Abstract. Recent biochemical studies of p190, a calmodulin (CM)-binding protein purified from vertebrate brain, have demonstrated that this protein, purified as a complex with bound CM, shares a number of properties with myosins (Espindola, E S., E. M. Esprearico, M. V. Coelho, A. R. Martins, E R. C. Costa, M. S. Mooseker, and R. E. Larson. 1992. J. Cell Biol. 118:359-368). To determine whether or not p190 was a member of the myosin family of proteins, a set of overlapping cDNAs encoding the full-length protein sequence of chicken brain p190 was isolated and sequenced. Verification that the deduced primary structure was that of p190 was demonstrated through microsequence analysis of a cyanogen bromide peptide generated from chick brain p190. The deduced primary structure of chicken brain p190 revealed that this 1,830-amino acid (aa) 212,509-D) protein is a member of a novel structural class of unconventional myosins that includes the gene products encoded by the dilute locus of mouse and the MYO2 gene of Saccharomyces cerevisiae. We have named the pl90-CM complex "myosin-V" based on the results of a detailed sequence comparison of the head domains of 29 myosin heavy chains (hc), which has revealed that this myosin, based on head structure, is the fifth of six distinct structural classes of myosin to be described thus far. Like the presumed products of the mouse dilute and yeast MYO2 genes, the head domain of chicken myosin-V hc (aa 1-764) is linked to a "neck" domain (aa 765-909) consisting of six tandem repeats of an ~23-aa "IQ-motify All known myosins contain at least one such motif at their head-tail junctions; these IQ-motifs may function as calmodulin or light chain binding sites. The tail domain of chicken myosin-V consists of an initial 511 aa predicted to form several segments of coiled-coil t~ helix followed by a terminal 410-aa globular domain (aa, 1,421-1,830). Interestingly, a portion of the tail domain (aa, 1,094-1,830) shares 58 % amino acid sequence identity with a 723-aa protein from mouse brain reported to be a glutamic acid decarboxylase. The neck region of chicken myosin-V, which contains the IQ-motifs, was demonstrated to contain the binding sites for CM by analyzing CM binding to bacterially expressed fusion proteins containing the head, neck, and tail domains. Immunolocalization of myosin-V in brain and in cultured cells revealed an unusual distribution for this myosin in both neurons and nonneuronal cells. Myosin-V staining was particularly intense in the cell bodies and dendrites of Purkinje cells. Double labeling with wheat germ agglutinin revealed colocalization of myosin-V with cytoplasmic, presumably Golgi-derived, membranes. In primary cultures of neurons and glia, myosin-V immunoreactivity had a punctate distribution more abundant in the region of the Golgi complex and at the tips of long processes such as growth cones. These results, together with the phenotypes of mutations described for the dilute and myo2 genes, suggest that the myosin-V family of unconventional myosins may be ...

Although the conventional myosins (myosins-II) responsible for processes such as muscle contraction and cytokinesis have been intensively studied, relatively little is known about the properties of the unconventional myosins. One widely distributed group of unconventional myosins, the class V myosins, have been suggested to play a role in organelle transport or membrane targeting (reviewed in Refs. 1 and 2). Brain myosin-V (BM-V 1 ), which was originally identified as a calmodulin (CaM)-binding protein in vertebrate brain, is the only member of this class of unconventional myosins to be purified and characterized biochemically (3-7). The class V myosins share a common domain structure consisting of an N-terminal head domain containing the actin-binding and ATP hydrolysis sites, an extended neck domain containing six IQ motifs (which form binding sites for CaM and/or related light chains), and a tail domain consisting of a region predicted to form coiled coils attached to a globular region of unknown function (7,8). The hypothetical functions of the class V myosins and their novel domain structure, particularly the extended neck domain, raise the obvious question of how the basic biochemical properties of the class V myosins compare with those of other types of myosins.Among the critically important properties of a molecular motor are its steady-state ATPase activity and the factors which regulate this activity. Known myosins are regulated either indirectly via actin-associated proteins such as troponin/ tropomyosin (as in vertebrate skeletal muscle myosin) or directly via the subunits of the myosin molecule itself. In the latter type of direct "myosin-linked" regulation, the light chains associated with the neck domain often function as the regulating subunits. The myosin light chains are all members of the CaM superfamily of proteins, although not all of them have retained the ability to bind to Ca 2ϩ . Vertebrate non-muscle and smooth muscle myosins-II are "turned on" by phosphorylation of their regulatory light chains by Ca 2ϩ /CaM-dependent myosin light chain kinase (9), whereas molluscan myosin-II and vertebrate brush border (BB) myosin-I are regulated by the direct binding of Ca 2ϩ to their light chains (10, 11). Like BB myosin-I, BM-V has multiple CaM light chains that remain bound in the absence of Ca 2ϩ (3,5,7). The neck domain of BM-V has been shown to be the precise site of CaM binding (8), suggesting that this myosin might also be directly regulated by Ca 2ϩ binding. The initial reports of BM-V ATPase activity also indicated that this protein is activated by Ca 2ϩ (5, 7), although the K ATPase for Ca 2ϩ and F-actin were not determined. The tail domains of myosins are unique to each class in the myosin superfamily and probably reflect the specific functions

Eukaryotic organisms rely on intracellular transport to position organelles and other components within their cells. Pigment cells provide an excellent model to study organelle transport as they specialize in the translocation of pigment granules in response to defined chemical signals. Pigment cells of lower vertebrates have traditionally been used as a model for these studies because these cells transport pigment organelles in a highly coordinated fashion, are easily cultured and transfected, are ideal for microsurgery, and are good for biochemical experiments, including in vitro analysis of organelle motility. Many important properties of organelle transport, for example, the requirement of two cytoskeletal filaments (actin and microtubules), the motor proteins involved, and the mechanisms of their regulation and interactions, have been studied using pigment cells of lower vertebrates. Genetic studies of mouse melanocytes allowed the discovery of essential elements involved in organelle transport including the myosin-Va motor and its receptor and adaptor molecules on the organelle surface. Future studies of pigment cells will contribute to our understanding of issues such as the cooperation among multiple motor proteins and the mechanisms of regulation of microtubule motors.

The discovery that the dilute gene encodes a class V myosin led to the hypothesis that this molecular motor is involved in melanosome transport and/or dendrite outgrowth in mammalian melanocytes. The present studies were undertaken to gain insight into the subcellular distribution of myosin-V in the melanoma cell line B16-F10, which is wildtype for the dilute gene. Immunofluorescence studies showed some degree of superimposed labeling of myosin-V with melanosomes that predominated at the cell periphery. A subcellular fraction highly enriched in melanosomes was also enriched in myosin-V based on Western blot analysis. Immunoelectron microscopy showed myosin-V labeling associated with melanosomes and other organelles. The stimulation of B16 cells with the ␣-melanocyte-stimulating hormone led to a significant increase in myosin-V expression. This is the first evidence that a cAMP signaling pathway might regulate the dilute gene expression. Immunofluorescence also showed an intense labeling of myosin-V independent of melanosomes that was observed within the dendrites and at the perinuclear region. Although the results presented herein are consistent with the hypothesis that myosin-V might act as a motor for melanosome translocation, they also suggest a broader cytoplasmic function for myosin-V, acting on other types of organelles or in cytoskeletal dynamics.

Regulation of intracellular transport plays a role in a number of processes, including mitosis, determination of cell polarity, and neuronal growth. In Xenopus melanophores, transport of melanosomes toward the cell center is triggered by melatonin, whereas their dispersion throughout the cytoplasm is triggered by melanocyte-stimulating hormone (MSH), with both of these processes mediated by cAMP-dependent protein kinase A (PKA) activity [1, 2]. Recently, the ERK (extracellular signal-regulated kinase) pathway has been implicated in regulating organelle transport and signaling downstream of melatonin receptor [3, 4]. Here, we directly demonstrate that melanosome transport is regulated by ERK signaling. Inhibition of ERK signaling by the MEK (MAPK/ERK kinase) inhibitor U0126 blocks bidirectional melanosome transport along microtubules, and stimulation of ERK by constitutively active MEK1/2 stimulates transport. These effects are specific because perturbation of ERK signaling has no effect on the movement of lysosomes, organelles related to melanosomes [5]. Biochemical analysis demonstrates that MEK and ERK are present on melanosomes and transiently activated by melatonin. Furthermore, this activation correlates with an increase in melanosome transport. Finally, direct inhibition of PKA transiently activates ERK, demonstrating that ERK acts downstream of PKA. We propose that signaling of organelle bound ERK is a key pathway that regulates bidirectional, microtubule-based melanosome transport.