FUNCTION: [Summary is not available for the mouse gene. This summary is for the human ortholog.] This gene encodes an unconventional myosin. This protein differs from other myosins in that it has a long N-terminal extension preceding the conserved motor domain. Studies in mice suggest that this protein is necessary for actin organization in the hair cells of the cochlea. Mutations in this gene have been associated with profound, congenital, neurosensory, nonsyndromal deafness. This gene is located within the Smith-Magenis syndrome region on chromosome 17. Read-through transcripts containing an upstream gene and this gene have been identified, but they are not thought to encode a fusion protein. Several alternatively spliced transcript variants have been described, but their full length sequences have not been determined. [provided by RefSeq, Jul 2008] PHENOTYPE: Mutations in this gene result in profound deafness and neurological behavior. [provided by MGI curators]
Figure 1. The typhoon mice exhibit ataxia and head tilt.
The typhoon phenotype was identified among N-ethyl-N-nitrosourea (ENU)-mutagenized G3 mice of the pedigree R5051, some of which showed ataxia and a head tilt (Figure 1). Some mice also ran in circles.
Nature of Mutation
Figure 2.Linkage mapping of the behavioral phenotypes using a recessive model of inheritance. Manhattan plot shows -log10 P values (Y-axis) plotted against the chromosome positions of 54 mutations (X-axis) identified in the G1 male of pedigree R5051. Phenotype data are shown for single locus linkage analysis without consideration of G2 dam identity. Horizontal pink and red lines represent thresholds of P = 0.05, and the threshold for P = 0.05 after applying Bonferroni correction, respectively.
Whole exome HiSeq sequencing of the G1 grandsire identified 54 mutations. The behavioral phenotypes were linked to two genes: Nlrp3 and Myo15. However, the mutation in Myo15 was presumed to be causative because the typhoon behavior phenotypes mimic other known alleles of Myo15 (see MGI for a list of Myo15 alleles as well as the entry for parker). The mutation in Myo15 a G to A transition at base pair 60,487,425(v38) on chromosome 11, or base pair 113,685 in the GenBank genomic region NC_000077 encoding Myo15; the mutation is within the donor splice site of intron 10. Linkage was found with a recessive model of inheritance (P = 1.981 x 10-5), wherein four variant homozygotes departed phenotypically from 20 homozygous reference mice and 18 heterozygous mice with (Figure 2). A substantial semidominant effect was also observed (P = 4.274 x 10-5).
The effect of the mutation at the cDNA and protein level have not examined, but the mutation is predicted to result in the use of a cryptic site in intron 10, resulting in a transcript that has an 8-base pair insertion on intron 10. This predicts a frame shifted protein product beginning after amino acid 1,386 of the protein, which is normally 3,511 amino acids in length, and terminating after the inclusion of 23 aberrant amino acids.
The donor splice site of intron 10, which is destroyed by the typhoon mutation, is indicated in blue lettering and the mutated nucleotide is indicated in red.
Figure 3. Domain structure of myosin XV. At its N-terminus, myosin XV has an N-terminal extension (NTE); isoform 3 does not have the NTE. Within the motor domain, myosin XV has an ATP- and actin-binding site. Two IQ motifs are located within the neck of myosin XV. The C-terminal tail contains two myosin tail homology 4 (MyTH4) domains, a Src homology 3 (SH3) domain, and a 4.1/ezrin/radixin/moesin (FERM) domain. A FERM-like domain is predicted based on sequence similarity to one in human myosin VIIa, however, the precise location in mouse myosin XV has not been documented. At the C-terminus, myosin XV has a PDZ-ligand motif (ITLL*). The typhoon mutation (red asterisk) is located in the donor splice site of intron 10. See the text for more details.
Myo15 encodes myosin XV (alternatively, Myosin XVa), a 3,511 amino acid member of the unconventional myosin family. Myosin XV has a proline-rich N-terminal extension that does not have sequence similarity to reported proteins and the function is unknown [Figure 3; amino acids 1-1200; SMART; (1-3)]. Myosin XV has a highly conserved motor domain (amino acids 1200-1884; NM_010862; SMART) following the N-terminal extension (2). The motor domain contains an adenosine triphosphate (ATP)- and an actin-binding site (amino acids 1299-1306 and 1776-1783, respectively; Uniprot) (4-6). The myosin neck region contains a variable number of light-chain binding (IQ) motifs (IQxxxRGxxxRK) and is linked to the motor domain by a converter region [(7); reviewed in (8;9)]. Myosin XV has two IQ motifs (amino acids 1909-1920; LQRCLRGFFIKR and amino acids 1932-1943; LQSRARGYLARQ) (5;8;9). The IQ motif is an α-helical structure that often mediates the binding of myosins to calmodulin, members of the EF-hand family of calcium-binding proteins, or myosin light chains [reviewed in (8;9)]. The myosin XV tail region is 1584 amino acids in length and has two myosin tail homology 4 (MyTH4) domains (amino acids 2049-2195 and 3031-3185; SMART), a band 4.1/ezrin/radixin/moesin (FERM)-like domain (amino acids 2687-2867, human myosin XVa), a Src homology 3 (SH3) domain (amino acids 2851-2933), and a FERM domain (alternatively, talin-like domain; amino acids 3188-3400) (1;2;10;11). The MyTH4 domain is proposed to function in microtubule binding as well as in actin binding to the plasma membrane (12). The FERM domain of myosin XV is proposed to be involved in anchoring myosin XV to the cell membrane (1). The function of the myosin XV SH3 domain is unknown, but it is proposed to mediate an intramolecular interaction with a region in the proline-rich N-terminal extension to regulate the activity of myosin XV (2). Myosin XV is unique among the myosins in that it has a predicted class I PDZ-ligand motif (ITLL*) at the C-terminus (11;13;14). The PDZ-ligand motif of myosin XV is required for association of myosin XV with whirlin as well as the localization of whirlin to stereocilia tips (13;15).
The typhoon mutation is predicted to result in a frame-shift and coding of a premature stop codon within exon 10. Premature truncation of myosin XV within exon 10 would result in loss in a portion of the motor domain and the domains following the motor domain.
Myosin XV functions in the assembly and maintenance of actin organization in hair cells of the inner ear by acting as a motor and carrier along the length of the actin filament within the hair cells (16). Myosin XV senses the tension between the plasma membrane and the actin filaments, a function that is necessary in the growth of the steocilia (11). Myosin XV contributes to the elongation of stereocilia by delivering whirlin, a multi-PDZ domain-containing scaffold protein, to the stereocilia tips (13). Both myosin XV and whirlin are required for the elongation and staircase formation of the stereocilia bundle and myosin XV and whirlin may function as part of a complex that modulates the growth of actin bundles in the stereocilia (11;13). Another proposed function of myosin XV is the maintenance of the hair cell mechanotransduction apparatus at the tips of the stereocilia (4;13). Two myosin XV mutant mouse models, shaker 2 (Myo15sh2; MGI:1857036) and shaker 2J (Myo15sh2J; MGI:1889795) have been characterized. Both the shaker 2 and shaker 2J mice have abnormally short stereocilia bundles on the inner ear hair cells compared to wild-type mice; the bundles are correctly positioned (1;4). As a result, the shaker 2 and shaker 2J homozygous mice are congenitally deaf and exhibit vestibular defects that cause head-tossing and circling behavior (2-4). Similar to the shaker 2J mice, the typhoon mice exhibit head-tossing and circling behavior. Anderson et al. propose that the myosin XVshaker 2J protein is unable to exert force on the actin cytoskeleton because the truncated protein is improperly anchored, resulting in a failure to form the scaffolding to form the normal stereocilia structure (1).