Tama Hasson

Effects of shaker-1 mutations on myosin-VIIa protein and mRNA expression

Numerous mammalian diseases have been found to be due to mutations in components of the actin cytoskeleton. Recently, mutations in the gene for an unconventional myosin, myosin-VIIa, were found to be the basis for the deafness and vestibular dysfunction observed in shaker-1 (sh1) mice and for a human deafness-blindness syndrome, Usher syndrome type 1B. Seven alleles of sh1 mice were analyzed to assess the affects of different myosin-VIIa mutations on both gene expression and tissue function. Myosin-VIIa is expressed in the inner ear and the retina, as well as the kidney, lung, and testis. Northern blot analysis indicated that myosin-VIIa mRNA expression, size, and stability were unaffected in the seven sh1 alleles. Immunoblot analysis showed that all seven alleles expressed some full-length myosin-VIIa protein. The range of expression, however, ran from sh1 [original], which expressed wild-type levels of protein, to two strains, sh1(4494SB) and sh1(4626SB), which expressed less than 1% of the normal level of myosin-VIIa protein. For the three alleles of sh1 that have been characterized and that have mutations in the motor domain, sh1 [original], sh1(816SB) and sh1(6J), the level of protein expression observed in these sh1 alleles correlated well with the predicted effects of the mutations on motor function. No change in retinal or testicular structure was observed at the light microscopic level during the life span of the seven sh1 alleles. Myosin-VIIa protein, when detectable, was observed to locate properly in the sh1 mice. On the basis of these results, we propose that the mutations in myosin-VIIa in the sh1 alleles leads to both motor dysfunction and to a protein destabilization phenotype.

Molecular motors, membrane movements and physiology: emerging roles for myosins.

Myosins are a large family of structurally diverse mechanoenzymes which, upon interaction with actin filaments, convert energy from ATP hydrolysis into mechanical force. Consistent with the ubiquitous association of actin with membranes, many of these novel myosins are membrane-associated. In the past two years, evidence has emerged that suggests roles for actin-based molecular motors in a wide range of membrane phenomena such as cell locomotion, phagocytosis, secretion, organelle transport, signal transduction and mechanoregulation of membrane protein function.

Vertebrate Unconventional Myosins

Myosinsarealargefamilyofstructurallydiversemechanoen-zymesthatbindtoF-actinandhydrolyzeATPtoproducemechan-icalforce.Phylogeneticanalysisofthemyosinmotordomainshasidentified 11 distinct classes of myosin, seven of which are ex-pressedinvertebrates(forreviewseeRef.1).Thesesevenmyosinclassesincludeconventionalmyosin,ormyosin-II,andsixmorerecentlyidentifiedunconventionalmyosinclasses,myosins-I,-V,-VI,-VII,-IX,and-X(seeFig.1).AttheNterminus,eachverte-brateunconventionalmyosincontainsaconservedmotordomainthatincludesboththeATPbindingandATP-sensitiveactinbind-ingsites(forreviewofthestructureofthemyosinmotordomainsee Ref. 2). Following the motor is a light chain (LC)

The growing family of myosin motors and their role in neurons and sensory cells

Biochemical and physiological evidence has suggested that myosins, both conventional and unconventional, are critical for neurosensory activities. In the past few years, this premise has been supported by genetic evidence that has shown that unconventional myosins are essential for the proper functioning of neurons, retina and the sensory cells of the inner ear.

Unconventional Myosins, the Basis for Deafness in Mouse and Man

Myosins are molecular motors that use the energy from ATP hydrolysis to generate force and move along actin filaments. Conventional myosin, or myosin-II, has the specialized ability to form bipolar filaments and is the basis for muscle contraction. Mutations in conventional myosins have been observed in man; dominant cardiomyopathies arise from mutations in P-cardiac myosin-II and other myosin-associated structural proteins (reviewed in Vikstrom and Leinwand 1996). Unlike myosin-II, unconventional myosins do not form bipolar filaments. Instead, these novel myosins serve in intracellular movements along actin filaments, primarily within nonmuscle cells. What the specific roles may be, however, has remained elusive. As described by Adato et al. (1997) in this issue of the Journal, as well as by other groups (Weil et al. 1995; Weston et al. 1996; Levy et al. 1997), mutations in one unconventional myosin, myosin-VIIa, lead to Usher disease, a syndromic recessive deafness. Mutations in myosin-VIIa can also lead to nonsyndromic recessive deafness in humans (Liu et al. 1997b; Weil et al. 1997) and result in deafness in the shaker-1 mutant mouse (Gibson et al. 1995). MyosinVIIa is not the only unconventional myosin that is the basis for deafness phenotypes. The gene for mouse myosin-VI encodes the deafness gene, Snells waltzer (Avraham et al. 1995).