Modification of loop 1 affects the nucleotide binding properties of Myo1c, the adaptation motor in the inner ear

Aberrant overexpression of myosin 1b in glioblastoma promotes angiogenesis via VEGF-myc-myosin 1b-Piezo1 axis

Glioblastoma (GBM) is the most aggressive intracranial malignancy with poor prognosis. Enhanced angiogenesis is an essential hallmark of GBM, which demonstrates extensive microvascular proliferation and abnormal vasculature. Here, we uncovered the key role of myosin 1b in angiogenesis and vascular abnormality in GBM. Myosin 1b is upregulated in GBM endothelial cells (ECs) compared to the paired nonmalignant brain tissue. In our study, we found that myosin 1b promotes migration, proliferation, and angiogenesis of human/mouse brain ECs. We also found that myosin 1b expression in ECs can be regulated by vascular endothelial growth factor (VEGF) signaling through myc. Moreover, myosin 1b promotes angiogenesis via Piezo1 by enhancing Ca2+ influx, in which process VEGF can be the trigger. In conclusion, our results identified myosin 1b as a key mediator in promoting angiogenesis via mechanosensitive ion channel component 1 (Piezo1) and suggested that VEGF/myc signaling pathway could be responsible for driving the changes of myosin 1b overexpression in GBM ECs.

Dilated cardiomyopathy mutation E525K in human beta-cardiac myosin stabilizes the interacting-heads motif and super-relaxed state of myosin

The auto-inhibited, super-relaxed (SRX) state of cardiac myosin is thought to be crucial for regulating contraction, relaxation, and energy conservation in the heart. We used single ATP turnover experiments to demonstrate that a dilated cardiomyopathy (DCM) mutation (E525K) in human beta-cardiac myosin increases the fraction of myosin heads in the SRX state (with slow ATP turnover), especially in physiological ionic strength conditions. We also utilized FRET between a C-terminal GFP tag on the myosin tail and Cy3ATP bound to the active site of the motor domain to estimate the fraction of heads in the closed, interacting-heads motif (IHM); we found a strong correlation between the IHM and SRX state. Negative stain electron microscopy and 2D class averaging of the construct demonstrated that the E525K mutation increased the fraction of molecules adopting the IHM. Overall, our results demonstrate that the E525K DCM mutation may reduce muscle force and power by stabilizing the auto-inhibited SRX state. Our studies also provide direct evidence for a correlation between the SRX biochemical state and the IHM structural state in cardiac muscle myosin. Furthermore, the E525 residue may be implicated in crucial electrostatic interactions that modulate this conserved, auto-inhibited conformation of myosin.

Alpha and beta myosin isoforms and human atrial and ventricular contraction

Human atrial and ventricular contractions have distinct mechanical characteristics including speed of contraction, volume of blood delivered and the range of pressure generated. Notably, the ventricle expresses predominantly β-cardiac myosin while the atrium expresses mostly the α-isoform. In recent years exploration of the properties of pure α- & β-myosin isoforms have been possible in solution, in isolated myocytes and myofibrils. This allows us to consider the extent to which the atrial vs ventricular mechanical characteristics are defined by the myosin isoform expressed, and how the isoform properties are matched to their physiological roles. To do this we Outline the essential feature of atrial and ventricular contraction; Explore the molecular structural and functional characteristics of the two myosin isoforms; Describe the contractile behaviour of myocytes and myofibrils expressing a single myosin isoform; Finally we outline the outstanding problems in defining the differences between the atria and ventricles. This allowed us consider what features of contraction can and cannot be ascribed to the myosin isoforms present in the atria and ventricles.

Kinetic adaptation of human Myo19 for active mitochondrial transport to growing filopodia tips

Myosins are actin-based molecular motors which are enzymatically adapted for their cellular functions such as transportation and membrane tethering. Human Myo19 affects mitochondrial motility, and promotes their localization to stress-induced filopodia. Therefore, studying Myo19 enzymology is essential to understand how this motor may facilitate mitochondrial motility. Towards this goal, we have purified Myo19 motor domain (Myo19-3IQ) from a human-cell expression system and utilized transient kinetics to study the Myo19-3IQ ATPase cycle. We found that Myo19-3IQ exhibits noticeable conformational changes (isomerization steps) preceding both ATP and ADP binding, which may contribute to nucleotide binding regulation. Notably, the ADP isomerization step and subsequent ADP release contribute significantly to the rate-limiting step of the Myo19-3IQ ATPase cycle. Both the slow ADP isomerization and ADP release prolong the time Myo19-3IQ spend in the strong actin binding state and hence contribute to its relatively high duty ratio. However, the predicted duty ratio is lower than required to support motility as a monomer. Therefore, it may be that several Myo19 motors are required to propel mitochondria movement on actin filaments efficiently. Finally, we provide a model explaining how Myo19 translocation may be regulated by the local ATP/ADP ratio, coupled to the mitochondria presence in the filopodia.

Crystal structure of the rigor-like human non-muscle myosin-2 motor domain

We determined the crystal structure of the motor domain of human non-muscle myosin 2B (NM-2B) in a nucleotide-free state and at a resolution of 2.8 Å. The structure shows the motor domain with an open active site and the large cleft that divides the 50 kDa domain in a closed state. Compared to other rigor-like myosin motor domain structures, our structure shows subtle but significant conformational changes in regions important for actin binding and mechanochemical coupling. Moreover, our crystal structure helps to rationalize the impact of myosin, heavy chain 9 (MYH9)-related disease mutations Arg709Cys and Arg709His on the kinetic and functional properties of NM-2B and of the closely related non-muscle myosin 2A (NM-2A).

Crystal structure of human myosin 1c--the motor in GLUT4 exocytosis: implications for Ca2+ regulation and 14-3-3 binding

Myosin 1c (Myo1c) plays a key role in supporting motile events that underlie cell migration, vesicle trafficking, insulin-stimulated glucose uptake and hearing. Here, we present the crystal structure of the human Myo1c motor in complex with its light chain calmodulin. Our structure reveals tight interactions of the motor domain with calmodulin bound to the first IQ motif in the neck region. Several of the calmodulin residues contributing to this interaction are also involved in Ca(2+) binding. Contact residues in the motor domain are linked to the central β-sheet and the HO helix, suggesting a mechanism for communicating changes in Ca(2+) binding in the neck region to the actin and nucleotide binding regions of the motor domain. The structural context and the chemical environment of Myo1c mutations that are involved in sensorineural hearing loss in humans are described and their impact on motor function is discussed. We show that a construct consisting of the motor domain of Myo1c and the first IQ motif is sufficient to establish a tight interaction with 14-3-3β (KD=0.9 μM) and present the model of a double-headed Myo1c-14-3-3 complex. This complex has been implicated in the exocytosis of glucose transporter 4 storage vesicles during insulin-stimulated glucose uptake.

Characterization of a novel ENU-generated myosin VI mutant mouse strain with congenital deafness and vestibular dysfunction

Myosin VI (Myo6) is known to play an important role in the mammalian auditory and vestibular systems. We have identified a novel N-ethyl-N-nitrosourea mutagenised mouse strain, charlie, carrying an intronic Myo6 splice site mutation. This mutation (IVS5+5G > A) results in skipping of exon 5, and is predicted to cause a frameshift and premature termination of the protein. We detected essentially no Myo6 transcript in tissue from charlie homozygous mutant mice (Myo6(chl/chl)). Myo6(chl/chl) mice exhibit vestibular dysfunction and profound hearing impairment when first tested at four weeks of age. Analysis of vestibular and cochlear hair cells by scanning electron microscopy and immunohistochemistry revealed highly disorganised hair bundles with irregular orientation and kinocilium position at postnatal stage P2-P3. Within a few weeks, the majority of hair cell stereocilia are missing, or fused and elongated, and degeneration of the sensory epithelium occurs. This novel mouse strain will be an important resource in elucidating the role myosin VI plays in the mammalian auditory system, as well as its non-auditory functions.

Myosin IC generates power over a range of loads via a new tension-sensing mechanism

Myosin IC (myo1c), a widely expressed motor protein that links the actin cytoskeleton to cell membranes, has been associated with numerous cellular processes, including insulin-stimulated transport of GLUT4, mechanosensation in sensory hair cells, endocytosis, transcription of DNA in the nucleus, exocytosis, and membrane trafficking. The molecular role of myo1c in these processes has not been defined, so to better understand myo1c function, we utilized ensemble kinetic and single-molecule techniques to probe myo1c's biochemical and mechanical properties. Utilizing a myo1c construct containing the motor and regulatory domains, we found the force dependence of the actin-attachment lifetime to have two distinct regimes: a force-independent regime at forces < 1 pN, and a highly force-dependent regime at higher loads. In this force-dependent regime, forces that resist the working stroke increase the actin-attachment lifetime. Unexpectedly, the primary force-sensitive transition is the isomerization that follows ATP binding, not ADP release as in other slow myosins. This force-sensing behavior is unique amongst characterized myosins and clearly demonstrates mechanochemical diversity within the myosin family. Based on these results, we propose that myo1c functions as a slow transporter rather than a tension-sensitive anchor.

Myo1c mutations associated with hearing loss cause defects in the interaction with nucleotide and actin

Three heterozygous missense mutations in the motor domain of myosin 1c (Myo1c), which mediates adaptation in the inner ear, are associated with bilateral sensorineural hearing loss in humans. With transient kinetic analyses, steady-state ATPase and motility assays, and homology modeling, we studied the interaction of these mutants with nucleotide and actin using a truncated construct, Myo1c(1IQ-SAH), which includes an artificial lever arm. Results indicate that mutation R156W, near switch 1, affects the nucleotide-binding pocket and the calcium binding by disrupting switch 1 movement. Mutation V252A, in the K helix of the upper 50 kDa domain, showed reduced actin affinity consistent with disruption of communication between the actin- and nucleotide-binding sites. T380M, in a Myo1c-specific insert in the HO linker, displayed aberrant changes in most kinetic parameters and uncoupling of the ATPase from motility. These data allow for an interpretation of how these mutations might affect adaptation.