Myosin VI Undergoes Cargo-Mediated Dimerization

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104

Although myosin VI has properties that would allow it to function optimally as a dimer, full-length myosin VI exists as a monomer in isolation. Based on the ability of myosin VI monomers to dimerize when held in close proximity, we postulated that cargo binding normally regulates dimerization of myosin VI. We tested this hypothesis by expressing a known dimeric cargo adaptor protein of myosin VI, optineurin, and the myosin VI-binding segment from a monomeric cargo adaptor protein, Dab2. In the presence of these adaptor proteins, full-length myosin VI has ATPase properties of a dimer, appears as a dimer in electron micrographs, and moves processively on actin filaments. The results support a model in which cargo binding exposes internal dimerization sequences within full-length myosin VI. Because, unexpectedly, a monomeric fragment of Dab2 triggers dimerization, it would appear that myosin VI is designed to function as a dimer in cells.Dab2 ͉ directionality ͉ motility ͉ optineurin ͉ unconventional myosin W ithin the myosin superfamily there are at least 35 classes of molecular motors that move along actin filaments (1). Myosin VI is the only class of myosin known to move toward the minus-end of actin filaments (2, 3). Myosin VI dimers take large and variable steps on actin (average of 30-36 nm) using a short lever arm and a unique lever arm extension (4-6). Not only is the myosin VI dimer capable of processive movement (i.e., can move as a single molecule) along an actin filament (4-6), it also functions as a load-dependent actin anchoring protein (7). Thus myosin VI can potentially fulfill a number of specialized cell biological functions (8-10). Paradoxically, although these functional features suggest that myosin VI is designed to work in cells as a dimer, myosin VI as isolated from cells is a monomer, and expressed full-length myosin VI is also monomeric (4,5,11).A number of cargo adaptor proteins that recruit myosin VI have been identified (8). For example, it has been demonstrated that optineurin is essential for myosin VI localization to the Golgi complex (12), and binds to a site within the globular tail of myosin VI. Dab2 (13,14) and Sap97 (15) mediate the recruitment of myosin VI to clathrin-coated pits and vesicles, whereas GipC serves this role on uncoated vesicles (16,17).The myosin VI molecule has discrete structural domains, as diagrammed in Fig. 1, using the terminology of Spink et al. (18). We have demonstrated that internal dimerization (probably coiled coil) occurs between residues 913 and 936 (6). However, we noted that this dimerization is weak and forms only if the molecules are held in close proximity (5, 6). We postulated that in vivo dimerization is initiated upon binding of myosin VI to a dimeric cargo, which would then trigger the weak internal dimerization (5, 6).Subsequently, it was shown that a headless myosin VI construct containing the entire tail and cargo-binding domains dimerized with relatively high affinity (18). Thus there may be two separate regions of the myosin VI molecule...

Section: Discussionsupporting

confidence: 77%

Section: Discussionmentioning

confidence: 66%

Section: Discussionmentioning

confidence: 99%

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Cargo binding induces dimerization of myosin VI

Phichith

Travaglia²,

Yang³

et al. 2009

105

104

“…The tail domain of myosin VI binds to the 39-residue fragment of the vesicle adaptor Dab2, which induces dimerization of the tail/Dab2 peptide complex (40). In the present study, it is suggested that, unlike myosin VI/ Dab2 binding, the binding of the targeting molecule, MyRip, does not directly promote dimer formation.…”

Section: Discussionmentioning

confidence: 57%

Cargo binding activates myosin VIIA motor function in cells

Sakai

Umeki

Ikebe

et al. 2011

Myosin VIIA, thought to be involved in human auditory function, is a gene responsible for human Usher syndrome type 1B, which causes hearing and visual loss. Recent studies have suggested that it can move processively if it forms a dimer. Nevertheless, it exists as a monomer in vitro, unlike the well-known two-headed processive myosin Va. Here we studied the molecular mechanism, which is currently unknown, of activating myosin VIIA as a cargo-transporting motor. Human myosin VIIA was present throughout cytosol, but it moved to the tip of filopodia upon the formation of dimer induced by dimer-inducing reagent. The forced dimer of myosin VIIA translocated its cargo molecule, MyRip, to the tip of filopodia, whereas myosin VIIA without the forced dimer-forming module does not translocate to the filopodial tips. These results suggest that dimer formation of myosin VIIA is important for its cargo-transporting activity. On the other hand, myosin VIIA without the forced dimerization module became translocated to the filopodial tips in the presence of cargo complex, i.e., MyRip/Rab27a, and transported its cargo complex to the tip. Coexpression of MyRip promoted the association of myosin VIIA to vesicles and the dimer formation. These results suggest that association of myosin VIIA monomers with membrane via the MyRip/Rab27a complex facilitates the cargo-transporting activity of myosin VIIA, which is achieved by cluster formation on the membrane, where it possibly forms a dimer. Present findings support that MyRip, a cargo molecule, functions as an activator of myosin VIIA transporter function.

“…However, the parallel coiled-coil dimerization model has been directly demonstrated only in myosin II and V (7,8). Instead of coiled-coil-mediated dimerization, certain unconventional myosins have been shown to undergo cargo bindingmediated dimerization (9,10). It is possible that the coiled-coil region interactions and cargo binding function synergistically in modulating the dimerization of some myosins.…”

mentioning

confidence: 99%

Antiparallel coiled-coil–mediated dimerization of myosin X

Lü¹,

Ye²,

Wei³

et al. 2012

Self Cite

Processive movements of unconventional myosins on actin filaments generally require motor dimerization. A commonly accepted myosin dimerization mechanism is via formation of a parallel coiled-coil dimer by a stretch of amino acid residues immediately carboxyl-terminal to the motor's lever-arm domain. Here, we discover that the predicted coiled-coil region of myosin X forms a highly stable, antiparallel coiled-coil dimer (anti-CC). Disruption of the anti-CC either by single-point mutations or by replacement of the anti-CC with a parallel coiled coil with a similar length compromised the filopodial induction activity of myosin X. We further show that the anti-CC and the single α-helical domain of myosin X are connected by a semirigid helical linker. The anti-CC-mediated dimerization may enable myosin X to walk on both single and bundled actin filaments.Myo10 | molecular motor | filopodial growth