Bidirectional pigment granule migration in isolated retinal pigment epithelial cells requires actin but not microtubules

Comparative transcriptome analysis reveals growth and molecular pathway of body color regulation in turbot (Scophthalmus maximus) exposed to different light spectrum

Fish body color changes play vital roles in adapting to ecological light environment and influencing market value. However, the initial mechanisms governing the changes remain unknown. Here, we scrutinized the impact of light spectrum on turbot (Scophthalmus maximus) body coloration, exposing them to red, blue, and full light spectra from embryo to 90 days post hatch. Transcriptome and quantitative real-time PCR (qRT-PCR) analyses were employed to elucidate underlying biological processes. The results showed that red light induced dimorphism in turbot juvenile skin pigmentation: some exhibited black coloration (Red_Black_Surface, R_B_S), while others displayed lighter skin (Red_White_Bottom, R_W_B), with red light leading to reduced skin lightness (L*) and body weight, particularly in R_B_S group. Transcriptomic and qRT-PCR analyses showcased upregulated gene expressions related to melanin synthesis in R_B_S individuals, notably tyrosinase (tyr), tyrosinase-related protein 1 (tyrp1), and dopachrome tautomerase (dct), alongside solute carrier family 24 member 5 (slc24a5) and oculocutaneous albinism type II (oca2) as pivotal regulators. Nervous system emerged as a critical mediator in spectral environment-driven color regulation. N-methyl d-aspartate (NMDA) glutamate receptor, and calcium signaling pathway emerged as pivotal links intertwining spectral conditions, neural signal transduction, and color regulation. The individual differences in NMDA glutamate receptor expression and subsequent neural excitability seemed responsible for dichromatic body coloration in red light-expose juveniles. This study provides new insights into the comprehending of fish adaptation to environment and methods for fish body color regulation and could potentially help enhance the economic benefit of fish farming industry.

Melanosome Motility in Fish Retinal Pigment Epithelial (RPE) Cells

Several model systems have been developed to investigate mechanisms and regulation of intracellular organelle motility. The fish retinal pigment epithelial (RPE) cell represents an unusual but simple system for the study of actin-dependent organelle motility. Primary cultures of RPE dissociated from the eye are amenable to motility studies using a simple perfusion chamber and conventional phase contrast microscopy. In vivo, melanin-containing pigment granules (melanosomes) within fish RPE migrate distances up to 100 μm in response to light flux. When sheets of RPE are removed from the eye and dissociated, they attach to the substrate with apical projections extending radially from the central cell body. Melanosomes can be chemically triggered to aggregate or disperse throughout the projections. Melanosome migration in RPE apical projections is dependent on actin filaments and thus renders this model system useful for investigations of actin-dependent organelle motility.

Protein kinase A regulation of pigment granule motility in retinal pigment epithelial cells from fish, Lepomis spp

Retinomotor movements include elongation and contraction of rod and cone photoreceptors, and mass migration of melanin-containing pigment granules (melanosomes) of the retinal pigment epithelium (RPE) within the eyes of fish, frogs, and other lower vertebrates. Eyes of these animals do not contain dilatable pupils; therefore the repositioning of the rods and cones and a moveable curtain of pigment granules serve to modulate light intensity within the eye. RPE from sunfish (Lepomis spp.) can be isolated from the eye and dissociated into single cells, allowing in vitro studies of the cytoskeletal and regulatory mechanisms of organelle movement. Pigment granule aggregation from distal tips of apical projections into the cell body can be triggered by the application of underivatized cAMP, and dispersion is effected by cAMP washout in the presence of dopamine. While the phenomenon of cAMP-dependent pigment granule aggregation in isolated RPE was described many years ago, whether cAMP acts through the canonical cAMP-PKA pathway to stimulate motility has never been demonstrated. Here, we show that pharmacological inhibition of PKA blocks pigment granule aggregation, and microinjection of protein kinase A catalytic subunit triggers pigment granule aggregation. Treatment with a cAMP agonist that activates the Rap GEF, Epac (Effector protein activated by cAMP), had no effect on pigment granule position. Taken together, these results confirm that cAMP activates RPE pigment granule motility by the canonical cAMP-PKA pathway. Isolated RPE cells labeled with antibodies against PKA RIIα and against PKA-phosphorylated serine/threonine amino acids show diffuse, punctate labeling throughout the RPE cell body and apical projections. Immunoblotting of RPE lysates using the anti-PKA substrate antibody demonstrated seven prominent bands; two bands in particular at 27 and 64 kD showed increased levels of phosphorylation in the presence of cAMP, indicating their phosphorylation could contribute to the pigment granule aggregation mechanism.

The cell biology of the retinal pigment epithelium

The retinal pigment epithelium (RPE), a monolayer of post-mitotic polarized epithelial cells, strategically situated between the photoreceptors and the choroid, is the primary caretaker of photoreceptor health and function. Dysfunction of the RPE underlies many inherited and acquired diseases that cause permanent blindness. Decades of research have yielded valuable insight into the cell biology of the RPE. In recent years, new technologies such as live-cell imaging have resulted in major advancement in our understanding of areas such as the daily phagocytosis and clearance of photoreceptor outer segment tips, autophagy, endolysosome function, and the metabolic interplay between the RPE and photoreceptors. In this review, we aim to integrate these studies with an emphasis on appropriate models and techniques to investigate RPE cell biology and metabolism, and discuss how RPE cell biology informs our understanding of retinal disease.

Melanosome Motility in Fish Retinal Pigment Epithelial (RPE) Cells

Several model systems have been developed to investigate mechanism and regulation of intracellular organelle motility. The fish retinal pigment epithelial (RPE) cell represents a novel yet simple system for the study of organelle motility. Primary cultures of dissociated RPE cells are easily prepared and amenable to motility studies. In vivo, melanin-containing pigment granules (melanosomes) within fish RPE migrate distances up to 100 μm in response to light flux. When dissociated from the epithelial layer and cultured in vitro, RPE cells attach to the substrate with the apical projections extending radially from the central cell body. Melanosomes can be chemically triggered to aggregate or disperse throughout the projections, and are easily observed using phase contrast microscopy. Melanosome migration in RPE apical projections is dependent on actin filaments, and thus renders this model system useful for investigations of actin-dependent organelle motility.

Orientation of actin filaments in teleost retinal pigment epithelial cells, and the effect of the lectin, Concanavalin A, on melanosome motility

Retinal pigment epithelial cells of teleosts contain numerous melanosomes (pigment granules) that exhibit light-dependent motility. In light, melanosomes disperse out of the retinal pigment epithelium (RPE) cell body (CB) into long apical projections that interdigitate with rod photoreceptors, thus shielding the photoreceptors from bleaching. In darkness, melanosomes aggregate through the apical projections back into the CB. Previous research has demonstrated that melanosome motility in the RPE CB requires microtubules, but in the RPE apical projections, actin filaments are necessary and sufficient for motility. We used myosin S1 labeling and platinum replica shadowing of dissociated RPE cells to determine actin filament polarity in apical projections. Actin filament bundles within RPE apical projections are uniformly oriented with barbed ends toward the distal tips. Treatment of RPE cells with the tetravalent lectin, Concanavalin A, which has been shown to suppress cortical actin flow by crosslinking of cell-surface proteins, inhibited melanosome aggregation and stimulated ectopic filopodia formation but did not block melanosome dispersion. The polarity orientation of F-actin in apical projections suggests that a barbed-end directed myosin motor could effect dispersion of melanosomes from the CB into apical projections. Inhibition of aggregation, but not dispersion, by ConA confirms that different actin-dependent mechanisms control these two processes and suggests that melanosome aggregation is sensitive to treatments previously shown to disrupt actin cortical flow.

Identification and localization of myosin superfamily members in fish retina and retinal pigmented epithelium

Myosins are cytoskeletal motors critical for generating the forces necessary for establishing cell structure and mediating actin-dependent cell motility. In each cell type a multitude of myosins are expressed, each myosin contributing to aspects of morphogenesis, transport, or motility occurring in that cell type. To examine the roles of myosins in individual retinal cell types, we first used polymerase chain reaction (PCR) screening to identify myosins expressed in retina and retinal pigmented epithelium (RPE), followed by immunohistochemistry to examine the cellular and subcellular localizations of seven of these expressed myosins. In the myosin PCR screen of cDNA from striped bass retina and striped bass RPE, we amplified 17 distinct myosins from eight myosin classes from retinal cDNA and 11 distinct myosins from seven myosin classes from RPE cDNA. By using antibodies specific for myosins IIA, IIB, IIIA, IIIB, VI, VIIA, and IXB, we examined the localization patterns of these myosins in retinas and RPE of fish, and in isolated inner/outer segment fragments of green sunfish photoreceptors. Each of the myosins exhibited unique expression patterns in fish retina. Individual cell types expressed multiple myosin family members, some of which colocalized within a particular cell type. Because much is known about the functions and properties of these myosins from studies in other systems, their cellular and subcellular localization patterns in the retina help us understand which roles they might play in the vertebrate retina and RPE.

Melanosome motility in fish retinal pigment epithelial cells

Several model systems have been developed to investigate intracellular organelle motility. A relatively novel system that is simple but useful for studying mechanisms of organelle motility is the fish retinal pigment epithelial (RPE) cell. Primary cultures of dissociated RPE cells are easily prepared and amenable to organelle motility studies. In vivo, melanin-containing pigment granules (melanosomes) within the RPE migrate long distances in response to light flux. When cultured in vitro, RPE cells attach to the substrate with the apical projections extending radially from the central cell body. Melanosomes can be chemically triggered to aggregate or disperse throughout the projections, and can be easily observed using phase contrast microscopy. Melanosome migration in RPE apical projections is dependent on actin filaments, and thus renders this model system useful for investigations of actin-dependent organelle motility.

The ternary Rab27a-Myrip-Myosin VIIa complex regulates melanosome motility in the retinal pigment epithelium

The retinal pigment epithelium (RPE) contains melanosomes similar to those found in the skin melanocytes, which undergo dramatic light-dependent movements in fish and amphibians. In mammals, those movements are more subtle and appear to be regulated by the Rab27a GTPase and the unconventional myosin, Myosin VIIa (MyoVIIa). Here we address the hypothesis that a recently identified Rab27a- and MyoVIIa-interacting protein, Myrip, promotes the formation of a functional tripartite complex. In heterologous cultured cells, all three proteins co-immunoprecipitated following overexpression. Rab27a and Myrip localize to the peripheral membrane of RPE melanosomes as observed by immunofluorescence and immunoelectron microscopy. Melanosome dynamics were studied using live-cell imaging of mouse RPE primary cultures. Wild-type RPE melanosomes exhibited either stationary or slow movement interrupted by bursts of fast movement, with a peripheral directionality trend. Nocodazole treatment led to melanosome paralysis, suggesting that movement requires microtubule motors. Significant and similar alterations in melanosome dynamics were observed when any one of the three components of the complex was missing, as studied in ashen- (Rab27a defective) and shaker-1 (MyoVIIa mutant)-derived RPE cells, and in wild-type RPE cells transduced with adenovirus carrying specific sequences to knockdown Myrip expression. We observed a significant increase in the number of motile melanosomes, exhibiting more frequent and prolonged bursts of fast movement, and inversion of directionality. Similar alterations were observed upon cytochalasin D treatment, suggesting that the Rab27a–Myrip–MyoVIIa complex regulates tethering of melanosomes onto actin filaments, a process that ensures melanosome movement towards the cell periphery.

Myosin II and Rho kinase activity are required for melanosome aggregation in fish retinal pigment epithelial cells

In the retinal pigment epithelium (RPE) of fish, melanosomes (pigment granules) migrate long distances through the cell body into apical projections in the light, and aggregate back into the cell body in the dark. RPE cells can be isolated from the eye, dissociated, and cultured as single cells in vitro. Treatment of isolated RPE cells with cAMP or the phosphatase inhibitor, okadaic acid (OA), stimulates melanosome aggregation, while cAMP or OA washout in the presence of dopamine triggers dispersion. Previous studies have shown that actin filaments are both necessary and sufficient for aggregation and dispersion of melanosomes within apical projections of isolated RPE. The role of myosin II in melanosome motility was investigated using the myosin II inhibitor, blebbistatin, and a specific rho kinase (ROCK) inhibitor, H-1152. Blebbistatin and H-1152 partially blocked melanosome aggregation triggered by cAMP in dissociated, isolated RPE cells and isolated sheets of RPE. In contrast, neither drug affected melanosome dispersion. In cells exposed to either blebbistatin or H-1152, then triggered to aggregate using OA, melanosome aggregation was completely inhibited. These results demonstrate that (1) melanosome aggregation and dispersion occur through different, actin-dependent mechanisms; (2) myosin II and ROCK activity are required for full melanosome aggregation, but not dispersion; (3) partial aggregation that occurred despite myosin II or ROCK inhibition suggests a second component of aggregation that is dependent on cAMP signaling, but independent of ROCK and myosin II.

Shroom2 (APXL) regulates melanosome biogenesis and localization in the retinal pigment epithelium

Shroom family proteins have been implicated in the control of the actin cytoskeleton, but so far only a single family member has been studied in the context of developing embryos. Here, we show that the Shroom-family protein,Shroom2 (previously known as APXL) is both necessary and sufficient to govern the localization of pigment granules at the apical surface of epithelial cells. In Xenopus embryos that lack Shroom2 function, we observed defects in pigmentation of the eye that stem from failure of melanosomes to mature and to associate with the apical cell surface. Ectopic expression of Shroom2 in naïve epithelial cells facilitates apical pigment accumulation, and this activity specifically requires the Rab27a GTPase. Most interestingly, we find that Shroom2, like Shroom3 (previously called Shroom),is sufficient to induce a dramatic apical accumulation of the microtubule-nucleating protein γ-tubulin at the apical surfaces of naïve epithelial cells. Together, our data identify Shroom2 as a central regulator of RPE pigmentation, and suggest that, despite their diverse biological roles, Shroom family proteins share a common activity. Finally,because the locus encoding human SHROOM2 lies within the critical region for two distinct forms of ocular albinism, it is possible that SHROOM2mutations may be a contributing factor in these human visual system disorders.

Myosin-V, Kinesin-1, and Kinesin-3 cooperate in hyphal growth of the fungus Ustilago maydis

Long-distance transport is crucial for polar-growing cells, such as neurons and fungal hyphae. Kinesins and myosins participate in this process, but their functional interplay is poorly understood. Here, we investigate the role of kinesin motors in hyphal growth of the plant pathogen Ustilago maydis. Although the microtubule plus-ends are directed to the hyphal tip, of all 10 kinesins analyzed, only conventional kinesin (Kinesin-1) and Unc104/Kif1A-like kinesin (Kinesin-3) were up-regulated in hyphae and they are essential for extended hyphal growth. deltakin1 and deltakin3 mutant hyphae grew irregular and remained short, but they were still able to grow polarized. No additional phenotype was detected in deltakin1rkin3 double mutants, but polarity was lost in deltamyo5rkin1 and deltamyo5rkin3 mutant cells, suggesting that kinesins and class V myosin cooperate in hyphal growth. Consistent with such a role in secretion, fusion proteins of green fluorescent protein and Kinesin-1, Myosin-V, and Kinesin-3 accumulate in the apex of hyphae, a region where secretory vesicles cluster to form the fungal Spitzenkörper. Quantitative assays revealed a role of Kin3 in secretion of acid phosphatase, whereas Kin1 was not involved. Our data demonstrate that just two kinesins and at least one myosin support hyphal growth.

Actin-dependent motility of melanosomes from fish retinal pigment epithelial (RPE) cells investigated using in vitro motility assays

Melanosomes (pigment granules) within retinal pigment epithelial (RPE) cells of fish and amphibians undergo massive migrations in response to light conditions to control light flux to the retina. Previous research has shown that melanosome motility within apical projections of dissociated fish RPE cells requires an intact actin cytoskeleton, but the mechanisms and motors involved in melanosome transport in RPE have not been identified. Two in vitro motility assays, the Nitella assay and the sliding filament assay, were used to characterize actin-dependent motor activity of RPE melanosomes. Melanosomes applied to dissected filets of the Characean alga, Nitella, moved along actin cables at a mean rate of 2 microm/min, similar to the rate of melanosome motility in dissociated, cultured RPE cells. Path lengths of motile melanosomes ranged from 9 to 37 microm. Melanosome motility in the sliding filament assay was much more variable, ranging from 0.4-33 microm/min; 70% of velocities ranged from 1-15 microm/min. Latex beads coated with skeletal muscle myosin II and added to Nitella filets moved in the same direction as RPE melanosomes, indicating that the motility is barbed-end directed. Immunoblotting using antibodies against myosin VIIa and rab27a revealed that both proteins are enriched on melanosome membranes, suggesting that they could play a role in melanosome transport within apical projections of fish RPE.

Role of myosin VIIa and Rab27a in the motility and localization of RPE melanosomes

Myosin VIIa functions in the outer retina, and loss of this function causes human blindness in Usher syndrome type 1B (USH1B). In mice with mutant Myo7a, melanosomes in the retinal pigmented epithelium (RPE) are distributed abnormally. In this investigation we detected many proteins in RPE cells that could potentially participate in melanosome transport, but of those tested, only myosin VIIa and Rab27a were found to be required for normal distribution. Two other expressed proteins, melanophilin and myosin Va, both of which are required for normal melanosome distribution in melanocytes, were not required in RPE, despite the association of myosin Va with the RPE melanosome fraction. Both myosin VIIa and myosin Va were immunodetected broadly in sections of the RPE, overlapping with a region of apical filamentous actin. Some 70-80% of the myosin VIIa in RPE cells was detected on melanosome membranes by both subcellular fractionation of RPE cells and quantitative immunoelectron microscopy, consistent with a role for myosin VIIa in melanosome motility. Time-lapse microscopy of melanosomes in primary cultures of mouse RPE cells demonstrated that the melanosomes move in a saltatory manner, interrupting slow movements with short bursts of rapid movement (>1 RR01183m/second). In RPE cells from Myo7a-null mice, both the slow and rapid movements still occurred, except that more melanosomes underwent rapid movements, and each movement extended approximately five times longer (and further). Hence, our studies demonstrate the presence of many potential effectors of melanosome motility and localization in the RPE, with a specific requirement for Rab27a and myosin VIIa, which function by transporting and constraining melanosomes within a region of filamentous actin. The presence of two distinct melanosome velocities in both control and Myo7a-null RPE cells suggests the involvement of at least two motors other than myosin VIIa in melanosome motility, most probably, a microtubule motor and myosin Va.

The role of Rab27a in the regulation of melanosome distribution within retinal pigment epithelial cells

Melanosomes within the retinal pigment epithelium (RPE) of mammals have long been thought to exhibit no movement in response to light, unlike fish and amphibian RPE. Here we show that the distribution of melanosomes within the mouse RPE undergoes modest but significant changes with the light cycle. Two hours after light onset, there is a threefold increase in the number of melanosomes in the apical processes that surround adjacent photoreceptors. In skin melanocytes, melanosomes are motile and evenly distributed throughout the cell periphery. This distribution is due to the interaction with the cortical actin cytoskeleton mediated by a tripartite complex of Rab27a, melanophilin, and myosin Va. In ashen (Rab27a null) mice RPE, melanosomes are unable to move beyond the adherens junction axis and do not enter apical processes, suggesting that Rab27a regulates melanosome distribution in the RPE. Unlike skin melanocytes, the effects of Rab27a are mediated through myosin VIIa in the RPE, as evidenced by the similar melanosome distribution phenotype observed in shaker-1 mice, defective in myosin VIIa. Rab27a and myosin VIIa are likely to be required for association with and movement through the apical actin cytoskeleton, which is a prerequisite for entry into the apical processes.

Pigment cells: a model for the study of organelle transport

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.

Abnormal phagocytosis by retinal pigmented epithelium that lacks myosin VIIa, the Usher syndrome 1B protein

Mutations in the myosin VIIa gene (MYO7A) cause Usher syndrome type 1B (USH1B), a major type of the deaf-blind disorder, Usher syndrome. We have studied mutant phenotypes in the retinas of Myo7a mutant mice (shaker1), with the aim of elucidating the role(s) of myosin VIIa in the retina and what might underlie photoreceptor degeneration in USH1B patients. A photoreceptor defect has been described. Here, we report that the phagocytosis of photoreceptor outer segment disks by the retinal pigment epithelium (RPE) is abnormal in Myo7a null mice. Both in vivo and in primary cultures of RPE cells, the transport of ingested disks out of the apical region is inhibited in the absence of Myo7a. The results with the cultured RPE cells were the same, irrespective of whether the disks came from wild-type or mutant mice, thus demonstrating that the RPE is the source of this defect. The inhibited transport seems to delay phagosome-lysosomal fusion, as the degradation of ingested disks was slower in mutant RPE. Moreover, fewer packets of disk membranes were ingested in vivo, possibly because retarded removal of phagosomes from the apical processes inhibited the ingestion of additional disk membranes. We conclude that Myo7a is required for the normal processing of ingested disk membranes in the RPE, primarily in the basal transport of phagosomes into the cell body where they then fuse with lysosomes. Because the phagocytosis of photoreceptor disks by the RPE has been shown to be critical for photoreceptor cell viability, this defect likely contributes to the progressive blindness in USH1B.

Actin-dependent, retrograde motility of surface-attached beads and aggregating pigment granules in dissociated teleost retinal pigment epithelial cells

Teleost retinal pigment epithelial (RPE) cells contain pigment granules within apical projections which undergo actin-dependent, bi-directional motility. Dissociated RPE cells in culture attach to the substrate and extend apical projections in a radial array from the central cell body. Pigment granules within projections can be triggered to aggregate or disperse by the presence or absence of 1 mM cAMP. Aminated, fluorescent latex beads attached to the dorsal surface of apical projections and moved in the retrograde direction, towards the cell body. Bead rates on RPE cells with aggregating or fully aggregated pigment granules were 2.2 +/- 0.5 and 2.6 +/- 0.2 microm/min (mean +/- SEM), respectively, similar to rates of aggregating (retrograde) pigment granule movement (2.0 +/- 0.4 microm/min). Bead rates were slightly slower on cells with fully dispersed or dispersing pigment granules (1.5 +/- 0.1 and 1.5 +/- 0.4 microm/min). Movements of surface-attached beads and aggregating pigment granules were closely correlated in the distal portions of apical projections, but were more independent of each other in proximal regions of the projections. The actin disrupting drug, cytochalasin D (CD), reversibly halted retrograde bead movements, suggesting that motility of surface-attached particles is actin-dependent. In contrast, the microtubule depolymerizing drug, nocodazole, had no effect on retrograde bead motility. The similar characteristics and actin-dependence of retrograde bead movements and aggregating pigment granules suggest a correlation between these two processes.

Regulation and expression of metazoan unconventional myosins

Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3-16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplification. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

The road less traveled: emerging principles of kinesin motor utilization

Proteins of the kinesin superfamily utilize a conserved catalytic motor domain to generate movements in a wide variety of cellular processes. In this review, we discuss the rapid expansion in our understanding of how eukaryotic cells take advantage of these proteins to generate force and movement in diverse functional contexts. We summarize several recent examples revealing that the simplest view of a kinesin motor protein binding to and translocating a cargo along a microtubule track is inadequate. In fact, this paradigm captures only a small subset of the many ways in which cells harness force production of the generation of intracellular movements and functions. We also highlight several situations where the catalytic kinesin motor domain may not be used to generate movement, but instead may be used in other biochemical and functional contexts. Finally, we review some recent ideas about kinesin motor regulation, redundancy, and cargo attachment strategies.

Cooperation between microtubule- and actin-based motor proteins

Organelle transport has been proposed to proceed in two steps: long-range transport along microtubules and local delivery via actin filaments. This model is supported by recent studies of pigment transport in several cell types and transport in neurons, and in several cases, class V myosin has been implicated as the actin-based motor. Mutations in mice (dilute) and yeast (myo2) have also implicated this class of myosin in organelle transport, and genetic interactions in yeast have indicated that a kinesin-related protein (Smy1p) plays a supporting role. This link between members of two different motor superfamilies has now taken a surprising turn: There is evidence for a physical interaction between class V myosins and kinesin or Smy1p in both mice and yeast.

Characterization of Xenopus RalB and its involvement in F-actin control during early development

We describe the characterization and a functional analysis in Xenopus development of RalB, a small G protein. RalB RNA and protein are detectable during oogenesis and early development, but the gene is expressed only weakly in adult tissues. The RalB transcripts are processed by poly(A) extension during oocyte maturation and up to the gastrulation stage. Microinjection of wild-type or mutant RalB RNAs was performed in fertilized eggs in order to gain insight into the function of RalB during development. We show that during cleavage stages the activated GTP form of RalB specifically induces a cortical reaction that affects the localization of pigment granules. The use of different drugs suggests that this reaction is dependent on the outer cortical actin array. The relation between F-actin and RalB was shown by confocal analysis. Injection of mRNAs encoding the mutated activated form of RalB leads, at dependent doses, to a blocking of gastrulation or defects in closing of neural folding structures. In contrast, the inactivated form blocks only the closing of neural tube. Altogether, these observations suggest that RalB is part of a regulatory pathway that may affect the blastomere cytoskeleton and take part in early development.

Myosin cooperates with microtubule motors during organelle transport in melanophores

Melanophores offer an outstanding system for the study of intracellular motility. These cells aggregate their pigment-filled melanosomes to the cell center or disperse them throughout the cytoplasm in response to hormonal modulation of intracellular cyclic AMP levels in order to effect color changes in lower vertebrates [1]. Previous work from our laboratory demonstrated a role for microtubule-based motors in melanosome transport and we succeeded in reconstituting their regulated motility along microtubules in vitro [2,3]. Here we demonstrate that, in addition to microtubule-mediated motility, melanosomes purified from Xenopus melanophores exhibit unidirectional movement along actin filaments in vitro as well. Immunoblotting analysis shows that these organelles possess the actin-based organelle motor, myosin-V. In vivo, melanosomes are able to slowly disperse in the absence of microtubules, and this slow dispersion requires the integrity of the actin cytoskeleton. Furthermore, in cells with dispersed pigment, disruption of filamentous actin induces a rapid, microtubule-dependent aggregation of melanosomes to the cell center. Our results, together with the accompanying paper by Rodionov et al. [4], demonstrate that the concerted efforts of both microtubule-based and actin-based motors are required for proper melanosome distribution in melanophores. This is the first example of a biochemically defined organelle in possession of both plus-end and minus-end directed microtubule motors and a myosin; coordinated activity of all three motors is essential for organelle motility in vivo.