A myosin III from Limulus eyes is a clock-regulated phosphoprotein

Myosin light chains: Teaching old dogs new tricks

The myosin holoenzyme is a multimeric protein complex consisting of heavy chains and light chains. Myosin light chains are calmodulin family members which are crucially involved in the mechanoenzymatic function of the myosin holoenzyme. This review examines the diversity of light chains within the myosin superfamily, discusses interactions between the light chain and the myosin heavy chain as well as regulatory and structural functions of the light chain as a subunit of the myosin holoenzyme. It covers aspects of the myosin light chain in the localization of the myosin holoenzyme, protein-protein interactions and light chain binding to non-myosin binding partners. Finally, this review challenges the dogma that myosin regulatory and essential light chain exclusively associate with conventional myosin heavy chains while unconventional myosin heavy chains usually associate with calmodulin.

What the clock tells the eye: lessons from an ancient arthropod

Circadian changes in visual sensitivity have been observed in a wide range of species, vertebrates, and invertebrates, but the processes impacted and the underlying mechanisms largely are unexplored. Among arthropods, effects of circadian signals on vision have been examined in most detail in the lateral compound eye (LE) of the American horseshoe crab, Limulus polyphemus, a chelicerate arthropod. As a consequence of processes influenced by a central circadian clock, Limulus can see at night nearly as well as they do during the day. The effects of the clock on horseshoe crab LE retinas are diverse and include changes in structure, gene expression, and rhabdom biochemistry. An examination of the known effects of circadian rhythms on LEs shows that the effects have three important outcomes: an increase in visual sensitivity at night, a rapid decrease in visual sensitivity at dawn, and maintenance of eyes in a relatively low state of sensitivity during the day, even in the dark. All three outcomes may be critically important for species' survival. Specific effects of circadian rhythms on vision will certainly vary with species and according to life styles. Studies of the circadian regulation of Limulus vision have revealed that these effects can be extremely diverse and profound and suggest that circadian clocks can play a critical role in the ability of animals to adapt to the dramatic daily changes in ambient illumination.

Opsin1-2, G(q)α and arrestin levels at Limulus rhabdoms are controlled by diurnal light and a circadian clock

Dark and light adaptation in photoreceptors involve multiple processes including those that change protein concentrations at photosensitive membranes. Light- and dark-adaptive changes in protein levels at rhabdoms have been described in detail in white-eyed Drosophila maintained under artificial light. Here we tested whether protein levels at rhabdoms change significantly in the highly pigmented lateral eyes of wild-caught Limulus polyphemus maintained in natural diurnal illumination and whether these changes are under circadian control. We found that rhabdomeral levels of opsins (Ops1-2), the G protein activated by rhodopsin (G(q)α) and arrestin change significantly from day to night and that nighttime levels of each protein at rhabdoms are significantly influenced by signals from the animal's central circadian clock. Clock input at night increases Ops1-2 and G(q)α and decreases arrestin levels at rhabdoms. Clock input is also required for a rapid decrease in rhabdomeral Ops1-2 beginning at sunrise. We found further that dark adaptation during the day and the night are not equivalent. During daytime dark adaptation, when clock input is silent, the increase of Ops1-2 at rhabdoms is small and G(q)α levels do not increase. However, increases in Ops1-2 and G(q)α at rhabdoms are enhanced during daytime dark adaptation by treatments that elevate cAMP in photoreceptors, suggesting that the clock influences dark-adaptive increases in Ops1-2 and G(q)α at Limulus rhabdoms by activating cAMP-dependent processes. The circadian regulation of Ops1-2 and G(q)α levels at rhabdoms probably has a dual role: to increase retinal sensitivity at night and to protect photoreceptors from light damage during the day.

Mouse class III myosins: kinase activity and phosphorylation sites

As class III unconventional myosins are motor proteins with an N-terminal kinase domain, it seems likely they play a role in both signaling and actin based transport. A growing body of evidence indicates that the motor functions of human class IIIA myosin, which has been implicated in progressive hearing loss, are modulated by intermolecular autophosphorylation. However, the phosphorylation sites have not been identified. We studied the kinase activity and phosphorylation sites of mouse class III myosins, mMyo3A and 3B, which are highly similar to their human orthologs. We demonstrate that the kinase domains of mMyo3A and 3B are active kinases, and that they have similar, if not identical, substrate specificities. We show that the kinase domains of these proteins autophosphorylate, and that they can phosphorylate sites within their myosin and tail domains. Using liquid chromatography-mass spectrometry, we identified phosphorylated sites in the kinase, myosin motor and tail domains of both mMyo3A and 3B. Most of the phosphorylated sites we identified and their consensus phosphorylation motifs are highly conserved among vertebrate class III myosins, including human class III myosins. Our findings are a major step toward understanding how the functions of class III myosins are regulated by phosphorylation.

S-crystallin and arginine kinase bind F-actin in light- and dark-adapted octopus retinas

PURPOSE

Rhabdomere microvilli dramatically reorganize in conditions of light and dark. This reorganization involves remodeling of the microvillus actin cytoskeleton. We are using the rhabdomeric retina of Octopus bimaculoides to identify actin-binding proteins that may be involved in this remodeling.

METHODS

Octopus were light-/dark-adapted, retinas separated into dorsal and ventral halves, and homogenized. Actin-binding proteins were recognized using F-actin overlay blot assays and selected proteins from the overlays were identified using N-terminal sequencing methods or mass spectroscopy. Protein concentrations were quantified and compared by statistical analysis.

RESULTS

Total protein gels of light-/dark-adapted, ventral/dorsal halves were almost identical except for a protein band at 26 kD. The relative amount of this protein in the dark was almost double that found in the light. The levels of other proteins did not vary significantly between the light and dark. F-actin overlays also showed matching patterns of actin-binding proteins except for the 26 kD protein. Although the 26 kD protein from light-adapted retinas transferred to the blotting membranes, it did not bind F-actin while the 26 kD protein on overlays from dark-adapted retinas always demonstrated F-actin binding. Besides the 26 kD protein, other proteins at 200 kD, 80 kD, 40 kD appeared on the overlays. These proteins and the 26 kD protein were sequenced and identified as hemocynanin, transitional ER ATPase, arginine kinase and S-crystallin, respectively.

CONCLUSIONS

The amount of S-crystallin present in the octopus retina is significantly greater in dark-adapted retinas and it binds to F-actin. In the light, the level of S-crystallin is greatly reduced and there is no apparent F-actin binding. No other studies, to our knowledge, show that S-crystallin binds to the actin cytoskeleton or that its expression is regulated by light. Arginine kinase may provide energy for cytoskeletal remodeling as it may in other neural tissues.

Structure of cone photoreceptors

Although outnumbered more than 20:1 by rod photoreceptors, cone cells in the human retina mediate daylight vision and are critical for visual acuity and color discrimination. A variety of human diseases are characterized by a progressive loss of cone photoreceptors but the low abundance of cones and the absence of a macula in non-primate mammalian retinas have made it difficult to investigate cones directly. Conventional rodents (laboratory mice and rats) are nocturnal rod-dominated species with few cones in the retina, and studying other animals with cone-rich retinas presents various logistic and technical difficulties. Originating in the early 1900s, past research has begun to provide insights into cone ultrastructure but has yet to afford an overall perspective of cone cell organization. This review summarizes our past progress and focuses on the recent introduction of special mammalian models (transgenic mice and diurnal rats rich in cones) that together with new investigative techniques such as atomic force microscopy and cryo-electron tomography promise to reveal a more unified concept of cone photoreceptor organization and its role in retinal diseases.

Cloning and distribution of myosin 3B in the mouse retina: differential distribution in cone outer segments

Class III myosins are important for the function and survival of photoreceptors and ciliary hair cells. Although vertebrates possess two class III myosin genes, myo3A and myo3B, recent studies have focused on Myo3A because mutations in the human gene are implicated in progressive hearing loss. Myo3B may compensate for defects in Myo3A, yet little is known about its distribution and function. This study focuses on Myo3B expression in the mouse retina. We cloned two variants of myo3B from mouse retina and determined that they are expressed early in retinal development. In this study we show for the first time in a mammal that both Myo3B and Myo3A proteins are present in inner segments of all photoreceptors. Myo3B is also present in outer segments of S opsin-immunoreactive cones but not M opsin dominant cones. Myo3B is also detected in rare cells of the inner nuclear layer and some ganglion cells. Myo3B may have diverse roles in retinal neurons. In photoreceptor inner segments Myo3B is positioned appropriately to prevent photoreceptor loss of function caused by Myo3A defects.

Isolation and expression of Pax6 and atonal homologues in the American horseshoe crab, Limulus polyphemus

Pax6 regulates eye development in many animals. In addition, Pax6 activates atonal transcription factors in both invertebrate and vertebrate eyes. Here, we investigate the roles of Pax6 and atonal during embryonic development of Limulus polyphemus rudimentary lateral, medial and ventral eyes, and the initiation of lateral ommatidial eye and medial ocelli formation. Limulus eye development is of particular interest because these animals hold a unique position in arthropod phylogeny and possess multiple eye types. Furthermore, the molecular underpinnings of eye development have yet to be investigated in chelicerates. We characterized a Limulus Pax6 gene, with multiple splice products and predicted protein isoforms, and one atonal homologue. Unexpectedly, neither gene is expressed in the developing eye types examined, although both genes are present in the lateral sense organ, a structure of unknown function.

ORYZA SATIVA MYOSIN XI B controls pollen development by photoperiod-sensitive protein localizations

Myosins are actin-based motor proteins responsible for various motility and signal transduction. Only a small set of myosin classes is present inplants, and little is known about their functions. Here we showed how a rice myosin gene controlled pollen development by sensing changed environmental factors. The analysis is based on a gene-trapped Ds insertion mutant Oryza sativa myosin XI B (osmyoXIB). This mutant showed male sterility under short day length (SD) conditions and fertility under long day length (LD) conditions. Under both SD and LD conditions, the OSMYOXIB transcript was detected in whole anthers. However, under SD conditions, the OSMYOXIB-GUS fusion protein was localized only in the epidermal layer of anthers due to the lack of 3'-untranslated region (3'-UTR) and to dilute (DIL) domain sequences following the Ds insertion. As a result, mutant pollen development was affected, leading to male sterility. By contrast, under LD conditions, the fusion protein was localized normally in anthers. Despite normal localization, the protein was only partially functional due to the lack of DIL domain sequences, resulting in limited recovery of pollen fertility. This study also provides a case for a novel molecular aspect of gene expression, i.e., cell layer-specific translation in anthers.

Kinetic mechanism of human myosin IIIA

Myosin IIIA is specifically expressed in photoreceptors and cochlea and is important for the phototransduction and hearing processes. In addition, myosin IIIA contains a unique N-terminal kinase domain and C-terminal tail actin-binding motif. We examined the kinetic properties of baculovirus expressed human myosin IIIA containing the kinase, motor, and two IQ domains. The maximum actin-activated ATPase rate is relatively slow (k(cat) = 0.77 +/- 0.08 s(-1)), and high actin concentrations are required to fully activate the ATPase rate (K(ATPase) = 34 +/- 11 microm). However, actin co-sedimentation assays suggest that myosin III has a relatively high steady-state affinity for actin in the presence of ATP (K(actin) approximately 7 microm). The rate of ATP binding to the motor domain is quite slow both in the presence and absence of actin (K(1)k(+2) = 0.020 and 0.001 microm(-1).s(-1), respectively). The rate of actin-activated phosphate release is more than 100-fold faster (85 s(-1)) than the k(cat), whereas ADP release in the presence of actin follows a two-step mechanism (7.0 and 0.6 s(-1)). Thus, our data suggest a transition between two actomyosin-ADP states is the rate-limiting step in the actomyosin III ATPase cycle. Our data also suggest the myosin III motor spends a large fraction of its cycle in an actomyosin ADP state that has an intermediate affinity for actin (K(d) approximately 5 microm). The long lived actomyosin-ADP state may be important for the ability of myosin III to function as a cellular transporter and actin cross-linker in the actin bundles of sensory cells.

Human myosin III is a motor having an extremely high affinity for actin

Myosin IIIA is expressed in photoreceptor cells and thought to play a critical role in phototransduction processes, yet its function on a molecular basis is largely unknown. Here we clarified the kinetic mechanism of the ATPase cycle of human myosin IIIA. The steady-state ATPase activity was markedly activated approximately 10-fold with very low actin concentration. The rate of ADP off from actomyosin IIIA was 10 times greater than the overall cycling rate, thus not a rate-determining step. The rate constant of the ATP hydrolysis step of the actin-dissociated form was very slow, but the rate was markedly accelerated by actin binding. The dissociation constant of the ATP-bound form of myosin IIIA from actin is submicromolar, which agrees well with the low K(actin). These results indicate that ATP hydrolysis predominantly takes place in the actin-bound form for actomyosin IIIA ATPase reaction. The obtained K(actin) was much lower than the previously reported one, and we found that the autophosphorylation of myosin IIIA dramatically increased the K(actin), whereas the V(max) was unchanged. Our kinetic model indicates that both the actin-attached hydrolysis and the P(i) release steps determine the overall cycle rate of the dephosphorylated form. Although the stable steady-state intermediates of actomyosin IIIA ATPase reaction are not typical strong actin-binding intermediates, the affinity of the stable intermediates for actin is much higher than conventional weak actin binding forms. The present results suggest that myosin IIIA can spend a majority of its ATP hydrolysis cycling time on actin.

Testing the role of calmodulin in the excitation of Limulus photoreceptors

The phototransduction cascade in Limulus ventral photoreceptors involves multiple second messengers, including Ca(2+) and cGMP. Light-induced Ca(2+) release from intracellular stores is an intermediate step, but the subsequent Ca(2+)-activated reaction remains to be determined. The possibility that Ca(2+)/calmodulin (Ca(2+)/CaM) might be involved is suggested by the high calmodulin content of the transducing lobe. To test whether CaM can excite the transduction cascade we injected a 25 microM Ca(2+)/CaM solution. This produced a rapid, brief depolarization similar to that produced by light, suggesting a role for CaM in the cascade. However, an important caveat is that Ca(2+) dissociating from the Ca(2+)/CaM complex might excite this process. Several control experiments argue against, but do not entirely eliminate this possibility. To test whether endogenous CaM has a function in excitation, trifluoperazine was pressure injected into the rhabdomeric region. The response to brief flashes was not affected, but the response to steady illumination was transiently attenuated by each injection. We conclude that calmodulin should be considered a candidate to couple intermediate and late stages of the transduction cascade.

Evolution of arthropod visual systems: Development of the eyes and central visual pathways in the horseshoe crab Limulus polyphemus Linnaeus, 1758 (Chelicerata, Xiphosura)

Despite ongoing interest into the architecture, biochemistry, and physiology of the visual systems of the xiphosuran Limulus polyphemus, their ontogenetic aspects have received little attention. Thus, we explored the development of the lateral eyes and associated neuropils in late embryos and larvae of these animals. The first external evidence of the lateral eyes was the appearance of white pigment spots-guanophores associated with the rudimentary photoreceptors-on the dorsolateral side of the late embryos, suggesting that these embryos can perceive light. The first brown pigment emerges in the eyes during the last (third) embryonic molt to the trilobite stage. However, ommatidia develop from this field of pigment toward the end of the larval trilobite stage so that the young larvae at hatching do not have object recognition. Double staining with the proliferation marker bromodeoxyuridine (BrdU) and an antibody against L. polyphemus myosin III, which is concentrated in photoreceptors of this species, confirmed previous reports that, in the trilobite larvae, new cellular material is added to the eye field from an anteriorly located proliferation zone. Pulse-chase experiments indicated that these new cells differentiate into new ommatidia. Examining larval eyes labeled for opsin showed that the new ommatidia become organized into irregular rows that give the eye field a triangular appearance. Within the eye field, the ommatidia are arranged in an imperfect hexagonal array. Myosin III immunoreactivity in trilobite larvae also revealed the architecture of the central visual pathways associated with the median eye complex and the lateral eyes. Double labeling with myosin III and BrdU showed that neurogenesis persists in the larval brain and suggested that new neurons of both the lamina and the medulla originate from a single common proliferation zone. These data are compared with eye development in Drosophila melanogaster and are discussed with regard to new ideas on eye evolution in the Euarthropoda.

Central regulation of photosensitive membrane turnover in the lateral eye of Limulus, II: octopamine acts via adenylate cyclase/cAMP-dependent protein kinase to prime the retina for transient rhabdom shedding

Why photoreceptors turn over a portion of their photoreceptive membrane daily is not clear; however, failure to do so properly leads to retinal degeneration in vertebrates and invertebrates. Little is known about the molecular mechanisms that regulate shedding and renewal of photoreceptive membrane. Photoreceptive cells in the lateral eye of the horseshoe crab Limulus turn over their photoreceptive membrane (rhabdom) in brief, synchronous burst in response to dawn each morning. Transient rhabdom shedding (TRS), the first phase of rhabdom turnover in Limulus, is triggered by dawn, but requires a minimum of 3-5 h of overnight priming from the central circadian clock (Chamberlain & Barlow, 1984). We determined previously that the clock primes the lateral eye for TRS using the neurotransmitter octopamine (OA) (Khadilkar et al., 2002), and report here that OA primes the eye for TRS through a G(s)-coupled, adenylate cyclase (AC)/cyclic adenosine 3',5'-monophosphate (cAMP)/cAMP-dependent protein kinase (PKA) signaling cascade. Long-term intraretinol injections (6-7 h @ 1.4 microl/min) of the AC activator forskolin, or the cAMP analogs Sp-cAMP[s] and 8-Br-cAmp primed the retina for TRS in eyes disconnected from the circadian clock, and/or in intact eyes during the day when the clock is quiescent. This suggests that OA primes the eye for TRS by stimulating an AC-mediated rise in intracellular cAMP concentration ([cAMP]i). Co-injection of SQ 22,536, an AC inhibitor, or the PKA inhibitors H-89 and PKI (14-22) with OA effectively antagonized octopaminergic priming by reducing the number of photoreceptors primed for TRS and the amount of rhabdom shed by those photoreceptors compared with eyes treated with OA alone. Our data suggest that OA primes the lateral eye for TRS in part through long-term phosphorylation of a PKA substrate.

Determination of human myosin III as a motor protein having a protein kinase activity

The class III myosin is the most divergent member of the myosin superfamily, having a domain with homology to a protein kinase. However, the function of class III myosin at a molecular level is not known at all, and it has been questioned whether it is actually an actin-based motor molecule. Here, we showed that human myosin III has an ATPase activity that is significantly activated by actin (20-fold) with Kactin of 112 microm and Vmax of 0.34 s-1, indicating the mechanoenzymatic activity of myosin III. Furthermore, we found that human myosin III has actin translocating activity (0.11 +/- 0.05 microm/s) using an in vitro actin gliding assay, and it moves toward the plus end of actin filaments. Myosin III containing calmodulin as the light chain subunit showed a protein kinase activity and underwent autophosphorylation. The autophosphorylation was the intramolecular process, and the sites were at the C-terminal end of the motor domain. Autophosphorylation significantly activated the kinase activity, although it did not affect the ATPase activity. The present study is the first report that clearly demonstrates that the class III myosin is an actin-based motor protein having a protein kinase activity.

Myo3A, one of two class III myosin genes expressed in vertebrate retina, is localized to the calycal processes of rod and cone photoreceptors and is expressed in the sacculus

The striped bass has two retina-expressed class III myosin genes, each composed of a kinase, motor, and tail domain. We report the cloning, sequence analysis, and expression patterns of the long (Myo3A) and short (Myo3B) class III myosins, as well as cellular localization and biochemical characterization of the long isoform, Myo3A. Myo3A (209 kDa) is expressed in the retina, brain, testis, and sacculus, and Myo3B (155 kDa) is expressed in the retina, intestine, and testis. The tails of these two isoforms contain two highly conserved domains, 3THDI and 3THDII. Whereas Myo3B has three IQ motifs, Myo3A has nine IQ motifs, four in its neck and five in its tail domain. Myo3A localizes to actin filament bundles of photoreceptors and is concentrated in the calycal processes. An anti-Myo3A antibody decorates the actin cytoskeleton of rod inner/outer segments, and this labeling is reduced by the presence of ATP. The ATP-sensitive actin association is a feature characteristic of myosin motors. The numerous IQ motifs may play a structural or signaling role in the Myo3A, and its localization to calycal processes indicates that this myosin mediates a local function at this site in vertebrate photoreceptors.

Circadian efferent input to Limulus eyes: anatomy, circuitry, and impact

Much is known about the anatomy of Limulus retinal efferent neurons and the structural and functional consequences of their activation. Retinal efferent axons arise from cell bodies located in the cheliceral ganglia of the brain, and they project out all of the optic nerves. Their unique neurosecretory-like terminals contact all cell types in lateral eye ommatidia, the retinular cells of the median eye, and the internal rhabdom of ventral photoreceptors. Lateral and median rudimentary photoreceptors are also innervated. The activity of the efferents is circadian. They are active during the subjective night and inactive during the subjective day. Activation of the efferents drives dramatic and diverse changes in the structure and function of Limulus eyes and causes the sensitivity and responsiveness of the eyes to light to increase at night. Relatively little is known about the molecular mechanisms that produce these structural and functional changes, but one efferent-activated biochemical cascade has been identified. The biogenic amine octopamine is released from efferent terminals, and an octopamine-stimulated rise in cAMP in photoreceptors, with a subsequent activation of cAMP-dependent protein kinase, mediates many of the known effects of efferent input. A photoreceptor-specific protein, myosin III, is phosphorylated in response to efferent input; this protein may play a role in the efferent stimulated changes in photoreceptor structure and function. Anatomical, biophysical, biochemical, and molecular approaches are now being effectively combined in studies of Limulus eyes; thus, this preparation should be particularly useful for further detailed investigations of mechanisms underlying the modulation of primary sensory cells by efferent input.

Multiple mechanisms of rhabdom shedding in the lateral eye of Limulus polyphemus

Rhabdom shedding in horseshoe crab lateral eye photoreceptors was studied with anti-opsin and anti-arrestin immunocytochemistry. Two, possibly three, distinct shedding mechanisms were revealed in animals maintained in natural lighting. Transient rhabdom shedding, triggered by dawn, is a brief, synchronous event that removes up to 10% of the rhabdom membrane. Whorls of rhabdomeral membrane break into vesicles and form compact multivesicular bodies. These debris particles are immunoreactive for opsin and are of a relatively uniform size, averaging approximately 2 microm(2) in area. Transient shedding requires that input from circadian efferent fibers to the retina precedes the light trigger, and cutting the optic nerve blocks efferent input and transient shedding. Light-driven rhabdom shedding is a progressive process. Rhabdomeral membrane is removed by coated vesicles that accumulate into loosely packed multivesicular bodies. These debris particles label with antibodies directed against opsin, arrestin, and adaptin, and they have a large distribution of sizes, averaging almost 6 microm(2) in area and ranging up to 25 microm(2) or more. The amount of rhabdomeral membrane removed by light-driven shedding has seasonal variation and depends on latitude. Light-driven shedding does not require circadian efferent input. A possible third shedding mechanism, light-independent shedding, is observed when transient shedding is blocked either by 48 hours of darkness or by cutting the optic nerve. Small particles, averaging 1.8 microm(2) in area, exhibiting opsin but not arrestin immunoreactivity can then be found in the cytoplasm surrounding the rhabdom. The nature of light-independent shedding is not yet clear.

A class III myosin expressed in the retina is a potential candidate for Bardet-Biedl syndrome

Class III myosins are actin-based motors with amino-terminal kinase domains. Expression of these motors is highly enhanced in retinal photoreceptors. As mutations in the gene encoding NINAC, a Drosophila melanogaster class III myosin, cause retinal degeneration, human homologs of this gene are potential candidates for human retinal disease. We have recently reported the cloning of MYO3A, a human myosin III expressed predominantly in the retina and retinal pigmented epithelium [1]. The map locus of MYO3A is close to, but does not overlap, that of human Usher's 1F [2]. Here we introduce a shorter class III myosin isoform, MYO3B, which is expressed in the retina, kidney, and testis. We describe the cDNA sequence, genomic organization, and splice variants of MYO3B expressed in the human retina. A product of 36 exons, MYO3B has several splice variants containing either one or two calmodulin binding (IQ) motifs in the neck domain and one of three predominant tail variations: a short tail ending just past the second IQ motif, or two alternatively spliced longer tails. MYO3B maps to 2q31.1-q31.2, a region that overlaps the locus for a Bardet-Biedl syndrome (BBS5) linked to markers at 2q31 [3].

Myr 8, a novel unconventional myosin expressed during brain development associates with the protein phosphatase catalytic subunits 1alpha and 1gamma1

Directed neuronal, astroglial, and oligodendroglial cell migrations comprise a prominent feature of mammalian brain development. Because molecular motor proteins have been implicated in a wide spectrum of processes associated with cell motility, we initiated studies to define the pool of myosins in migrating cerebellar granule neurons and type-1 neocortical astrocytes. Our analyses identified two isoforms of a novel unconventional myosin, which we have cloned, sequenced, and designated myr 8a and 8b (eighth unconventional myosin from rat). Phylogenetic analysis indicates that myr 8 myosins comprise a new class of myosins, which we have designated class XVI. The head domain contains a large N-terminal extension composed of multiple ankyrin repeats, which are implicated in mediating an association with the protein phosphatase 1 (PP1) catalytic subunits 1alpha and 1gamma. The motor domain is followed by a single putative light-chain binding domain. The tail domain of myr 8a is comparatively short with a net positive charge, whereas the tail domain of myr 8b is extended, bears an overall neutral charge, and reveals several stretches of poly-proline residues. Neither the myr 8a nor the myr 8b sequence reveals alpha-helical coiled-coil motifs, suggesting that these myosins exist as monomers. Both immunoblot and Northern blot analyses indicate that myr 8b is the predominant isoform expressed in brain, principally at developmental time periods. The structural features and restricted expression patterns suggest that members of this novel class of unconventional myosins comprise a mechanism to target selectively the protein phosphatase 1 catalytic subunits 1alpha and/or 1gamma in developing brain.

Immunocytochemical localization of opsin, visual arrestin, myosin III, and calmodulin in Limulus lateral eye retinular cells and ventral photoreceptors

The photoreceptors of the horseshoe crab Limulus polyphemus are classical preparations for studies of the photoresponse and its modulation by circadian clocks. An extensive literature details their physiology and ultrastructure, but relatively little is known about their biochemical organization largely because of a lack of antibodies specific for Limulus photoreceptor proteins. We developed antibodies directed against Limulus opsin, visual arrestin, and myosin III, and we have used them to examine the distributions of these proteins in the Limulus visual system. We also used a commercial antibody to examine the distribution of calmodulin in Limulus photoreceptors. Fixed frozen sections of lateral eye were examined with conventional fluorescence microscopy; ventral photoreceptors were studied with confocal microscopy. Opsin, visual arrestin, myosin III, and calmodulin are all concentrated at the photosensitive rhabdomeral membrane, which is consistent with their participation in the photoresponse. Opsin and visual arrestin, but not myosin III or calmodulin, are also concentrated in extra-rhabdomeral vesicles thought to contain internalized rhabdomeral membrane. In addition, visual arrestin and myosin III were found widely distributed in the cytosol of photoreceptors, suggesting that they have functions in addition to their roles in phototransduction. Our results both clarify and raise new questions about the functions of opsin, visual arrestin, myosin III, and calmodulin in photoreceptors and set the stage for future studies of the impact of light and clock signals on the structure and function of photoreceptors.

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.

Cloning and chromosomal localization of a human class III myosin

Two class III myosins have been identified to date: NINAC from Drosophila melanogaster and MyoIII(Lim) from Limulus polyphemus. Both have N-terminal kinase domains and are expressed exclusively in photoreceptors. Mutations in NINAC have been shown to alter the photoresponse and compromise photoreceptor survival. We report the cloning and chromosomal localization of a human class III myosin, MYO3A, from retina and a retinal pigment epithelial cell line. Human MYO3A (which we will refer to simply as MYO3A) possesses an N-terminal kinase domain and three consensus calmodulin-binding (IQ) motifs, two in the neck and one in the tail domain. We detected two MYO3A splice variants differing by 52 amino acids near the kinase/myosin junction. On Northern blots, MYO3A probes detected a 6. 5-kb transcript in human and monkey retina, in a cultured human RPE cell line (RPE-19), and at much lower levels in human pancreas. A somatic hybrid panel PCR screen localized MYO3A to human chromosome 10, and a radiation hybrid screen further localized it proximal to marker D10S197, which is located at 10p11.1 on the human cytogenetic map. Since mutations in NINAC have been shown to alter the photoresponse and compromise photoreceptor survival, the human homologue MYO3A may also play a role in photoreceptor function and/or maintenance.

Are class III and class IX myosins motorized signalling molecules?

Myosins exist that are fused to domains that harbour signalling activities. Class III myosins (NINAC) are protein kinases that play important roles in phototransduction. Class IX myosins inactivate the small G-protein Rho that acts as molecular switch. Because these myosins interact via their myosin head domain with actin filaments, they link signal transduction to the actin cytoskeleton. The exact motor properties of these myosins, however, remain to be determined.

Modulation of the light response by cAMP in Drosophila photoreceptors

Phototransduction in Drosophila is mediated by a G-protein-coupled phospholipase C transduction cascade in which each absorbed photon generates a discrete electrical event, the quantum bump. In whole-cell voltage-clamp recordings, cAMP, as well as its nonhydrolyzable and membrane-permeant analogs 8-bromo-cAMP (8-Br-cAMP) and dibutyryl-cAMP, slowed down the macroscopic light response by increasing quantum bump latency, without changes in bump amplitude or duration. In contrast, cGMP or 8-Br-cGMP had no effect on light response amplitude or kinetics. None of the cyclic nucleotides activated any channels in the plasma membrane. The effects of cAMP were mimicked by application of the non-specific phosphodiesterase inhibitor IBMX and the adenylyl cyclase activator forskolin; zaprinast, a specific cGMP-phosphodiesterase inhibitor, was ineffective. Bump latency was also increased by targeted expression of either an activated G(s) alpha subunit, which increased endogenous adenylyl cyclase activity, or an activated catalytic protein kinase A (PKA) subunit. The action of IBMX was blocked by pretreatment with the PKA inhibitor H-89. The effects of cAMP were abolished in mutants of the ninaC gene, suggesting this nonconventional myosin as a possible target for PKA-mediated phosphorylation. Dopamine (10 microM) and octopamine (100 microM) mimicked the effects of cAMP. These results indicate the existence of a G-protein-coupled adenylyl cyclase pathway in Drosophila photoreceptors, which modulates the phospholipase C-based phototransduction cascade.

Role of posttranscriptional regulation in circadian clocks: lessons from Drosophila

Incredible progress has been made in the last few years in our understanding of the molecular mechanisms underlying circadian clocks. Many of the recent insights have been gained by the isolation and characterization of novel clock mutants and their associated gene products. As might be expected based on theoretical considerations and earlier studies that indicated the importance of temporally regulated macromolecular synthesis for the manifestation of overt rhythms, daily oscillations in the levels of "clock" RNAs and proteins are a pervasive feature of these timekeeping devices. How are these molecular rhythms generated and synchronized? Recent evidence accumulated from a wide variety of model organisms, ranging from bacteria to mammals, points toward an emerging trend; at the "heart" of circadian oscillators lies a cell autonomous transcriptional feedback loop that is composed of alternatively functioning positive and negative elements. Nonetheless, it is also clear that to bring this transcriptional feedback loop to "life" requires important contributions from posttranscriptional regulatory schemes. For one thing, there must be times in the day when the activities of negative-feedback regulators are separated from the activities of the positive regulators they act on, or else the oscillatory potential of the system will be dissipated, resulting in a collection of molecules at steady state. This review mainly summarizes the role of posttranscriptional regulation in the Drosophila melanogaster time-keeping mechanism. Accumulating evidence from Drosophila and other systems suggests that posttranscriptional regulatory mechanisms increase the dynamic range of circadian transcriptional feedback loops, overlaying them with appropriately timed biochemical constraints that not only engender these loops with precise daily periods of about 24 h, but also with the ability to integrate and respond rapidly to multiple environmental cues such that their phases are aligned optimally to the prevailing external conditions.

Identification and immunolocalization of actin cytoskeletal components in light- and dark-adapted octopus retinas

Photoreceptors in the octopus retina are of the rhabdomeric type, with rhabdomeres arising from the plasma membrane on opposite sides of the cylindrical outer segment. Each rhabdomere microvillus has an actin filament core, but other actin-binding proteins have not been identified. We used immunoblotting techniques to identify actin-binding proteins in octopus retinal extracts and immunofluorescence microscopy to localize the same proteins in fixed tissue. Antibodies directed against alpha-actinin and vinculin recognized single protein bands on immunoblots of octopus retinal extract with molecular weights comparable to the same proteins in other tissues. Anti-filamin identified two closely spaced bands similar in molecular weight to filamin in other species. Antibodies to the larger of the Drosophila ninaC gene products, p174, identified two bands lower in molecular weight than p174. Anti-villin localized a band that was significantly less in molecular weight than villin found in other cells. Epifluorescence and confocal microscopy were used to map the location of the same actin-binding proteins in dark- and light-adapted octopus photoreceptors and other retinal cells. Antibodies to most of the actin-binding proteins showed heavy staining of the photoreceptor proximal/supportive cell region accompanied by rhabdom membrane and rhabdom tip staining, although subtle differences were detected with individual antibodies. In dark-adapted retinas anti-alpha-actinin stained the photoreceptor proximal/supportive cell region where an extensive junctional complex joins these two cell types, but in the light, immunoreactivity extended above the junctional complex into the rhabdom bases. Most antibodies densely stained the rhabdom tips but anti-villin exhibited a striated pattern of localization at the tips. We believe that the actin-binding proteins identified in the octopus retina may play a significant role in the formation of new rhabdomere microvilli in the dark. We speculate that these proteins and actin remain associated with an avillar membrane that connects opposing sets of rhabdomeres in light-adapted retinas. Association of these cytoskeletal proteins with the avillar membrane would constitute a pool of proteins that could be recruited for rapid microvillus formation from the previously avillar region.

Unconventional myosins at the crossroad of signal transduction and cytoskeleton remodeling

The cytoplasm of eukaryotic cells is a complex milieu and unraveling how its unique cytoarchitecture is achieved and maintained is a central theme in modern cell biology. The actin cytoskeleton is essential for the maintenance of cell shape and locomotion, and also provides tracks for active intracellular transport. Myosins, the actin-dependent motor proteins form a superfamily of at least 15 structural classes and have been identified in a wide variety of organisms, making the presence of actin and myosins a hallmark feature of eukaryotes. Direct connections of myosins to a variety of cellular tasks are now emerging, such as in cytokinesis, phagocytosis, endocytosis, polarized secretion and exocytosis, axonal transport. Recent studies reveal that myosins also play an essential role in many aspects of signal transduction and neurosensation.