myosin II contractility and cell retraction. To coordinate migration, these processes must be carefully regulated. The myosin Myo9b, a Rho GTPase-activating protein (GAP), negatively regulates Rho activity and deletion of Myo9b in leukocytes impairs cell migration through increased Rho activity. However, it is not known whether cell motility is regulated by global or local inhibition of Rho activity by Myo9b. Here, we addressed this question by using Myo9b-deficient macrophage-like cells that expressed different recombinant Myo9b constructs. We found that Myo9b accumulates in lamellipodial extensions generated by Rac-induced actin polymerization as a function of its motor activity. Deletion of Myo9b in HL-60-derived macrophages altered cell morphology and impaired cell migration. Reintroduction of Myo9b or Myo9b motor and GAP mutants revealed that local GAP activity rescues cell morphology and migration. In summary, Rac activation leads to actin polymerization and recruitment of Myo9b, which locally inhibits Rho activity to enhance directional cell migration.

The R249Q hypertrophic cardiomyopathy myosin mutation decreases contractility in Drosophila by impeding force production

KEY POINTS

Hypertrophic cardiomyopathy (HCM) is a genetic disease that causes thickening of the heart's ventricular walls and is a leading cause of sudden cardiac death. HCM is caused by missense mutations in muscle proteins including myosin, but how these mutations alter muscle mechanical performance in largely unknown. We investigated the disease mechanism for HCM myosin mutation R249Q by expressing it in the indirect flight muscle of Drosophila melanogaster and measuring alterations to muscle and flight performance. Muscle mechanical analysis revealed R249Q decreased muscle power production due to slower muscle kinetics and decreased force production; force production was reduced because fewer mutant myosin cross-bridges were bound simultaneously to actin. This work does not support the commonly proposed hypothesis that myosin HCM mutations increase muscle contractility, or causes a gain in function; instead, it suggests that for some myosin HCM mutations, hypertrophy is a compensation for decreased contractility.

ABSTRACT

Hypertrophic cardiomyopathy (HCM) is an inherited disease that causes thickening of the heart's ventricular walls. A generally accepted hypothesis for this phenotype is that myosin heavy chain HCM mutations increase muscle contractility. To test this hypothesis, we expressed an HCM myosin mutation, R249Q, in Drosophila indirect flight muscle (IFM) and assessed myofibril structure, skinned fibre mechanical properties, and flight ability. Mechanics experiments were performed on fibres dissected from 2-h-old adult flies, prior to degradation of IFM myofilament structure, which started at 2 days old and increased with age. Homozygous and heterozygous R249Q fibres showed decreased maximum power generation by 67% and 44%, respectively. Decreases in force and work and slower overall muscle kinetics caused homozygous fibres to produce less power. While heterozygous fibres showed no overall slowing of muscle kinetics, active force and work production dropped by 68% and 47%, respectively, which hindered power production. The muscle apparent rate constant 2πb decreased 33% for homozygous but increased for heterozygous fibres. The apparent rate constant 2πc was greater for homozygous fibres. This indicates that R249Q myosin is slowing attachment while speeding up detachment from actin, resulting in less time bound. Decreased IFM power output caused 43% and 33% decreases in Drosophila flight ability and 19% and 6% drops in wing beat frequency for homozygous and heterozygous flies, respectively. Overall, our results do not support the increased contractility hypothesis. Instead, our results suggest the ventricular hypertrophy for human R249Q mutation is a compensatory response to decreases in heart muscle power output.

Protein phosphatase 2A is crucial for sarcomere organization in Caenorhabditis elegans striated muscle

Protein phosphatase 2A (PP2A) is a heterotrimer composed of single catalytic and scaffolding subunits and one of several possible regulatory subunits. We identified PPTR-2, a regulatory subunit of PP2A, as a binding partner for the giant muscle protein UNC-89 (obscurin) in Caenorhabditis elegans. PPTR-2 is required for sarcomere organization when its paralogue, PPTR-1, is deficient. PPTR-2 localizes to the sarcomere at dense bodies and M-lines, colocalizing with UNC-89 at M-lines. PP2A components in C. elegans include one catalytic subunit LET-92, one scaffolding subunit (PAA-1), and five regulatory subunits (SUR-6, PPTR-1, PPTR-2, RSA-1, and CASH-1). In adult muscle, loss of function in any of these subunits results in sarcomere disorganization. rsa-1 mutants show an interesting phenotype: one of the two myosin heavy chains, MHC A, localizes as closely spaced double lines rather than single lines. This "double line" phenotype is found in rare missense mutants of the head domain of MHC B myosin, such as unc-54(s74). Analysis of phosphoproteins in the unc-54(s74) mutant revealed two additional phosphoserines in the nonhelical tailpiece of MHC A. Antibodies localize PPTR-1, PAA-1, and SUR-6 to I-bands and RSA-1 to M-lines and I-bands. Therefore, PP2A localizes to sarcomeres and functions in the assembly or maintenance of sarcomeres.

A Failure to Communicate: MYOSIN RESIDUES INVOLVED IN HYPERTROPHIC CARDIOMYOPATHY AFFECT INTER-DOMAIN INTERACTION

Our molecular modeling studies suggest a charge-dependent interaction between residues Glu-497 in the relay domain and Arg-712 in the converter domain of human β-cardiac myosin. To test the significance of this putative interaction, we generated transgenic Drosophila expressing indirect flight muscle myosin with charge reversal mutations in the relay (E496R) or converter (R713E). Each mutation yielded dramatic reductions in myosin Ca-ATPase activity (~80%) as well as in basal (~67%) and actin-activated (~84%) Mg-ATPase activity. E496R myosin-induced in vitro actin-sliding velocity was reduced by 71% and R713E myosin permitted no actin motility. Indirect flight muscles of late pupae from each mutant displayed disrupted myofibril assembly, with adults having severely abnormal myofibrils and no flight ability. To understand the molecular basis of these defects, we constructed a putative compensatory mutant that expresses myosin with both E496R and R713E. Intriguingly, ATPase values were restored to ~73% of wild-type and actin-sliding velocity increased to 40%. The double mutation suppresses myofibril assembly defects in pupal indirect flight muscles and dramatically reduces myofibril disruption in young adults. Although sarcomere organization is not sustained in older flies and flight ability is not restored in homozygotes, young heterozygotes fly well. Our results indicate that this charge-dependent interaction between the myosin relay and converter domains is essential to the mechanochemical cycle and sarcomere assembly. Furthermore, the same inter-domain interaction is disrupted when modeling human β-cardiac myosin heavy chain cardiomyopathy mutations E497D or R712L, implying that abolishing this salt bridge is one cause of the human disease.

Biochemical and bioinformatic analysis of the myosin-XIX motor domain

Mitochondrial dynamics are dependent on both the microtubule and actin cytoskeletal systems. Evidence for the involvement of myosin motors has been described in many systems, and until recently a candidate mitochondrial myosin transport motor had not been described in vertebrates. Myosin-XIX (MYO19) was predicted to represent a novel class of myosin and had previously been shown to bind to mitochondria and increase mitochondrial network dynamics when ectopically expressed. Our analyses comparing ∼40 MYO19 orthologs to ∼2000 other myosin motor domain sequences identified instances of homology well-conserved within class XIX myosins that were not found in other myosin classes, suggesting MYO19-specific mechanochemistry. Steady-state biochemical analyses of the MYO19 motor domain indicate that Homo sapiens MYO19 is a functional motor. Insect cell-expressed constructs bound calmodulin as a light chain at the predicted stoichiometry and displayed actin-activated ATPase activity. MYO19 constructs demonstrated high actin affinity in the presence of ATP in actin-co-sedimentation assays, and translocated actin filaments in gliding assays. Expression of GFP-MYO19 containing a mutation impairing ATPase activity did not enhance mitochondrial network dynamics, as occurs with wild-type MYO19, indicating that myosin motor activity is required for mitochondrial motility. The measured biochemical properties of MYO19 suggest it is a high-duty ratio motor that could serve to transport mitochondria or anchor mitochondria, depending upon the cellular microenvironment.

Alpha-cardiac myosin heavy chain (MYH6) mutations affecting myofibril formation are associated with congenital heart defects

Congenital heart defects (CHD) are collectively the most common form of congenital malformation. Studies of human cases and animal models have revealed that mutations in several genes are responsible for both familial and sporadic forms of CHD. We have previously shown that a mutation in MYH6 can cause an autosomal dominant form of atrial septal defect (ASD), whereas others have identified mutations of the same gene in patients with hypertrophic and dilated cardiomyopathy. In the present study, we report a mutation analysis of MYH6 in patients with a wide spectrum of sporadic CHD. The mutation analysis of MYH6 was performed in DNA samples from 470 cases of isolated CHD using denaturing high-performance liquid chromatography and sequence analysis to detect point mutations and small deletions or insertions, and multiplex amplifiable probe hybridization to detect partial or complete copy number variations. One non-sense mutation, one splicing site mutation and seven non-synonymous coding mutations were identified. Transfection of plasmids encoding mutant and non-mutant green fluorescent protein-MYH6 fusion proteins in mouse myoblasts revealed that the mutations A230P and A1366D significantly disrupt myofibril formation, whereas the H252Q mutation significantly enhances myofibril assembly in comparison with the non-mutant protein. Our data indicate that functional variants of MYH6 are associated with cardiac malformations in addition to ASD and provide a novel potential mechanism. Such phenotypic heterogeneity has been observed in other genes mutated in CHD.

Mutating the converter-relay interface of Drosophila myosin perturbs ATPase activity, actin motility, myofibril stability and flight ability

We used an integrative approach to probe the significance of the interaction between the relay loop and converter domain of the myosin molecular motor from Drosophila melanogaster indirect flight muscle. During the myosin mechanochemical cycle, ATP-induced twisting of the relay loop is hypothesized to reposition the converter, resulting in cocking of the contiguous lever arm into the pre-power stroke configuration. The subsequent movement of the lever arm through its power stroke generates muscle contraction by causing myosin heads to pull on actin filaments. We generated a transgenic line expressing myosin with a mutation in the converter domain (R759E) at a site of relay loop interaction. Molecular modeling suggests that the interface between the relay loop and converter domain of R759E myosin would be significantly disrupted during the mechanochemical cycle. The mutation depressed calcium as well as basal and actin-activated MgATPase (V(max)) by approximately 60% compared to wild-type myosin, but there is no change in apparent actin affinity (K(m)). While ATP or AMP-PNP (adenylyl-imidodiphosphate) binding to wild-type myosin subfragment-1 enhanced tryptophan fluorescence by approximately 15% or approximately 8%, respectively, enhancement does not occur in the mutant. This suggests that the mutation reduces lever arm movement. The mutation decreases in vitro motility of actin filaments by approximately 35%. Mutant pupal indirect flight muscles display normal myofibril assembly, myofibril shape, and double-hexagonal arrangement of thick and thin filaments. Two-day-old fibers have occasional "cracking" of the crystal-like array of myofilaments. Fibers from 1-week-old adults show more severe cracking and frayed myofibrils with some disruption of the myofilament lattice. Flight ability is reduced in 2-day-old flies compared to wild-type controls, with no upward mobility but some horizontal flight. In 1-week-old adults, flight capability is lost. Thus, altered myosin function permits myofibril assembly, but results in a progressive disruption of the myofilament lattice and flight ability. We conclude that R759 in the myosin converter domain is essential for normal ATPase activity, in vitro motility and locomotion. Our results provide the first mutational evidence that intramolecular signaling between the relay loop and converter domain is critical for myosin function both in vitro and in muscle.

Myosin 1G (Myo1G) is a haematopoietic specific myosin that localises to the plasma membrane and regulates cell elasticity

Immune cells navigate through different environments where they experience different mechanical forces. Responses to external forces are determined by the mechanical properties of a cell and they depend to a large extent on the actin-rich cell cortex. We report here that Myo1G, a previously uncharacterised member of class I myosins, is expressed specifically in haematopoietic tissues and cells. It is associated with the plasma membrane. This association is dependent on a conserved PH-domain-like myosin I tail homology motif and the head domain. However, the head domain does not need to be a functional motor. Knockdown of Myo1G in Jurkat cells decreased cell elasticity significantly. We propose that Myo1G regulates cell elasticity by deformations of the actin network at the cell cortex.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

An MYH9 human disease model in flies: site-directed mutagenesis of the Drosophila non-muscle myosin II results in hypomorphic alleles with dominant character

We investigated whether or not human disease-causing, amino acid substitutions in MYH9 could cause dominant phenotypes when introduced into the sole non-muscle myosin II heavy chain in Drosophila melanogaster (zip/MyoII). We characterized in vivo the effects of four MYH9-like mutations in the myosin rod-R1171C, D1430N, D1847K and R1939X-which occur at highly conserved residues. These engineered mutant heavy chains resulted in D. melanogaster non-muscle myosin II with partial wild-type function. In a wild-type genetic background, mutant heavy chains were overtly recessive and hypomorphic: each was able to substitute partially for endogenous non-muscle myosin II heavy chain in animals lacking zygotically produced heavy chain (but the penetrance of rescue was below Mendelian expectation). Moreover, each of the four mutant heavy chains exhibits dominant characteristics when expressed in a sensitized genetic background (flies heterozygous for RhoA mutations). Thus, these zip/MyoII(MYH9) alleles function, like certain other hypomorphic alleles, as excellent bait in screens for genetic interactors. Our conjecture is that these mutations in D. melanogaster behave comparably to their parent mutations in humans. We further characterized these zip/MyoII(MYH9) alleles, and found that all were capable of correct spatial and temporal localization in animals lacking zygotic expression of wild-type zip/MyoII. In vitro, we demonstrate that mutant heavy chains can dimerize with endogenous, wild-type heavy chains, fold into coiled-coil structures and assemble into higher-order structures. Our work further supports D. melanogaster as a model system for investigating the basis of human disease.

The Myosin IXb motor activity targets the myosin IXb RhoGAP domain as cargo to sites of actin polymerization

Myosin IXb (Myo9b) is a single-headed processive myosin that exhibits Rho GTPase-activating protein (RhoGAP) activity in its tail region. Using live cell imaging, we determined that Myo9b is recruited to extending lamellipodia, ruffles, and filopodia, the regions of active actin polymerization. A functional motor domain was both necessary and sufficient for targeting Myo9b to these regions. The head domains of class IX myosins comprise a large insertion in loop2. Deletion of the large Myo9b head loop 2 insertion abrogated the enrichment in extending lamellipodia and ruffles, but enhanced significantly the enrichment at the tips of filopodia and retraction fibers. The enrichment in the tips of filopodia and retraction fibers depended on four lysine residues C-terminal to the loop 2 insertion and the tail region. Fluorescence recovery after photobleaching and photoactivation experiments in lamellipodia revealed that the dynamics of Myo9b was comparable to that of actin. The exchange rates depended on the Myo9b motor region and motor activity, and they were also dependent on the turnover of F-actin. These results demonstrate that Myo9b functions as a motorized RhoGAP molecule in regions of actin polymerization and identify Myo9b head sequences important for in vivo motor properties.

Conservation rules, their breakdown, and optimality in Caenorhabditis sinusoidal locomotion

Undulatory locomotion is common to nematodes as well as to limbless vertebrates, but its control is not understood in spite of the identification of hundred of genes involved in Caenorhabditis elegans locomotion. To reveal the mechanisms of nematode undulatory locomotion, we quantitatively analysed the movement of C. elegans with genetic perturbations to neurons, muscles, and skeleton (cuticle). We also compared locomotion of different Caenorhabditis species. We constructed a theoretical model that combines mechanics and biophysics, and that is constrained by the observations of propulsion and muscular velocities, as well as wavelength and amplitude of undulations. We find that normalized wavelength is a conserved quantity among wild-type C. elegans individuals, across mutants, and across different species. The velocity of forward propulsion scales linearly with the velocity of the muscular wave and the corresponding slope is also a conserved quantity and almost optimal; the exceptions are in some mutants affecting cuticle structure. In theoretical terms, the optimality of the slope is equivalent to the exact balance between muscular and visco-elastic body reaction bending moments. We find that the amplitude and frequency of undulations are inversely correlated and provide a theoretical explanation for this fact. These experimental results are valid both for young adults and for all larval stages of wild-type C. elegans. In particular, during development, the amplitude scales linearly with the wavelength, consistent with our theory. We also investigated the influence of substrate firmness on motion parameters, and found that it does not affect the above invariants. In general, our biomechanical model can explain the observed robustness of the mechanisms controlling nematode undulatory locomotion.

Calcium transients regulate patterned actin assembly during myofibrillogenesis

The highly ordered arrangement of sarcomeric myosin during striated muscle development requires spontaneous calcium (Ca(2+)) transients. Here, we show that blocking transients also compromises patterned assembly of actin thin filaments, titin, and capZ. Because a conserved temporal assembly pattern has been described for these proteins, selective inhibitors of either thick or thin filament formation were used to determine their relative temporal interdependencies. For example, inhibition of myosin light chain kinase (MLCK) by application of a specific inhibitory peptide or phorbol myistate acetate (PMA) disrupts myosin assembly without significantly affecting formation of actin bands. The MLCK inhibitor ML-7, however, disrupted actin as well as myosin. Surprisingly, agents that interfere with actin dynamics, such as cytochalasin D, produced only minor organizational disruptions in actin, capZ, and titin staining. However, cytochalasin D and other actin disrupting compounds significantly perturbed myosin organization. The results indicate that (1) Ca(2+) transients regulate one or more of the earliest steps in sarcomere formation, (2) mature actin filaments can assemble independently of myosin band formation, and (3) myosin thick filament assembly is extremely sensitive to disruption of either the actin or titin filament systems.

Mutations in the relay loop region result in dominant-negative inhibition of myosin II function in Dictyostelium

Dominant-negative inhibition is a powerful genetic tool for the characterization of gene function in vivo, based on the specific impairment of a gene product by the coexpression of a mutant version of the same gene product. We describe the detailed characterization of two myosin constructs containing either point mutations F487A or F506G in the relay region. Dictyostelium cells transformed with F487A or F506G myosin are unable to undergo processes that require myosin II function, including fruiting-body formation, normal cytokinesis and growth in suspension. Our results show that the dominant-negative inhibition of myosin function is caused by disruption of the communication between active site and lever arm, which blocks motor activity completely, and perturbation of the communication between active site and actin-binding site, leading to an approximately 100-fold increase in the mutants' affinity for actin in the presence of ATP.

Stop making nonSense: the C. elegans smg genes

Cells monitor the quality of their mRNAs and degrade any transcripts that are poorly or incompletely translated. In the nematode Caenorhabditis elegans, degradation by the mRNA surveillance pathway depends on seven smg genes. Three of these genes also have a role in a second mRNA degradation pathway called RNA interference (RNAi), which is triggered by double-stranded RNA (dsRNA). Here I describe what is known about the smg genes and their potential functions in these two mRNA degradation pathways.

Functional role for the class IX myosin myr5 in epithelial cell infection by Shigella flexneri

Efficient control of Shigella-induced, rho-dependent cytoskeletal rearrangements seems to be required to shape the delicate cellular structures associated with bacterial invasion of epithelial cells. We therefore studied a class IX myosin and rho antagonist, the GTPase-activating protein (GAP) myr5, for a potential role in the bacterial entry process. We show that myr5 is recruited into bacterial entry spots. The recruitment pattern resembled that of rhoC or ezrin, but not rhoA, rac or CDC42, while in vitro GAP activity of myr5 was similar for rhoA, B or C. Analysis of myr5 mutants suggested that GTPase- or ATP-binding activites are not required for Shigella-induced recruitment of this atypical myosin to the bacterial entry site. Functional studies revealed a potential dual role of the myosin functions and the GAP module of myr5 for bacterial internalization.

Homozygous mutation in cardiac troponin T: implications for hypertrophic cardiomyopathy

BACKGROUND

Mutations in the gene that encode cardiac troponin T (cTnT) account for approximately 15% of cases of familial hypertrophic cardiomyopathy (HCM). These mutations are associated with a particularly severe form of HCM characterized by a high incidence of sudden death and a poor overall prognosis, despite subclinical or mild left ventricular hypertrophy.

METHODS AND RESULTS

We evaluated a family with HCM and multiple occurrences of sudden death in children. DNA samples were isolated from peripheral blood or paraffin-embedded tissue, and all protein-encoding exons of the cTnT gene were sequenced. A mutation was identified in exon 11 and is predicted to substitute a phenylalanine-for-serine mutation at residue 179 (Ser(179)Phe) in cTnT. Both parents and 3 of 4 surviving and clinically unaffected children were heterozygous for this mutation; another clinically unaffected child did not carry the mutation. Genetic analysis of DNA from a child who died suddenly at age 17 years demonstrated he was homozygous for this mutation. A review of his echocardiogram revealed profound left and right ventricular hypertrophy.

CONCLUSIONS

An homozygous Ser(179)Phe mutation in cTnT causes a severe form of HCM characterized by striking morphological abnormalities and juvenile lethality. In contrast, the natural history of the heterozygous mutation is benign. These studies emphasize the relevance of genetic diagnosis in hypertrophic cardiomyopathy and provide a new perspective on the clinical consequences of troponin T mutations.

Unproductively spliced ribosomal protein mRNAs are natural targets of mRNA surveillance in C. elegans

Messenger RNA surveillance, the selective and rapid degradation of mRNAs containing premature stop codons, occurs in all eukaryotes tested. The biological role of this decay pathway, however, is not well understood. To identify natural substrates of mRNA surveillance, we used a cDNA-based representational difference analysis to identify mRNAs whose abundance increases in Caenorhabditis elegans smg(-) mutants, which are deficient for mRNA surveillance. Alternatively spliced mRNAs of genes encoding ribosomal proteins L3, L7a, L10a, and L12 are abundant natural targets of mRNA surveillance. Each of these genes expresses two distinct mRNAs. A productively spliced mRNA, whose abundance does not change in smg(-) mutants, encodes a normal, full-length, ribosomal protein. An unproductively spliced mRNA, whose abundance increases dramatically in smg(-) mutants, contains premature stop codons because of incomplete removal of an alternatively spliced intron. In transgenic animals expressing elevated quantities of RPL-12, a greater proportion of endogenous rpl-12 transcript is spliced unproductively. Thus, RPL-12 appears to autoregulate its own splicing, with unproductively spliced mRNAs being degraded by mRNA surveillance. We demonstrate further that alternative splicing of rpl introns is conserved among widely diverged nematodes. Our results suggest that one important role of mRNA surveillance is to eliminate unproductive by-products of gene regulation.

Molecular motors--a paradigm for mutant analysis

Molecular motors perform essential functions in the cell and have the potential to provide insights into the basis of many important processes. A unique property of molecular motors is their ability to convert energy from ATP hydrolysis into work, enabling the motors to bind to and move along cytoskeletal filaments. The mechanism of energy conversion by molecular motors is not yet understood and may lead to the discovery of new biophysical principles. Mutant analysis could provide valuable information, but it is not obvious how to obtain mutants that are informative for study. The analysis presented here points out several strategies for obtaining mutants by selection from molecular or genetic screens, or by rational design. Mutants that are expected to provide important information about the motor mechanism include ATPase mutants, which interfere with the nucleotide hydrolysis cycle, and uncoupling mutants, which unlink basic motor activities and reveal their interdependence. Natural variants can also be exploited to provide unexpected information about motor function. This general approach to uncovering protein function by analysis of informative mutants is applicable not only to molecular motors, but to other proteins of interest.

Direct involvement of yeast type I myosins in Cdc42-dependent actin polymerization

The generation of cortical actin filaments is necessary for processes such as cell motility and cell polarization. Several recent studies have demonstrated that Wiskott-Aldrich syndrome protein (WASP) family proteins and the actin-related protein (Arp) 2/3 complex are key factors in the nucleation of actin filaments in diverse eukaryotic organisms. To identify other factors involved in this process, we have isolated proteins that bind to Bee1p/Las17p, the yeast WASP-like protein, by affinity chromatography and mass spectroscopic analysis. The yeast type I myosins, Myo3p and Myo5p, have both been identified as Bee1p-interacting proteins. Like Bee1p, these myosins are essential for cortical actin assembly as assayed by in vitro reconstitution of actin nucleation sites in permeabilized yeast cells. Analysis using this assay further demonstrated that the motor activity of these myosins is required for the polymerization step, and that actin polymerization depends on phosphorylation of myosin motor domain by p21-activated kinases (PAKs), downstream effectors of the small guanosine triphosphatase, Cdc42p. The type I myosins also interact with the Arp2/3 complex through a sequence at the end of the tail domain homologous to the Arp2/3-activating region of WASP-like proteins. Combined deletions of the Arp2/3-interacting domains of Bee1p and the type I myosins abolish actin nucleation sites at the cortex, suggesting that these proteins function redundantly in the activation of the Arp2/3 complex.

High frequency of cryptic deleterious mutations in Caenorhabditis elegans

Deleterious mutations with very small phenotypic effects could be important for several evolutionary phenomena, but the extent of their contribution has been unknown. Fitness effects of induced mutations in lines of Caenorhabditis elegans were measured using a system for which the number of deleterious point mutations in the DNA can be estimated. In fitness assays, only about 4 percent of the deleterious mutations fixed in each line were detectable. The remaining 96 percent, though cryptic, are significant for mutation load and, potentially, for the evolution of sex.

Structural analysis of the titin gene in hypertrophic cardiomyopathy: identification of a novel disease gene

Hypertrophic cardiomyopathy (HCM) is characterized by ventricular hypertrophy accompanied by myofibrillar disarrays. Molecular genetic analyses have revealed that mutations in 8 different genes cause HCM. Mutations in these disease genes, however, could be found in about half of HCM patients, suggesting that there are other unknown disease gene(s). Because the known disease genes encode sarcomeric proteins expressed in the cardiac muscle, we searched for a disease-associated mutation in the titin gene in 82 HCM patients who had no mutation in the known disease genes. A G to T transversion in codon 740, from CGC to CTC, replacing Arginine with Leucine was found in a patient. This mutation was not found in more than 500 normal chromosomes and increased the binding affinity of titin to alpha-actitin in the yeast two-hybrid assay. These observations suggest that the titin mutation may cause HCM in this patient via altered affinity to alpha-actinin.

Assembly of thick filaments and myofibrils occurs in the absence of the myosin head

We investigated the importance of the myosin head in thick filament formation and myofibrillogenesis by generating transgenic Drosophila lines expressing either an embryonic or an adult isoform of the myosin rod in their indirect flight muscles. The headless myosin molecules retain the regulatory light-chain binding site, the alpha-helical rod and the C-terminal tailpiece. Both isoforms of headless myosin co-assemble with endogenous full-length myosin in wild-type muscle cells. However, rod polypeptides interfere with muscle function and cause a flightless phenotype. Electron microscopy demonstrates that this results from an antimorphic effect upon myofibril assembly. Thick filaments assemble when the myosin rod is expressed in mutant indirect flight muscles where no full-length myosin heavy chain is produced. These filaments show the characteristic hollow cross-section observed in wild type. The headless thick filaments can assemble with thin filaments into hexagonally packed arrays resembling normal myofibrils. However, thick filament length as well as sarcomere length and myofibril shape are abnormal. Therefore, thick filament assembly and many aspects of myofibrillogenesis are independent of the myosin head and these processes are regulated by the myosin rod and tailpiece. However, interaction of the myosin head with other myofibrillar components is necessary for defining filament length and myofibril dimensions.

Unc-45 mutations in Caenorhabditis elegans implicate a CRO1/She4p-like domain in myosin assembly

The Caenorhabditis elegans unc-45 locus has been proposed to encode a protein machine for myosin assembly. The UNC-45 protein is predicted to contain an NH2-terminal domain with three tetratricopeptide repeat motifs, a unique central region, and a COOH-terminal domain homologous to CRO1 and She4p. CRO1 and She4p are fungal proteins required for the segregation of other molecules in budding, endocytosis, and septation. Three mutations that lead to temperature-sensitive (ts) alleles have been localized to conserved residues within the CRO1/She4p-like domain, and two lethal alleles were found to result from stop codon mutations in the central region that would prevent translation of the COOH-terminal domain. Electron microscopy shows that thick filament accumulation in vivo is decreased by approximately 50% in the CB286 ts mutant grown at the restrictive temperature. The thick filaments that assemble have abnormal structure. Immunofluorescence and immunoelectron microscopy show that myosins A and B are scrambled, in contrast to their assembly into distinct regions at the permissive temperature and in wild type. This abnormal structure correlates with the high degree of instability of the filaments in vitro as reflected by their extremely low yields and shortened lengths upon isolation. These results implicate the UNC-45 CRO1/She4p-like region in the assembly of myosin isoforms in C. elegans and suggest a possible common mechanism for the function of this UCS (UNC-45/CRO1/She4p) protein family.

Familial hypertrophic cardiomyopathy: from mutations to functional defects

Hypertrophic cardiomyopathy is characterized by left and/or right ventricular hypertrophy, which is usually asymmetric and involves the interventricular septum. Typical morphological changes include myocyte hypertrophy and disarray surrounding the areas of increased loose connective tissue. Arrhythmias and premature sudden deaths are common. Hypertrophic cardiomyopathy is familial in the majority of cases and is transmitted as an autosomal-dominant trait. The results of molecular genetics studies have shown that familial hypertrophic cardiomyopathy is a disease of the sarcomere involving mutations in 7 different genes encoding proteins of the myofibrillar apparatus: ss-myosin heavy chain, ventricular myosin essential light chain, ventricular myosin regulatory light chain, cardiac troponin T, cardiac troponin I, alpha-tropomyosin, and cardiac myosin binding protein C. In addition to this locus heterogeneity, there is a wide allelic heterogeneity, since numerous mutations have been found in all these genes. The recent development of animal models and of in vitro analyses have allowed a better understanding of the pathophysiological mechanisms associated with familial hypertrophic cardiomyopathy. One can thus tentatively draw the following cascade of events: The mutation leads to a poison polypeptide that would be incorporated into the sarcomere. This would alter the sarcomeric function that would result (1) in an altered cardiac function and then (2) in the alteration of the sarcomeric and myocyte structure. Some mutations induce functional impairment and support the pathogenesis hypothesis of a "hypocontractile" state followed by compensatory hypertrophy. Other mutations induce cardiac hyperfunction and determine a "hypercontractile" state that would directly induce cardiac hypertrophy. The development of other animal models and of other mechanistic studies linking the genetic mutation to functional defects are now key issues in understanding how alterations in the basic contractile unit of the cardiomyocyte alter the phenotype and the function of the heart.

Hypertrophic obstructive cardiomyopathy due to a novel T-to-A transition at codon 624 in the beta-myosin heavy chain (beta-MHC) gene possibly related to the sudden death

Many missense mutations in the beta-myosin heavy chain have been reported in patients with hypertrophic obstructive cardiomyopathy (HOCM). However, the controversy is present whether the mutation accompanying the change of electric charge is related with poorer prognosis. The proband, a 48-year-old female, of the family was diagnosed clinically as HOCM, and a structural analysis of the cardiac beta-MHC gene showed that the proband and her junior daughter had a novel mutation with T to A transition in codon 624 replacing tyrosine with asparagine, which was not present in her husband, elder daughter and son. The proband's husband, son and two daughters were healthy except that the ECG of junior daughter (15-year-old) showed complete right bundle branch block. Proband's mother died suddenly after the delivery of the proband and the proband also collapsed suddenly. The occurrence of sudden death in proband and her mother suggested that HOCM with this novel mutation might be associated with a high risk of sudden death irrespective of the absence of charge alteration.

Point mutations in human beta cardiac myosin heavy chain have differential effects on sarcomeric structure and assembly: an ATP binding site change disrupts both thick and thin filaments, whereas hypertrophic cardiomyopathy mutations display normal assembly

Hypertrophic cardiomyopathy is a human heart disease characterized by increased ventricular mass, focal areas of fibrosis, myocyte, and myofibrillar disorganization. This genetically dominant disease can be caused by mutations in any one of several contractile proteins, including beta cardiac myosin heavy chain (beta MHC). To determine whether point mutations in human beta MHC have direct effects on interfering with filament assembly and sarcomeric structure, full-length wild-type and mutant human beta MHC cDNAs were cloned and expressed in primary cultures of neonatal rat ventricular cardiomyocytes (NRC) under conditions that promote myofibrillogenesis. A lysine to arginine change at amino acid 184 in the consensus ATP binding sequence of human beta MHC resulted in abnormal subcellular localization and disrupted both thick and thin filament structure in transfected NRC. Diffuse beta MHC K184R protein appeared to colocalize with actin throughout the myocyte, suggesting a tight interaction of these two proteins. Human beta MHC with S472V mutation assembled normally into thick filaments and did not affect sarcomeric structure. Two mutant myosins previously described as causing human hypertrophic cardiomyopathy, R249Q and R403Q, were competent to assemble into thick filaments producing myofibrils with well defined I bands, A bands, and H zones. Coexpression and detection of wild-type beta MHC and either R249Q or R403Q proteins in the same myocyte showed these proteins are equally able to assemble into the sarcomere and provided no discernible differences in subcellular localization. Thus, human beta MHC R249Q and R403Q mutant proteins were readily incorporated into NRC sarcomeres and did not disrupt myofilament formation. This study indicates that the phenotype of myofibrillar disarray seen in HCM patients which harbor either of these two mutations may not be directly due to the failure of the mutant myosin heavy chain protein to assemble and form normal sarcomeres, but may rather be a secondary effect possibly resulting from the chronic stress of decreased beta MHC function.

Expression and functional assessment of a truncated cardiac troponin T that causes hypertrophic cardiomyopathy. Evidence for a dominant negative action

Mutations in the beta-myosin heavy chain gene are believed to cause hypertrophic cardiomyopathy (HCM) by acting as dominant negative alleles. In contrast, a truncated cardiac troponin T (TnT) that causes HCM implies that altered stoichiometry of contractile proteins may also cause cardiac hypertrophy. Wild-type and HCM-mutant (truncated) TnT were studied in a novel quail myotube expression system. Unexpectedly, antibody staining demonstrated incorporation of both forms of human cardiac TnT into the sarcomeres of quail myotubes. Functional studies of wild type and mutant transfected myotubes of normal appearance revealed that calcium-activated force of contraction was normal upon incorporation of wild type TnT, but greatly diminished for the mutant TnT. These findings indicate that HCM-causing mutations in TnT and beta-myosin heavy chain share abnormalities in common, acting as dominant negative alleles that impair contractile performance. This diminished force output is the likely stimulus for hypertrophy in the human heart.

Structure-function analysis of the motor domain of myosin

Motor proteins perform a wide variety of functions in all eukaryotic cells. Recent advances in the structural and mutagenic analysis of the myosin motor has led to insights into how these motors transduce chemical energy into mechanical work. This review focuses on the analysis of the effects of myosin mutations from a variety of organisms on the in vivo and in vitro properties of this ubiquitous motor and illustrates the positions of these mutations on the high-resolution three-dimensional structure of the myosin motor domain.

Contractile protein mutations and heart disease

Mutations in several muscle structural proteins (the myosin heavy chain, alpha tropomyosin, cardiac troponin T and myosin binding protein C) result in a genetically dominant heart disease, hypertrophic cardiomyopathy. Biochemical data from studies of mutant myosin suggest a dominant-negative mechanism for inheritance of this disease. The most likely primary defect is sarcomere dysfunction, which is followed by the major clinical symptoms.

A myosin missense mutation, not a null allele, causes familial hypertrophic cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is characterized by myocardial hypertrophy of unknown etiology. Missense mutations of the cardiac beta-myosin-heavy-chain (beta-MHC) gene that may be responsible for cardiac hypertrophy have been detected in patients with HCM. On the other hand, gross structural abnormalities in the cardiac beta-MHC gene, ie, an alpha/beta hybrid gene and partial deletion of the gene, have also been reported. The direct correlation between gross abnormalities and development of HCM is not well understood.

METHODS AND RESULTS

We analyzed the structure of the cardiac beta-MHC gene from patients with HCM by using polymerase chain reaction-DNA conformation polymorphism analysis and found two sequence variations in exons 3 and 22 in one patient. These sequence variations at codon 54 (exon 3; nonsense mutation) and codon 870 (exon 22; Arg-to-His mutation) were identified by direct sequencing and dot-blot hybridization with allele-specific oligonucleotide probes. Relatives of this patient were examined for the mutations. It was revealed that the missense mutation was inherited from the affected father and the nonsense mutation from the unaffected grandmother through the unaffected mother. In addition, the missense mutation was also found in seven other patients from two other unrelated multiplex HCM families.

CONCLUSIONS

The Arg870His mutation was suggested to cause HCM. In contrast, the gene with the nonsense mutation would encode for a cardiac beta-MHC protein of only 53 amino acid residues, which may be too short to be incorporated into the thick filament assembly of cardiac myosin chains and showed no dominant phenotype of heart disease. This is the first report of a nonsense mutation in the human cardiac beta-MHC gene.

Imprecise excision of the Caenorhabditis elegans transposon Tc1 creates functional 5' splice sites

Imprecise excision of the Caenorhabditis elegans transposon Tc1 from a specific site of insertion within the unc-54 myosin heavy chain gene generates either wild-type or partial phenotypic revertants. Wild-type revertants and one class of partial revertants contain insertions of four nucleotides in the unc-54 third exon (Tc1 "footprints"). Such revertants express large amounts of functional unc-54 myosin despite having what would appear to be frameshifting insertions in the unc-54 third exon. We demonstrate that these Tc1 footprints act as efficient 5' splice sites for removal of the unc-54 third intron. Splicing of these new 5' splice sites to the normal third intron splice acceptor removes the Tc1 footprint from the mature mRNA and restores the normal translational reading frame. Partial revertant unc-54(r661), which contains a single nucleotide substitution relative to the wild-type gene, is spliced similarly, except that the use of its new 5' splice site creates a frameshift in the mature mRNA rather than removing one. In all of these revertants, two alternative 5' splice sites are available to remove intron 3. We determined the relative efficiency with which each alternative 5' splice site is used by stabilizing frameshifted mRNAs with smg(-) genetic backgrounds. In all cases, the upstream member of the two alternative sites is used preferentially (> 75% utilization). This may reflect an inherent preference of the splicing machinery for the upstream member of two closely spaced 5' splice sites. Creation of new 5' splice sites may be a general characteristic of Tc1 insertion and excision events.

Imprecise excision of the Caenorhabditis elegans transposon Tc1 creates functional 5' splice sites

Imprecise excision of the Caenorhabditis elegans transposon Tc1 from a specific site of insertion within the unc-54 myosin heavy chain gene generates either wild-type or partial phenotypic revertants. Wild-type revertants and one class of partial revertants contain insertions of four nucleotides in the unc-54 third exon (Tc1 "footprints"). Such revertants express large amounts of functional unc-54 myosin despite having what would appear to be frameshifting insertions in the unc-54 third exon. We demonstrate that these Tc1 footprints act as efficient 5' splice sites for removal of the unc-54 third intron. Splicing of these new 5' splice sites to the normal third intron splice acceptor removes the Tc1 footprint from the mature mRNA and restores the normal translational reading frame. Partial revertant unc-54(r661), which contains a single nucleotide substitution relative to the wild-type gene, is spliced similarly, except that the use of its new 5' splice site creates a frameshift in the mature mRNA rather than removing one. In all of these revertants, two alternative 5' splice sites are available to remove intron 3. We determined the relative efficiency with which each alternative 5' splice site is used by stabilizing frameshifted mRNAs with smg(-) genetic backgrounds. In all cases, the upstream member of the two alternative sites is used preferentially (> 75% utilization). This may reflect an inherent preference of the splicing machinery for the upstream member of two closely spaced 5' splice sites. Creation of new 5' splice sites may be a general characteristic of Tc1 insertion and excision events.

Transient posterior localization of a kinesin fusion protein reflects anteroposterior polarity of the Drosophila oocyte

BACKGROUND

During oogenesis in Drosophila, determinants that will dictate abdomen and germline formation are localized to the 'polar plasm' in the posterior of the oocyte. Assembly of the polar plasm involves the sequential localization of several messenger RNAs and proteins to the posterior of the oocyte, beginning with the localization of oskar mRNA and Staufen protein during stages 8 and 9 of oogenesis. The mechanism by which these two early components accumulate at the posterior is not known. We have investigated whether directed transport along microtubules could be used to accomplish this localization.

RESULTS

We have made a fusion protein composed of the bacterial beta-galactosidase enzyme as a reporter, joined to part of the plus-end-directed microtubule motor, kinesin, and have found that the fusion protein transiently localizes to the posterior of the oocyte during stages 8 and 9 of oogenesis. Treatment with the microtubule-depolymerizing agent colchicine prevents both the localization of the fusion protein and the posterior transport of oskar mRNA and Staufen protein. Furthermore, the fusion protein localizes normally in oocytes mutant for either oskar and staufen, but not in other mutants in which oskar mRNA and Staufen protein are mislocalized.

CONCLUSIONS

Association with a plus-end-directed microtubule motor can promote posterior localization of a reporter protein during oogenesis. The genetic requirements for this localization and its sensitivity to colchicine, both of which are shared with the posterior transport of oskar mRNA and Staufen protein, suggest that similar mechanism may function in both processes.

Trimethylpsoralen induces small deletion mutations in Caenorhabditis elegans

To examine the mutagenic spectrum of 4,5',8-trimethylpsoralen (TMP) in Caenorhabditis elegans, we isolated mutations in the unc-22 and pal-1 genes following TMP mutagenesis and analyzed them for restriction fragment length polymorphisms by Southern blot. Eleven of 21 unc-22 mutations exhibited restriction fragment length polymorphisms, 8 of which were deletions of between 0.10 and 15 kb in length. Both of two pal-1 mutations were also small deletions within this size range. Comparison of our results with previous studies on mutagenesis by gamma-rays and x-rays suggests that the mutagenic spectrum of TMP may be similar. TMP should be useful in generating mutations that cause complete loss of function of single genes and that are likely to result in allele-specific DNA polymorphisms.

The molecular basis of genetic dominance

Studies of mutagenesis in many organisms indicate that the majority (over 90%) of mutations are recessive to wild type. If recessiveness represents the 'default' state, what are the distinguishing features that make a minority of mutations give rise to dominant or semidominant characters? This review draws on the rapid expansion in knowledge of molecular and cellular biology to classify the molecular mechanisms of dominant mutation. The categories discussed include (1) reduced gene dosage, expression, or protein activity (haploinsufficiency); (2) increased gene dosage; (3) ectopic or temporally altered mRNA expression; (4) increased or constitutive protein activity; (5) dominant negative effects; (6) altered structural proteins; (7) toxic protein alterations; and (8) new protein functions. This provides a framework for understanding the basis of dominant genetic phenomena in humans and other organisms.

Prognostic implications of novel beta cardiac myosin heavy chain gene mutations that cause familial hypertrophic cardiomyopathy

Three novel beta cardiac myosin heavy chain (MHC) gene missense mutations, Phe513Cys, Gly716Arg, and Arg719Trp, which cause familial hypertrophic cardiomyopathy (FHC) are described. One mutation in exon 15 (Phe513Cys) does not alter the charge of the encoded amino acid, and affected family members have a near normal life expectancy. The Gly716Arg mutation (exon 19; charge change of +1) causes FHC in three family members, one of whom underwent transplantation for heart failure. The Arg719Trp mutation (exon 19; charge change of -1) was found in four unrelated FHC families with a high incidence of premature death and an average life expectancy in affected individuals of 38 yr. A comparable high frequency of disease-related deaths in four families with the Arg719Trp mutation suggests that this specific gene defect directly accounts for the observed malignant phenotype. Further, the significantly different life expectancies associated with the Arg719Trp vs. Phe513Cys mutation (P < 0.001) support the hypothesis that mutations which alter the charge of the encoded amino acid affect survival more significantly than those that produce a conservative amino acid change.

mRNA surveillance by the Caenorhabditis elegans smg genes

mRNAs that contain premature stop codons are unstable in most eukaryotes, but the mechanism of their degradation is largely unknown. We demonstrate that functions of the six C. elegans smg genes are necessary for rapid turnover of nonsense mutant mRNAs of the unc-54 myosin heavy chain gene. Nonsense alleles of unc-54 express mRNAs that are unstable in smg(+) genetic backgrounds but have normal or near normal stability in smg(-) backgrounds. smg mutations also stabilize mRNA of unc-54(r293), a small deletion that removes the unc-54 polyadenylation site and expresses an aberrant mRNA. Most unc-54 nonsense mutations are recessive in both smg(+) and smg(-) genetic backgrounds. However, four specific alleles are recessive when smg(+) and dominant when smg(-). These smg-dependent dominant alleles express nonsense mutant polypeptides that disrupt thick filament and/or sarcomere assembly. All four alleles are predicted to express nonsense fragment polypeptides that contain most of the myosin globular head domain without an attached rod segment. By degrading messages that contain premature stop codons, the smg genes eliminate mRNAs that encode potentially toxic protein fragments. We propose that this system of mRNA turnover protects cells from their own errors of transcription, mRNA processing, or mRNA transport.

Site-selected insertion of the transposon Tc1 into a Caenorhabditis elegans myosin light chain gene

We used the polymerase chain reaction to detect insertions of the transposon Tc1 into mlc-2, one of two Caenorhabditis elegans regulatory myosin light chain genes. Our goals were to develop a general method to identify mutations in any sequenced gene and to establish the phenotype of mlc-2 loss-of-function mutants. The sensitivity of the polymerase chain reaction allowed us to identify nematode populations containing rare Tc1 insertions into mcl-2. mlc-2::Tc1 mutants were subsequently isolated from these populations by a sib selection procedure. We isolated three mutants with Tc1 insertions within the mlc-2 third exon and a fourth strain with Tc1 inserted in nearby noncoding DNA. To demonstrate the generality of our procedure, we isolated two additional mutants with Tc1 insertions within hlh-1, the C. elegans MyoD homolog. All of these mutants are essentially wild type when homozygous. Despite the fact that certain of these mutants have Tc1 inserted within exons of the target gene, these mutations may not be true null alleles. All three of the mlc-2 mutants contain mlc-2 mRNA in which all or part of Tc1 is spliced from the pre-mRNA, leaving small in-frame insertions or deletions in the mature message. There is a remarkable plasticity in the sites used to splice Tc1 from these mlc-2 pre-mRNAs; certain splice sites used in the mutants are very different from typical eukaryotic splice sites.

Independent domains of the Sdc-3 protein control sex determination and dosage compensation in C. elegans

sdc-3 is an early-acting regulatory gene that controls both sex determination and X chromosome dosage compensation in C. elegans. It is unique among sdc genes in that its sex determination and dosage compensation functions act independently. The molecular analysis reported here demonstrates that separate domains of the Sdc-3 protein control these two developmental processes. Sequence analysis of 16 sdc-3 alleles reveals that the dosage compensation mutations specifically eliminate a pair of zinc finger motifs at the carboxyl terminus of Sdc-3, while the sex determination mutations after a region with limited homology to the ATP-binding domain of myosin. Null mutations, which disrupt both processes, abort translation of Sdc-3 prior to both domains. Analysis of site-directed changes confirms the functional significance of the two separate regions in sex determination and dosage compensation and reveals that an additional region, undetected by genetic analysis, is also required for proper dosage compensation.

Site-selected insertion of the transposon Tc1 into a Caenorhabditis elegans myosin light chain gene

We used the polymerase chain reaction to detect insertions of the transposon Tc1 into mlc-2, one of two Caenorhabditis elegans regulatory myosin light chain genes. Our goals were to develop a general method to identify mutations in any sequenced gene and to establish the phenotype of mlc-2 loss-of-function mutants. The sensitivity of the polymerase chain reaction allowed us to identify nematode populations containing rare Tc1 insertions into mcl-2. mlc-2::Tc1 mutants were subsequently isolated from these populations by a sib selection procedure. We isolated three mutants with Tc1 insertions within the mlc-2 third exon and a fourth strain with Tc1 inserted in nearby noncoding DNA. To demonstrate the generality of our procedure, we isolated two additional mutants with Tc1 insertions within hlh-1, the C. elegans MyoD homolog. All of these mutants are essentially wild type when homozygous. Despite the fact that certain of these mutants have Tc1 inserted within exons of the target gene, these mutations may not be true null alleles. All three of the mlc-2 mutants contain mlc-2 mRNA in which all or part of Tc1 is spliced from the pre-mRNA, leaving small in-frame insertions or deletions in the mature message. There is a remarkable plasticity in the sites used to splice Tc1 from these mlc-2 pre-mRNAs; certain splice sites used in the mutants are very different from typical eukaryotic splice sites.

Characteristics and prognostic implications of myosin missense mutations in familial hypertrophic cardiomyopathy

BACKGROUND

Familial hypertrophic cardiomyopathy is characterized by a variable degree of myocardial hypertrophy and a wide range of symptoms. Different mutations in the beta cardiac myosin heavy-chain gene have been identified in three affected families. However, neither the proportion of cases attributable to myosin mutations nor the effects of different mutations on clinical outcome are known.

METHODS

Using a ribonuclease protection assay, we screened the beta cardiac myosin heavy-chain genes of probands from 25 unrelated families with familial hypertrophic cardiomyopathy; this assay is a sensitive method for detecting the presence and location of mutations. We further defined the mutations by analyzing their nucleotide sequences. The clinical features of the disease were compared in families with various myosin mutations.

RESULTS

Seven mutations in the beta cardiac myosin heavy-chain gene were identified in 12 of the 25 families. All were missense mutations (i.e., causing the substitution of a single amino acid) clustered in the head and head-rod junction regions of the molecule. Six mutations resulted in a change in the charge of the amino acid. Patients with mutations that changed the charge of the altered amino acid (such as that from arginine to glutamine at nucleotide 403 or from arginine to cysteine at nucleotide 453) had a significantly shorter life expectancy (mean age at death, 33 years), whereas patients with the one mutation that did not produce a change in charge (Val606Met) had nearly normal survival. However, patients with different mutations did not differ appreciably in their clinical manifestations of familial hypertrophic cardiomyopathy.

CONCLUSIONS

Different missense mutations in the beta cardiac myosin heavy-chain gene can be identified in approximately 50 percent of families with hypertrophic cardiomyopathy. In those families, a definite genetic diagnosis can be made in all members. Since the location of a mutation or its DNA-sequence alteration (or both) appears to influence survival, we suggest that the precise definition of the disease-causing mutation can provide important prognostic information about affected members.