Purification and characterization of a mammalian myosin I

Myosin isoform expressed in metastatic prostate cancer stimulates cell invasion

During metastasis, tumor cells migrate out of their original tissue to invade other organs. Secretion of exosomes and metalloproteases is essential for extracellular matrix remodeling, enabling migration through tissue barriers. Metastatic prostate cancer is differentiated by expression of the rare isoform A of the molecular motor myosin IC, however the function of this isoform remained unknown. Here we show that it contributes causatively to the invasive motility of prostate cancer cells. We found that the isoform associates with metalloprotease-containing exosomes and stimulates their secretion. While the data show that myosin IC is involved in prostate cancer cell migration, migration outside extracellular matrix in vitro proves little affected specifically by isoform A. Nevertheless, this isoform stimulates invasion through extracellular matrix, pointing to a critical role in secretion. Both the secretion and invasion depend on the integrity of the motor and lipid-binding domains of the protein. Our results demonstrate how myosin IC isoform A is likely to function in metastasis, driving secretion of exosomes that enable invasion of prostate cancer cells across extracellular matrix barriers. The new data identify a molecule suitable for a mechanistically grounded development into a marker and target for prognosis, detection, and treatment of invasive prostate cancer.

Calcium-regulated import of myosin IC into the nucleus

Myosin IC is a molecular motor involved in intracellular transport, cell motility, and transcription. Its mechanical properties are regulated by calcium via calmodulin binding, and its functions in the nucleus depend on import from the cytoplasm. The import has recently been shown to be mediated by the nuclear localization signal located within the calmodulin-binding domain. In the present paper, it is demonstrated that mutations in the calmodulin-binding sequence shift the intracellular distribution of myosin IC to the nucleus. The redistribution is displayed by isoform B, described originally as the "nuclear myosin," but is particularly pronounced with isoform C, the normally cytoplasmic isoform. Furthermore, experimental elevation of the intracellular calcium concentration induces a rapid import of myosin into the nucleus. The import is blocked by the importin β inhibitor importazole. These findings are consistent with a mechanism whereby calmodulin binding prevents recognition of the nuclear localization sequence by importin β, and the steric inhibition of import is released by cell signaling leading to the intracellular calcium elevation. The results establish a mechanistic connection between the calcium regulation of the motor function of myosin IC in the cytoplasm and the induction of its import into the nucleus. © 2016 Wiley Periodicals, Inc.

Tissue specific expression of myosin IC isoforms

Background

Myosin IC is a single headed member of the myosin superfamily that localizes to the cytoplasm and the nucleus and is implicated in a variety of processes in both compartments. We recently identified a novel isoform of myosin IC and showed that the MYOIC gene in mammalian cells encodes three isoforms (isoforms A, B, and C) that differ only in the addition of short isoform-specific N-terminal peptides. The expression pattern of the isoforms and the mechanisms of expression regulation remain unknown.

Results

To determine the expression patterns of myosin IC isoforms, we performed a comprehensive expression analysis of the two myosin IC isoforms (isoform A and B) that contain isoform-specific sequences. By immunoblotting with isoform-specific antibodies and by qRT-PCR with isoform-specific primer we demonstrate that myosin IC isoforms A and B have distinct expression patterns in mouse tissues. Specifically, we show that myosin IC isoform A is expressed in a tissue specific pattern, while myosin IC isoform B is ubiquitously expressed at comparable levels in mouse tissues.

Conclusions

The differences in the expression profile of the myosin IC isoforms indicate a tissue-specific MYOIC gene regulation and further suggest that the myosin IC isoforms, despite their high sequence homology, might have tissue-specific and isoform-specific functions.

Cavin-3 dictates the balance between ERK and Akt signaling

Cavin-3 is a tumor suppressor protein of unknown function. Using both in vivo and in vitro approaches, we show that cavin-3 dictates the balance between ERK and Akt signaling. Loss of cavin-3 increases Akt signaling at the expense of ERK, while gain of cavin-3 increases ERK signaling at the expense Akt. Cavin-3 facilitates signal transduction to ERK by anchoring caveolae to the membrane skeleton of the plasma membrane via myosin-1c. Caveolae are lipid raft specializations that contain an ERK activation module and loss of the cavin-3 linkage reduces the abundance of caveolae, thereby separating this ERK activation module from signaling receptors. Loss of cavin-3 promotes Akt signaling through suppression of EGR1 and PTEN. The in vitro consequences of the loss of cavin-3 include induction of Warburg metabolism (aerobic glycolysis), accelerated cell proliferation, and resistance to apoptosis. The in vivo consequences of cavin-3 knockout are increased lactate production and cachexia. DOI:http://dx.doi.org/10.7554/eLife.00905.001.

The myosin motor Myo1c is required for VEGFR2 delivery to the cell surface and for angiogenic signaling

Vascular endothelial growth factor receptor-2 (VEGFR2) is a receptor tyrosine kinase that is expressed in endothelial cells and regulates angiogenic signal transduction under both physiological and pathological conditions. VEGFR2 turnover at the plasma membrane (PM) is regulated by its transport through endocytic and secretory transport pathways. Short-range cargo trafficking along actin filaments is commonly regulated by motor proteins of myosin superfamily. In the current study, performed in primary human endothelial cells, we demonstrate that unconventional myosin 1c (Myo1c; class I family member) regulates the localization of VEGFR2 at the PM. We further demonstrate that the recruitment of VEGFR2 to the PM and its colocalization with Myo1c and caveolin-1 occur in response to VEGF-A (VEGF) stimulation. In addition, VEGF-induced delivery of VEGFR2 to the cell surface requires Myo1c; surface VEGFR2 levels are reduced in the absence of Myo1c and, more importantly, are restored by the overexpression of wild-type but not mutant Myo1c. Subcellular density gradient fractionation revealed that partitioning of VEGFR2 into caveolin-1- and Myo1c-enriched membrane fractions is dependent on VEGF stimulation. Myo1c depletion resulted in increased VEGF-induced VEGFR2 transport to the lysosomes for degradation and was rescued by applying either brefeldin A, which blocks trafficking between the endoplasmic reticulum and the Golgi complex, or dynasore, an inhibitor of dynamin-mediated endocytosis. Myo1c depletion also reduced VEGF-induced VEGFR2 phosphorylation at Y1175 and phosphorylation-dependent activation of ERK1/2 and c-Src kinase, leading to reduced cell proliferation and cell migration. This is the first report demonstrating that Myo1c is an important mediator of VEGF-induced VEGFR2 delivery to the cell surface and plays a role in angiogenic signaling.

Precipitation of kidney myosin IIA and IIB by freezing

Actomyosin precipitation is a critical step in the purification of myosins. In this work, the objective was to precipitate rat kidney actomyosin and isolate myosin by freezing and thawing the soluble fraction. Kidney was homogenized in imidazole buffer, centrifuged at 45000 g for 30 min, and the supernatant was frozen at -20°C for 48 h. The supernatant was thawed at 4°C, centrifuged at 45000 g for 30 min and the precipitate washed twice with imidazole buffer pH 7.0 (with and without Triton X-100, respectively). The resulting precipitate presented a polypeptide profile in SDS/PAGE characteristic of actomyosin and expressed Mg- and K/EDTA-ATPase activity. The actomyosin complex was solubilized with ATP and Mg, and the main polypeptide, p200, was purified in a DEAE-Sepharose column. p200 was marked with anti-myosin II, co-sedimented with F-actin in the absence, but not in the presence, of ATP and was identified by MS/MS with a high Mascot score for myosin IIA. The analysis identified peptides exclusive of myosin IIB, but detected no peptides exclusive of myosin IIC.

CaMKII-mediated phosphorylation of the myosin motor Myo1c is required for insulin-stimulated GLUT4 translocation in adipocytes

The unconventional myosin Myo1c has been implicated in insulin-regulated GLUT4 translocation to the plasma membrane in adipocytes. We show that Myo1c undergoes insulin-dependent phosphorylation at S701. Phosphorylation was accompanied by enhanced 14-3-3 binding and reduced calmodulin binding. Recombinant CaMKII phosphorylated Myo1c in vitro and siRNA knockdown of CaMKIIdelta abolished insulin-dependent Myo1c phosphorylation in vivo. CaMKII activity was increased upon insulin treatment and the CaMKII inhibitors CN21 and KN-62 or the Ca(2+) chelator BAPTA-AM blocked insulin-dependent Myo1c phosphorylation and insulin-stimulated glucose transport in adipocytes. Myo1c ATPase activity was increased after CaMKII phosphorylation in vitro and after insulin stimulation of CHO/IR/IRS-1 cells. Expression of wild-type Myo1c, but not S701A or ATPase dead mutant K111A, rescued the inhibition of GLUT4 translocation by siRNA-mediated Myo1c knockdown. These data suggest that insulin regulates Myo1c function via CaMKII-dependent phosphorylation, and these events play a role in insulin-regulated GLUT4 trafficking in adipocytes likely involving Myo1c motor activity.

Myosin I and adaptation of mechanical transduction by the inner ear

Twenty years ago, the description of hair-cell stereocilia as actin-rich structures led to speculation that myosin molecules participated in mechanical transduction in the inner ear. In 1987, Howard and Hudspeth proposed specifically that a myosin I might mediate adaptation of the transduction current carried by hair cells, the sensory cells of the ear. We exploited the myosin literature to design tests of this hypothesis and to show that the responsible isoform is myosin 1c. The identification of this myosin as the adaptation motor would have been impossible without thorough experimentation on other myosins, particularly muscle myosins. The sliding-filament hypothesis for muscle contraction has thus led to a detailed understanding of the behaviour of hair cells.

Expression of the unconventional myosin Myo1c alters sodium transport in M1 collecting duct cells

Epithelial cells rely on proper targeting of cellular components to perform their physiological function. This dynamic process utilizes the cytoskeleton and involves movement of vesicles to and from the plasma membrane, thus traversing the actin cortical cytoskeleton. Studies support both direct interaction of actin with channels and an indirect mechanism whereby actin may serve as a track in the final delivery of the channel to the plasma membrane. Actin-dependent processes are often mediated via a member of the myosin family of proteins. Myosin I family members have been implicated in multiple cellular events occurring at the plasma membrane. In these studies, we investigated the function of the unconventional myosin I Myo1c in the M1 mouse collecting duct cell line. Myo1c was observed to be concentrated at or near the plasma membrane, often in discrete membrane domains. To address the possible role of Myo1c in channel regulation, we expressed a truncated Myo1c, lacking ATP and actin domains, in M1 cells and compared electrophysiological responses to control M1 cells, M1 cells expressing the empty vector, and M1 cells expressing the full-length Myo1c construct. Interestingly, cells expressing the Myo1c constructs had modulated antidiuretic hormone (ADH)-stimulated short-circuit current and showed little inhibition of short-circuit current with amiloride addition. Evaluation of enhanced green fluorescent protein-Myo1c constructs supports the importance of the IQ region in targeting the Myo1c to its respective cellular domain. These data are consistent with Myo1c participating in the regulation of the Na+ channel after ADH stimulation.

Myosin-1c, the hair cell's adaptation motor

Given their prominent actin-rich subcellular specializations, it is no surprise that mechanosensitive hair cells of the inner ear exploit myosin molecules-the only known actin-dependent molecular motors-to carry out exotic but essential tasks. Recent experiments have confirmed that an unconventional myosin isozyme, myosin-1c, is a component of the hair cell's adaptation-motor complex. This complex carries out slow adaptation, provides tension to sensitize transduction channels, and may participate in assembly of the transduction apparatus. This review focuses on the detailed operation of the adaptation motor and the functional consequences of the incorporation of this specific myosin isozyme into the motor complex.

Human deafness mutation of myosin VI (C442Y) accelerates the ADP dissociation rate

The missense mutation of Cys(442) to Tyr of myosin VI causes progressive postlingual sensorineural deafness. Here we report the affects of the C442Y mutation on the kinetics of the actomyosin ATP hydrolysis mechanism and motor function of myosin VI. The largest changes in the kinetic mechanism of ATP hydrolysis produced by the C442Y mutation are about 10-fold increases in the rate of ADP dissociation from both myosin VI and actomyosin VI. The rates of ADP dissociation from acto-C442Y myosin VI-ADP and C442Y myosin VI-ADP are 20-40 times more rapid than the steady state rates and cannot be the rate-limiting steps of the hydrolysis mechanism in the presence or absence of actin. The 2-fold increase in the actin gliding velocity of C442Y compared with wild type (WT) may be explained at least in part by the more rapid rate of ADP dissociation. The C442Y myosin VI has a significant increase ( approximately 10-fold) in the steady state ATPase rate in the absence of actin relative to WT myosin VI. The steady state rate of actin-activated ATP hydrolysis is unchanged by the C442Y mutation at low (<10(-7) m) calcium but is calcium-sensitive with a 1.6-fold increase at high ( approximately 10(-4) m) calcium that does not occur with WT. The actin gliding velocity of the C442Y mutant decreases significantly at low surface density of myosin VI, suggesting that the mutation hampers the processive movement of myosin VI.

Glucose transporter recycling in response to insulin is facilitated by myosin Myo1c

Insulin stimulates glucose uptake in muscle and adipocytes by signalling the translocation of GLUT4 glucose transporters from intracellular membranes to the cell surface. The translocation of GLUT4 may involve signalling pathways that are both independent of and dependent on phosphatidylinositol-3-OH kinase (PI(3)K). This translocation also requires the actin cytoskeleton, and the rapid movement of GLUT4 along linear tracks may be mediated by molecular motors. Here we report that the unconventional myosin Myo1c is present in GLUT4-containing vesicles purified from 3T3-L1 adipocytes. Myo1c, which contains a motor domain, three IQ motifs and a carboxy-terminal cargo domain, is highly expressed in primary and cultured adipocytes. Insulin enhances the localization of Myo1c with GLUT4 in cortical tubulovesicular structures associated with actin filaments, and this colocalization is insensitive to wortmannin. Insulin-stimulated translocation of GLUT4 to the adipocyte plasma membrane is augmented by the expression of wild-type Myo1c and inhibited by a dominant-negative cargo domain of Myo1c. A decrease in the expression of endogenous Myo1c mediated by small interfering RNAs inhibits insulin-stimulated uptake of 2-deoxyglucose. Thus, myosin Myo1c functions in a PI(3)K-independent insulin signalling pathway that controls the movement of intracellular GLUT4-containing vesicles to the plasma membrane.

Calmodulin binding to recombinant myosin-1c and myosin-1c IQ peptides

BACKGROUND

Bullfrog myosin-1c contains three previously recognized calmodulin-binding IQ domains (IQ1, IQ2, and IQ3) in its neck region; we identified a fourth IQ domain (IQ4), located immediately adjacent to IQ3. How calmodulin binds to these IQ domains is the subject of this report.

RESULTS

In the presence of EGTA, calmodulin bound to synthetic peptides corresponding to IQ1, IQ2, and IQ3 with Kd values of 2-4 microM at normal ionic strength; the interaction with an IQ4 peptide was much weaker. Ca2+ substantially weakened the calmodulin-peptide affinity for all of the IQ peptides except IQ3. To reveal how calmodulin bound to the linearly arranged IQ domains of the myosin-1c neck, we used hydrodynamic measurements to determine the stoichiometry of complexes of calmodulin and myosin-1c. Purified myosin-1c and T701-Myo1c (a myosin-1c fragment with all four IQ domains and the C-terminal tail) each bound 2-3 calmodulin molecules. At a physiologically relevant temperature (25 degrees C) and under low-Ca2+ conditions, T701-Myo1c bound two calmodulins in the absence and three calmodulins in the presence of 5 microM free calmodulin. Ca2+ dissociated nearly all calmodulins from T701-Myo1c at 25 degrees C; one calmodulin was retained if 5 microM free calmodulin was present.

CONCLUSIONS

We inferred from these data that at 25 degrees C and normal cellular concentrations of calmodulin, calmodulin is bound to IQ1, IQ2, and IQ3 of myosin-1c when Ca2+ is low. The calmodulin bound to one of these IQ domains, probably IQ2, is only weakly associated. Upon Ca2+ elevation, all calmodulin except that bound to IQ3 should dissociate.

Dual regulation of mammalian myosin VI motor function

Myosin VI is expressed in a variety of cell types and is thought to play a role in membrane trafficking and endocytosis, yet its motor function and regulation are not understood. The present study clarified mammalian myosin VI motor function and regulation at a molecular level. Myosin VI ATPase activity was highly activated by actin with K(actin) of 9 microm. A predominant amount of myosin VI bound to actin in the presence of ATP unlike conventional myosins. K(ATP) was much higher than those of other known myosins, suggesting that myosin VI has a weak affinity or slow binding for ATP. On the other hand, ADP markedly inhibited the actin-activated ATPase activity, suggesting a high affinity for ADP. These results suggested that myosin VI is predominantly in a strong actin binding state during the ATPase cycle. p21-activated kinase 3 phosphorylated myosin VI, and the site was identified as Thr(406). The phosphorylation of myosin VI significantly facilitated the actin-translocating activity of myosin VI. On the other hand, Ca(2+) diminished the actin-translocating activity of myosin VI although the actin-activated ATPase activity was not affected by Ca(2+). Calmodulin was not dissociated from the heavy chain at high Ca(2+), suggesting that a conformational change of calmodulin upon Ca(2+) binding, but not its physical dissociation, determines the inhibition of the motility activity. The present results revealed the dual regulation of myosin VI by phosphorylation and Ca(2+) binding to calmodulin light chain.

Ischemia disrupts myosin I beta in renal tubules

In these studies we have examined rat kidneys biochemically and microscopically to determine where myosin I beta is located before, during, and after an acute ischemic injury. Myosin I beta is present in multiple tubule segments including the brush border (BB) of the proximal tubule cell (PTC). Its distribution is severely altered by a 15-min renal artery clamp. Myosin I beta is present in the urine during reflow and is found in the numerous cellular blebs arising from the damaged PTC and other tubules. Two hours of reflow result in a decrease in BB myosin I beta staining and an increase in its cytoplasmic staining. Interestingly, the return of the F-actin in the BB precedes the return of the myosin I beta, suggesting that this myosin I isoform may not play a role in rebuilding the microvilli after an ischemic injury. A nonstructural role for this myosin, such as transport or channel regulation, is supported by its presence in many tubule segments, all of which have transport and channel requirements but do not all contain microvilli.

Motor function and regulation of myosin X

Myosin X is a member of the diverse myosin superfamily that is ubiquitously expressed in various mammalian tissues. Although its association with actin in cells has been shown, little is known about its biochemical and mechanoenzymatic function at the molecular level. We expressed bovine myosin X containing the entire head, neck, and coiled-coil domain and purified bovine myosin X in Sf9 cells. The Mg(2+)-ATPase activity of myosin X was significantly activated by actin with low K(ATP). The actin-activated ATPase activity was reduced at Ca(2+) concentrations above pCa 5 in which 1 mol of calmodulin light chain dissociates from the heavy chain. Myosin X translocates F-actin filaments with the velocity of 0.3 microm/s with the direction toward the barbed end. The actin translocating activity was inhibited at concentrations of Ca(2+) at pCa 6 in which no calmodulin dissociation takes place, suggesting that the calmodulin dissociation is not required for the inhibition of the motility. Unlike class V myosin, which shows a high affinity for F-actin in the presence of ATP, the K(actin) of the myosin X ATPase was much higher than that of myosin V. Consistently nearly all actin dissociated from myosin X in the presence of ATP. ADP did not significantly inhibit the actin-activated ATPase activity of myosin X, suggesting that the ADP release step is not rate-limiting. These results suggest that myosin X is a nonprocessive motor. Consistently myosin X failed to support the actin translocation at low density in an in vitro motility assay where myosin V, a processive motor, supports the actin filament movement.

Myosin VIIA is specifically associated with calmodulin and microtubule-associated protein-2B (MAP-2B)

Myosin VIIA is a motor molecule with a conserved head domain and tail region unique to myosin VIIA, which probably defines its unique function in vivo. In an attempt to further characterize myosin VIIA function we set out to identify molecule(s) that specifically associate with it. We demonstrate that 17 and 55 kDa proteins from mouse kidney and cochlea co-purify with myosin VIIA on affinity columns carrying immobilized anti-myosin VIIA antibody. N-terminal sequencing and immunoblotting analysis identified the 17 kDa protein as calmodulin, whereas MS and immunoblotting analysis identified the 55 kDa protein as microtubule-associated protein-2B (MAP-2B). Myosin VIIA can also be co-immunoprecipitated from kidney homogenate using anti-calmodulin or anti-MAP2 (recognizing isoforms 2A and 2B) antibodies, confirming the strong association between calmodulin and myosin VIIA and between MAP-2B and myosin VIIA. Myosin VIIA binds to calmodulin with an apparent K(d) of 10(-9) M. Scatchard analysis of the binding of myosin VIIA to MAP-2B provided evidence for two binding sites, with K(d) values of 10(-10) and 10(-9) M, which have been mapped to medial and C-terminal tail domains of myosin VIIA. The characterization of the interaction of calmodulin and MAP-2B with myosin VIIA provides new insights into the function of myosin VIIA.

Ca(2+)-dependent regulation of the motor activity of myosin V

Mouse myosin V constructs were produced that consisted of the myosin motor domain plus either one IQ motif (M5IQ1), two IQ motifs (M5IQ2), a complete set of six IQ motifs (SHM5), or the complete IQ motifs plus the coiled-coil domain (thus permitting formation of a double-headed structure, DHM5) and expressed in Sf9 cells. The actin-activated ATPase activity of all constructs except M5IQ1 was inhibited above pCa 5, but this inhibition was completely reversed by addition of exogenous calmodulin. At the same Ca(2+) concentration, 2 mol of calmodulin from SHM5 and DHM5 or 1 mol of calmodulin from M5IQ2 were dissociated, suggesting that the inhibition of the ATPase activity is due to dissociation of calmodulin from the heavy chain. However, the motility activity of DHM5 and M5IQ2 was completely inhibited at pCa 6, where no dissociation of calmodulin was detected. Inhibition of the motility activity was not reversed by the addition of exogenous calmodulin. These results indicate that inhibition of the motility is due to conformational changes of calmodulin upon the Ca(2+) binding to the high affinity site but is not due to dissociation of calmodulin from the heavy chain.

Regulation of the enzymatic and motor activities of myosin I

Myosins I were the first unconventional myosins to be purified and they remain the best characterized. They have been implicated in various motile processes, including organelle translocation, ion channel gating and cytoskeletal reorganization but their exact cellular functions are still unclear. All members of the myosin I family, from yeast to man, have three structural domains: a catalytic head domain that binds ATP and actin; a tail domain believed to be involved in targeting the myosins to specific subcellular locations and a junction or neck domain that connects them and interacts with light chains. In this review we discuss how each of these three domains contributes to the regulation of myosin I enzymatic activity, motor activity and subcellular localization.

Overlapping distribution of the 130- and 110-kDa myosin I isoforms on rat liver membranes

The biochemical and mechanochemical properties and localization of myosin I suggest the involvement of these small members of the myosin superfamily in some aspects of intracellular motility in higher cells. We have determined by quantitative immunoblotting with isoform-specific antibodies that the 130-kDa myosin I (myr 1 gene product) and 110-kDa myosin I (myr 2 gene product) account for 0.5 and 0.4%, respectively, of total rat liver protein. Immunoblot analyses reveal that the 130- and 110-kDa myosins I are found in several purified subcellular fractions from rat liver. The membrane-associated 130-kDa myosin I is found at the highest concentration in the plasma membrane (28 ng/microg plasma membrane protein) followed by the endoplasmic reticulum-like mitochondria-associated membrane fraction (MAM; 10 ng/microg MAM protein), whereas the 110-kDa myosin I is found at the highest concentration in Golgi (50 ng/¿g Golgi protein) followed by plasma membrane (20 ng/microg) and MAM (7 ng/microg). Our analyses indicate that myosin I is peripherally associated with Golgi and MAM and its presence in these fractions is not a consequence of myosin I bound to contaminating actin filaments. Although found in relatively low concentrations in microsomes, because of the abundance of microsomes, in liver most of the membrane-associated myosin I is associated with microsomes. Neither myosin I isoform is detected in purified mitochondria. This is the first quantitative analysis addressing the cellular distribution of these mammalian class I myosins.

Localization of myosin-Ibeta near both ends of tip links in frog saccular hair cells

Current evidence suggests that the adaptation motor of mechanoelectrical transduction in vertebrate hair cells is myosin-Ibeta. Previously, confocal and electron microscopy of bullfrog saccular hair cells using an anti-myosin-Ibeta antibody labeled the tips of stereocilia. We have now done quantitative immunoelectron microscopy to test whether myosin-Ibeta is enriched at or near the side plaques of tip links, the proposed sites of adaptation, using hair bundles that were serially sectioned parallel to the macular surface. The highest particle density occurred at stereocilia bases, close to the cuticular plate. Also, stereocilia of differing lengths had approximately the same number of total particles, suggesting equal targeting of myosin-Ibeta to all stereocilia. Finally, particles tended to clump in clusters of two to five particles in the distal two-thirds of stereocilia, suggesting a tendency for self-assembly of myosin-Ibeta. As expected from fluorescence microscopy, particle density was high in the distal 1 micrometer of stereocilia. If myosin-Ibeta is the adaptation motor, a difference should exist in particle density between regions containing the side plaque and those excluding it. Averaging of particle distributions revealed two regions with approximately twice the average density: at the upper ends of tip links in a 700-nm-long region centered approximately 100 nm above the side plaque, and at the lower ends of tip links within the tip plaques. Controls demonstrated no such increase. The shortest stereocilia, which lack side plaques, showed no concentration rise on their sides. Thus, the specific localization of myosin-Ibeta at both ends of tip links supports its role as the adaptation motor.

Localization of myosin-Ibeta near both ends of tip links in frog saccular hair cells

Current evidence suggests that the adaptation motor of mechanoelectrical transduction in vertebrate hair cells is myosin-Ibeta. Previously, confocal and electron microscopy of bullfrog saccular hair cells using an anti-myosin-Ibeta antibody labeled the tips of stereocilia. We have now done quantitative immunoelectron microscopy to test whether myosin-Ibeta is enriched at or near the side plaques of tip links, the proposed sites of adaptation, using hair bundles that were serially sectioned parallel to the macular surface. The highest particle density occurred at stereocilia bases, close to the cuticular plate. Also, stereocilia of differing lengths had approximately the same number of total particles, suggesting equal targeting of myosin-Ibeta to all stereocilia. Finally, particles tended to clump in clusters of two to five particles in the distal two-thirds of stereocilia, suggesting a tendency for self-assembly of myosin-Ibeta. As expected from fluorescence microscopy, particle density was high in the distal 1 micrometer of stereocilia. If myosin-Ibeta is the adaptation motor, a difference should exist in particle density between regions containing the side plaque and those excluding it. Averaging of particle distributions revealed two regions with approximately twice the average density: at the upper ends of tip links in a 700-nm-long region centered approximately 100 nm above the side plaque, and at the lower ends of tip links within the tip plaques. Controls demonstrated no such increase. The shortest stereocilia, which lack side plaques, showed no concentration rise on their sides. Thus, the specific localization of myosin-Ibeta at both ends of tip links supports its role as the adaptation motor.

High affinity Ca2+ binding sites of calmodulin are critical for the regulation of myosin Ibeta motor function

We coexpressed myosin Ibeta heavy chain with three different calmodulin mutants in which the two Ca2+-binding sites of the two N-terminal domain (E12Q), C-terminal domain (E34Q), or all four sites (E1234Q) are mutated in order to define the importance of these Ca2+ binding sites to the regulation of myosin Ibeta. The calmodulin mutated at the two Ca2+ binding sites in N-terminal domain and C-terminal domain lost its lower affinity Ca2+ binding site and higher affinity Ca2+ binding site, respectively. We found that, based upon the change in the actin-activated ATPase activities and actin translocating activities, myosin Ibeta with E12Q calmodulin has the regulatory characteristics similar to myosin Ibeta containing wild-type calmodulin, while myosin Ibeta with E34Q or E1234Q calmodulin lose all Ca2+ regulation. While the increase in myosin Ibeta ATPase activity paralleled the dissociation of 1 mol of calmodulin from myosin Ibeta heavy chain for both wild type (above pCa 5) and E12Q calmodulin (above pCa 6), the Ca2+ level required for the inhibition of actin-translocating activity of myosin Ibeta was lower than that required for dissociation of calmodulin, suggesting that the conformational change induced by the binding of Ca2+ at the high affinity site but not the dissociation of calmodulin is critical for the inhibition of the motor activity. Our results suggest that the regulation of unconventional myosins by Ca2+ is directly mediated by the Ca2+ binding to calmodulin, and that the C-terminal pair of Ca2+-binding sites are critical for this regulation.

Myosin Ibeta is located at tip link anchors in vestibular hair bundles

Recent studies have suggested that myosin Ibeta mediates the adaptation of mechanoelectrical transduction in vestibular hair cells. An important prediction of this hypothesis is that myosin Ibeta should be found in the side insertional plaque, an osmiophilic hair bundle structure that anchors tip links and is thought to house the adaptation motor. To determine whether myosin Ibeta was situated properly to perform adaptation, we used immunofluorescence and immunoelectron microscopy with the monoclonal antibody mT2 to examine the distribution of myosin Ibeta in hair bundles of the bullfrog utricle. Although utricular hair cells differ in their rates and extent of adaptation [Baird RA (1994) Comparative transduction mechanisms of hair cells in the bullfrog utriculus. II. Sensitivity and response dynamics to hair bundle displacement. J Neurophysiol 71:685-705.], myosin Ibeta was present in all hair bundles, regardless of adaptation kinetics. Confirming that, nevertheless, it was positioned properly to mediate adaptation, myosin Ibeta was found at significantly higher levels in the side insertional plaque. Myosin Ibeta was also present at elevated levels at the second tip link anchor of a hair bundle, the tip insertional plaque, found at the tip of a stereocilium. These data support myosin Ibeta as the adaptation motor and are consistent with the suggestion that the motor serves to restore tension applied to transduction channels to an optimal level, albeit with different kinetics in different cell types.

Myosin Ibeta is located at tip link anchors in vestibular hair bundles

Recent studies have suggested that myosin Ibeta mediates the adaptation of mechanoelectrical transduction in vestibular hair cells. An important prediction of this hypothesis is that myosin Ibeta should be found in the side insertional plaque, an osmiophilic hair bundle structure that anchors tip links and is thought to house the adaptation motor. To determine whether myosin Ibeta was situated properly to perform adaptation, we used immunofluorescence and immunoelectron microscopy with the monoclonal antibody mT2 to examine the distribution of myosin Ibeta in hair bundles of the bullfrog utricle. Although utricular hair cells differ in their rates and extent of adaptation [Baird RA (1994) Comparative transduction mechanisms of hair cells in the bullfrog utriculus. II. Sensitivity and response dynamics to hair bundle displacement. J Neurophysiol 71:685-705.], myosin Ibeta was present in all hair bundles, regardless of adaptation kinetics. Confirming that, nevertheless, it was positioned properly to mediate adaptation, myosin Ibeta was found at significantly higher levels in the side insertional plaque. Myosin Ibeta was also present at elevated levels at the second tip link anchor of a hair bundle, the tip insertional plaque, found at the tip of a stereocilium. These data support myosin Ibeta as the adaptation motor and are consistent with the suggestion that the motor serves to restore tension applied to transduction channels to an optimal level, albeit with different kinetics in different cell types.

The ATPase activity of Myr3, a rat myosin I, is allosterically inhibited by its own tail domain and by Ca2+ binding to its light chain calmodulin

We purified Myr3 (third unconventional myosin from rat), a mammalian "amoeboid" subclass myosin I, from rat liver. The heavy chain of purified Myr3 is associated with a single calmodulin light chain. Myr3 exhibits K/EDTA-ATPase and Mg-ATPase activity. The Mg-ATPase activity is stimulated by increasing F-actin concentrations in a complex triphasic manner similar to the Mg-ATPase activity of myosin I molecules from protozoa. Although purified Myr3 was observed to cross-link actin filaments, it bound in an ATP regulated manner to F-actin, and no evidence for a nucleotide-independent high affinity actin binding site that could explain the triphasic activation pattern was obtained. Micromolar concentrations of free Ca2+ reversibly inhibit the Mg-ATPase activity of Myr3 by binding to its light chain calmodulin, which remains bound to the Myr3 heavy chain irrespective of the free Ca2+ concentration. Polyclonal antibodies and Fab fragments directed against the tail domain were found to stimulate the Mg-ATPase activity. A similar stimulation of the Myr3 Mg-ATPase activity is observed upon proteolytic removal of the very C-terminal SH3 domain. These results demonstrate that Myr3 is subject to negative regulation by free calcium and its own tail domain and possibly positive regulation by a tail-domain binding partner.

Evidence for the presence of myosin I in the nucleus

We produced and affinity-purified polyclonal antibodies to adrenal myosin I. These antibodies recognize adrenal myosin I by Western blot analysis (116 kDa) and inhibit the actin-activated ATPase activity of purified adrenal myosin I. They also recognize a 120-kDa protein in extracts prepared from many different cell lines. Fluorescence microscopy demonstrated the presence of immunoreactive material in the perinuclear region, the leading edges, and the nuclei of 3T3 cells. Fluorescence microscopy also demonstrated nuclear staining in mouse oocytes at the germinal vesicle stage and in the pronuclei during fertilization. Confocal and immunoelectron microscopy confirmed the intranuclear localization. Electron microscopy also demonstrated staining of structures in nucleoli that are thought to be associated with rDNA transcription. Western blot analyses revealed the presence of the 120-kDa protein in extracts prepared from nuclei that are apparently free of cytosolic contamination. The same nuclear protein binds 125I-calmodulin and is photoaffinity labeled with [alpha-32P]ATP. The 120-kDa protein was partially purified from twice washed nuclei using ammonium sulfate fractionation and gel filtration chromatography. Column fractions containing 120-kDa protein as revealed by Western blot analysis also contain K+-EDTA ATPase activity. The 120-kDa protein was also shown to bind actin in the absence, but not the presence, of ATP. Since K+-EDTA ATPase activity, actin, and ATP binding are defining features of the members of the myosin superfamily of proteins, we propose that the 120-kDa protein is a previously undescribed myosin I isoform that is an intranuclear actin-based molecular motor.

Improved electrophoresis and transfer of picogram amounts of protein with hemoglobin

During electrophoresis and electroblotting to transfer membranes, picogram amounts of protein can react irreversibly with the polyacrylamide matrix, preventing complete electrophoresis and efficient electroblotting. Bovine hemoglobin, but not other potential carrier proteins, mitigates this protein loss by migrating with or ahead of other proteins and scavenging reactive groups. Inclusion of 5 micrograms of hemoglobin in sample wells increases by 4-fold the amount of a radiolabeled test protein, myosin I beta, found at its appropriate 120-kDa position in sodium dodecyl sulfate-polyacrylamide gels. For electroblotting, incubating the gel with 0.25 mg/ml hemoglobin prior to transfer improves mobilization of picogram amounts of radiolabeled myosin I beta out of the gel by about 6-fold. For picogram amounts of proteins, therefore, approximately 20-fold more protein transfers to a blotting membrane when hemoglobin is used during both electrophoresis and transfer. This effect is general: transfer of radiolabeled Drosophila embryo proteins is improved dramatically by including hemoglobin in the pretransfer incubation solution. We suggest that electroblot-based detection of small amounts of protein, particularly when in the absence of other potential carrier proteins, can be improved substantially by using hemoglobin.

Intracellular distributions and putative functions of calcium-binding proteins in the bullfrog vestibular otolith organs

Hair cells in the bullfrog vestibular otolith organs were immunolabeled by monoclonal and polyclonal antisera against calbindin (CaB), calmodulin (CaM), calretinin (CaR), and parvalbumin (PA). S-100, previously shown to immunolabel striolar hair cells in fish vestibular organs, only weakly immunolabeled hair cells in the bullfrog vestibular otolith organs. Immunolabeling was not detected in supporting cells. With the exception of CaR, myelinated axons and unmyelinated nerve terminals were immunolabeled by all of the above antisera. Immunolabeling was seen in all saccular hair cells, although hair cells at the macular margins were immunolabeled more intensely for CaB, CaM, and PA than more centrally located hair cells. As the macula margins are known to be a growth zone, this labeling pattern suggests that marginal hair cells up-regulate their calcium-binding proteins during hair cell development. In the utriculus, immunolabeling for CaM and PA was generally restricted to striolar hair cells. CaR immunolabeling was restricted to the stereociliary array. Immunolabeling for other calcium-binding proteins was generally seen in both the cell body and hair bundles of hair cells, although this labeling was often localized to the stereociliary array and the apical portion of the cell body. CaM and PA immunolabeling in the stereociliary array in saccular and utricular striolar cells suggests a functional role for these proteins in mechanoelectric transduction and adaptation.

ATPase activity of myosin in hair bundles of the bullfrog's sacculus

Mechanoelectrical transduction by a hair cell displays adaptation, which is thought to occur as myosin-based molecular motors within the mechanically sensitive hair bundle adjust the tension transmitted to transduction channels. To assess the enzymatic capabilities of the myosin isozymes in hair bundles, we examined the actin-dependent ATPase activity of bundles isolated from the bullfrog's sacculus. Separation of 32P-labeled inorganic phosphate from unreacted [gamma-32P]ATP by thin-layer chromatography enabled us to measure the liberation of as little as 0.1 fmol phosphate. To distinguish the Mg(2+)-ATPase activity of myosin isozymes from that of other hair-bundle enzymes, we inhibited the interaction of hair-bundle myosin with actin and determined the reduction in ATPase activity. N-ethylmaleimide (NEM) decreased neither physiologically measured adaptation nor the nucleotide-hydrolytic activity of a 120-kDa protein thought to be myosin 1 beta. The NEM-insensitive, actin-activated ATPase activity of myosin increased from 1.0 fmol x s-1 in 1 mM EGTA to 2.3 fmol x s-1 in 10 microM Ca2+. This activity was largely inhibited by calmidazolium, but was unaffected by the addition of exogenous calmodulin. These results, which indicate that hair bundles contain enzymatically active, Ca(2+)-sensitive myosin molecules, are consistent with the role of Ca2+ in adaptation and with the hypothesis that myosin forms the hair cell's adaptation motor.

Function of myosin-V in filopodial extension of neuronal growth cones

The molecular mechanisms underlying directed motility of growth cones have not been determined. The role of myosin-V, an unconventional myosin, in growth cone dynamics was examined by chromophore-assisted laser inactivation (CALI). CALI of purified chick brain myosin-V absorbed onto nitrocellulose-coated cover slips inhibited the ability of myosin-V to translocate actin filaments. CALI of myosin-V in growth cones of chick dorsal root ganglion neurons resulted in rapid filopodial retraction. The rate of filopodial extension was significantly decreased, whereas the rate of filopodial retraction was not affected, which suggests a specific role for myosin-V in filopodial extension.

Enzymatic characterization and functional domain mapping of brain myosin-V

The actin binding and ATPase properties, as well as the functional domain structure of chick brain myosin-V, a two-headed, unconventional myosin, is reported here. Compared to conventional myosin from skeletal muscle, brain myosin-V exhibits low K-EDTA- and Ca-ATPase activities (1.8 and 0.8 ATP/s per head). The physiologically relevant Mg-ATPase is also low (approximately 0.3 ATP/s), unless activated by the presence of both F-actin and Ca2+ (Vmax of 27 ATP/s). Ca2+ stimulates the actin-activated Mg-ATPase over a narrow concentration range between 1 and 3 microM. In the presence of saturating Ca2+ and 75 mM KCl, surprisingly low concentrations of F-actin activate the Mg-ATPase in a hyperbolic manner (KATPase of 1.3 microM). Brain myosin-V also binds with relatively high affinity (compared to other known myosins) to F-actin in the presence of ATP, as assayed by cosedimentation. Digestion of brain myosin-V with calpain yielded a 65-kDa head domain fragment that cosediments with actin in an ATP-sensitive manner and a 80-kDa tail fragment that does not interact with F-actin. The 80-kDa fragment results from cleavage one residue beyond the proline-, glutamate-, serine-, threonine-rich region. Our data indicate that the Mg-ATPase cycle of brain myosin-V is tightly regulated by Ca2+, probably via direct binding to the calmodulin light chains in the neck domain, which like brush border myosin-I, results in partial (approximately 30%) dissociation of the calmodulin associated with brain myosin-V. The effect of Ca2+ binding, which appears to relieve suppression by the neck domain, can be mimicked by calpain cleavage near the head/neck junction.

Brush border myosin-I truncated in the motor domain impairs the distribution and the function of endocytic compartments in an hepatoma cell line

Myosins I, a ubiquitous monomeric class of myosins that exhibits actin-based motor properties, are associated with plasma and/or vesicular membranes and have been suggested as players for trafficking events between cell surface and intracellular membranous structures. To investigate the function of myosins 1, we have transfected a mouse hepatoma cell line (BWTG3) with cDNAs encoding the chicken brush border myosin-I (BBMI) and two variants truncated in the motor domain. One variant is deleted of the first 446 amino acids and thereby lacks the ATP binding site, whereas the other is deleted of the entire motor domain and lacks the ATP and actin binding sites. We have observed (i) that significant amounts of the truncated variants are recovered with membrane fractions after cell fractionation, (ii) that they codistribute with a compartment containing alpha2-macroglobulin internalized for 30 min as determined by fluorescent microscopy, (iii) that the production of BBMI-truncated variants impairs the distribution of the acidic compartment and ligands internalized for 30 min, and (iv) that the production of the truncated variant containing the actin binding site decreases the rate of alpha2-macroglobulin degradation whereas the production of the variant lacking the ATP binding site and the actin binding site increases the rate of a2-macroglobulin degradation. These observations indicate that the two truncated variants have a dominant negative effect on the distribution and the function of the endocytic compartments. We propose that an unidentified myosin-I might contribute to the distribution of endocytic compartments in a juxtanuclear position and/or to the regulation of the delivery of ligands to the degradative compartment in BWTG3 cells.

Localization of the rat myosin I molecules myr 1 and myr 2 and in vivo targeting of their tail domains

Myr 1 is a widely distributed mammalian myosin I molecule related to brush border myosin 1. A second widely distributed myosin I molecule similar to myr 1 and brush border myosin I, called myr 2, has now been identified. Specific antibodies and expression of epitope-tagged molecules were used to determine the subcellular localization of myr 1 and myr 2 in NRK cells. Myr 1 was detected at the plasma membrane and was particularly enriched in cell protrusions like lamellipodia, membrane ruffles and filopodia. In dividing cells myr 1 localized to the cleavage furrow. Myr 2 was localized in a discrete punctate pattern in resting cells and in cells undergoing cytokinesis. In subcellular fractionation experiments myr 1 and myr 2 were both partly soluble and partly associated with smooth membranes of medium density. The tail domains of myosin I molecules have been proposed to interact with a receptor and thereby determine the subcellular localization. To test this hypothesis we expressed the tail domains of myr 1 and myr 2 that lack the F-actin-binding myosin head domain in NRK cells. These tail domains also partly copurified with smooth membranes of medium density and immunolocalized similar to the respective endogenous myosin I; however, they exhibited a lower affinity for membranes and an increased diffuse cytosolic localization. These results suggest that the tail domains of myr 1 and myr 2 are sufficient for subcellular targeting but that their head domains also contribute significantly to maintaining a proper subcellular localization.

Recombinant expression of the brush border myosin I heavy chain

Although the specific functions of myosin I motors are not known, their localization to membrane structures suggests a function in membrane motility. Different myosin I isoforms in the same cell or in different cells can possess different localizations. To determine if the localization and biochemical activity of the best-characterized mammalian myosin I, chicken intestinal epithelium brush border myosin I, was dependent on determinants of the membrane or actin cytoskeleton specific to epithelial cells, we transfected the cDNA for the heavy chain of this myosin into COS cells. Transient transfection of COS cells with the chicken brush border myosin heavy chain resulted in the production of recombinant myosin I. Recombinant brush border myosin I localized to protrusions of the plasma membrane, particularly at spreading edges, and also to unknown cytoplasmic structures. Some cells expressing particularly high levels of brush border myosin I possessed a highly irregular surface. Recombinant brush border myosin I purified from COS cells bound to actin filaments in an ATP-dependent manner and decorated actin filaments to form a characteristic appearance. The recombinant myosin also catalyzed calcium-sensitive, actin-activated MgATPase activity similar to that of the native enzyme. Thus, any cellular factor required for the general membrane localization or biochemical activity of brush border myosin I is present in COS cells as well as intestinal epithelium.

Regulation of actomyosin and contraction in smooth muscle

Unlike striated muscle cells, smooth muscle cells do not have an organized sarcomeric structure. However, all smooth muscle cells contain the contractile proteins, myosin, actin, and tropomyosin. Polymorphism of the myosin heavy chain exists in smooth muscle cells. Two myosin heavy chain (MHC) isoforms, SM1 (204 kDa) and SM2 (200 kDa), are present in smooth muscle cells; however, their ratios vary in smooth muscles from different sources. The hypertrophy of the urinary bladder induced by partial outlet obstruction in rabbits is associated with an alteration of the SM1-to-SM2 ratio from 1:3 to 1:1. Both heavy chains react with polyclonal antibody against smooth muscle myosin; however, antibody prepared against a peptide from the C-terminal region of the SM2 heavy chain cross-reacts only with the SM2 heavy chain. Removal of the obstruction reverses the bladder to normal mass with a concomitant change in the SM1-to-SM2 ratio back to 1:3. The expression of the SM1 mRNA is increased in response to obstruction-induced hypertrophy, and it also returns to normal upon removal of the obstruction. Urinary bladder smooth muscle contains predominantly gamma-actin. Obstruction-induced hypertrophy of the bladder smooth muscle is associated with an increase in the gamma-actin at both protein and mRNA levels. The beta-non-muscle actin is decreased and the alpha-smooth muscle actin is unchanged in response to obstruction-induced bladder hypertrophy. Contraction of all smooth muscles involves similar mechanisms. This review describes our current understanding of the mechanisms regulating contraction of the smooth muscle of the urinary bladder.

Differential calmodulin binding to three myosin-1 isoforms from liver

We have previously purified and characterized two myosin-1 isoforms from rat liver (molecular masses 130 kDa and 110 kDa; L. M. Coluccio and C. Conaty (1993) Cell Motil. Cytoskel. 24, 189–199). Here, we describe the purification and characterization from liver of a third myosin-1 (molecular mass 105 kDa) and determine the number of calmodulin molecules associated with each of these three myosin-1 isoforms. The 105 kDa polypeptide, solubilized from liver homogenates with the addition of ATP, co-sediments with F-actin, co-purifies with calmodulin, and binds calmodulin in the presence of EGTA. Antibodies directed against chicken intestinal brush border myosin-1 cross-react with the 105 kDa polypeptide on immunoblots. Partial peptide sequence analysis indicates that the polypeptide corresponds with an MM1 gamma gene product that represents a myosin-1 isoform cloned from mouse brain (Sherr et al. (1993) J. Cell Biol. 120, 1405–1416). A comparison of calmodulin binding to the now three isolated forms of myosin-1 in liver shows that in solution the 105 kDa and 110 kDa polypeptides bind two molecules of calmodulin each whereas the 130 kDa binds six molecules of calmodulin.

Domain structure of a mammalian myosin I beta

We have determined the primary structure of a myosin I (called mammalian myosin I beta, MMI beta) from bovine brain and identified its functional domains. The protein was previously purified from brain and adrenal gland. Several constructs were generated and expressed in Escherichia coli as glutathione S-transferase fusion proteins and the recombinant proteins were recognized by monoclonal antibodies that recognize either "head" or "tail" domains of native myosin I. A gel overlay method was used to confirm that calmodulin binds to the consensus calmodulin-binding sequence in MMI beta. Binding assays were used to detect interaction with anionic phospholipid vesicles. We conclude that MMI beta consists of an amino-terminal 80.5-kDa domain that contains the ATP- and actin-binding sites, followed by an 8.5-kDa domain with three calmodulin-binding sequences and a basic 30-kDa carboxyl-terminal tail segment that binds to anionic phospholipids and membranes.

Rat myr 4 defines a novel subclass of myosin I: identification, distribution, localization, and mapping of calmodulin-binding sites with differential calcium sensitivity

We report the identification and characterization of myr 4 (myosin from rat), the first mammalian myosin I that is not closely related to brush border myosin I. Myr 4 contains a myosin head (motor) domain, a regulatory domain with light chain binding sites and a tail domain. Sequence analysis of myosin I head (motor) domains suggested that myr 4 defines a novel subclass of myosin I's. This subclass is clearly different from the vertebrate brush border myosin I subclass (which includes myr 1) and the myosin I subclass(es) identified from Acanthamoeba castellanii and Dictyostelium discoideum. In accordance with this notion, a detailed sequence analysis of all myosin I tail domains revealed that the myr 4 tail is unique, except for a newly identified myosin I tail homology motif detected in all myosin I tail sequences. The Ca(2+)-binding protein calmodulin was demonstrated to be associated with myr 4. Calmodulin binding activity of myr 4 was mapped by gel overlay assays to the two consecutive light chain binding motifs (IQ motifs) present in the regulatory domain. These two binding sites differed in their Ca2+ requirements for optimal calmodulin binding. The NH2-terminal IQ motif bound calmodulin in the absence of free Ca2+, whereas the COOH-terminal IQ motif bound calmodulin in the presence of free Ca2+. A further Ca(2+)-dependent calmodulin binding site was mapped to amino acids 776-874 in the myr 4 tail domain. These results demonstrate a differential Ca2+ sensitivity for calmodulin binding by IQ motifs, and they suggest that myr 4 activity might be regulated by Ca2+/calmodulin. Myr 4 was demonstrated to be expressed in many cell lines and rat tissues with the highest level of expression in adult brain tissue. Its expression was developmentally regulated during rat brain ontogeny, rising 2-3 wk postnatally, and being maximal in adult brain. Immunofluorescence localization demonstrated that myr 4 is expressed in subpopulations of neurons. In these neurons, prominent punctate staining was detected in cell bodies and apical dendrites. A punctate staining that did not obviously colocalize with the bulk of F-actin was also observed in C6 rat glioma cells. The observed punctate staining for myr 4 is reminiscent of a membranous localization.

Molecular motors and cell motility in the brain

The advent of video computer-enhanced microscopy has provided a new vision of cell migrations, growth cones, and fast axonal transport in the nervous system. In images obtained in studies of fast transport in isolated axoplasm from the squid giant axon, a virtual torrent of membrane traffic could be seen moving in both directions. Similarly, examination of growth cones and cell migrations in vitro and in vivo revealed properties of cell motility that were previously unsuspected. Evidence has accumulated that many of these activities are driven by a variety of microtubule and microfilament based motors.

A novel myosin I from bovine adrenal gland

A 3.5 kb cDNA clone was isolated from bovine adrenal gland cDNA library. The clone contained a full-length 3.1 kb open reading frame, encoding a novel myosin I. The deduced amino acid sequence was highly homologous to other known myosin Is in the N-terminal 2 kb region which corresponds to the myosin head domain, while no strong homology was detected in the tail region. The head-tail junction contained the Ca(2+)-independent calmodulin binding consensus sequence, suggesting that the novel myosin I binds calmodulin. This was confirmed by calmodulin overlay which showed the binding of 125I-calmodulin to the recombinant myosin I expressed in E. coli. Northern blots with probes from head and tail regions of this myosin I revealed that this novel myosin I is widely distributed among various tissues.

Intracellular calcium and cAMP regulate directional pigment movements in teleost erythrophores

Teleost pigment cells (erythrophores and melanophores) are useful models for studying the regulation of rapid, microtubule-dependent organelle transport. Previous studies suggest that melanophores regulate the direction of pigment movements via changes in intracellular cAMP (Rozdzial and Haimo, 1986a; Sammak et al., 1992), whereas erythrophores may use calcium- (Ca(2+)-) based regulation (Luby-Phelps and Porter, 1982; McNiven and Ward, 1988). Despite these observations, there have been no direct measurements in intact erythrophores or any cell type correlating changes of intracellular free Ca2+ ([Ca2+]i) with organelle movements. Here we demonstrate that extracellular Ca2+ is necessary and that a Ca2+ influx via microinjection is sufficient to induce pigment aggregation in erythrophores, but not melanophores of squirrel fish. Using the Ca(2+)-sensitive indicator, Fura-2, we demonstrate that [Ca2+]i rises dramatically concomitant with aggregation of pigment granules in erythrophores, but not melanophores. In addition, we find that an erythrophore stimulated to aggregate pigment will immediately transmit a rise in [Ca2+]i to neighboring cells, suggesting that these cells are electrically coupled. Surprisingly, we find that a fall in [Ca2+]i is not sufficient to induce pigment dispersion in erythrophores, contrary to the findings obtained with the ionophore and lysed-cell models (Luby-Phelps and Porter, 1982; McNiven and Ward, 1988). We find that a rise in intracellular cAMP ([cAMP]i) induces pigment dispersion, and that this dispersive stimulus can be overridden by an aggregation stimulus, suggesting that both high [cAMP]i and low [Ca2+]i are necessary to produce pigment dispersion in erythrophores.

Myosin I localizes to the midbody region during mammalian cytokinesis

During cytokinesis, daughter cells are cleaved in two by the constriction of an actin-rich contractile ring which encircles the equator of the dividing cell. Filamentous myosin II is present in the contractile ring and necessary for constriction of the furrow, as shown in several cell types [Satterwhite and Pollard, 1992: Curr. Opin. Cell Biol. 4:43-52]. However, no functional role nor distinctive localization has been previously identified for non-filamentous "unconventional" myosins, such as myosin I, during cytokinesis. Using antibodies to adrenal medullary myosin I, we report that myosin I is localized in 3T3 fibroblasts to the mid-equatorial plane during late-cytokinesis, as well as to the polar edges as previously described in ameboid cells [Fukui et al., 1989: Nature 341:328-331]. Confocal microscopy revealed that myosin I is concentrated at the midbody region in a nearly continuous transverse disk, extending from the cortical region of the furrow through the midbody itself. These findings suggest that, in addition to the accepted role of filamentous myosin II in constriction of the contractile ring, nonfilamentous myosin I might contribute to motile events occurring late in cytokinesis.

Differential regulation of skeletal muscle myosin-II and brush border myosin-I enzymology and mechanochemistry by bacterially produced tropomyosin isoforms

In this report, we have compared the physical properties and actin-binding characteristics of several bacterially produced nonmuscle and striated muscle tropomyosins, and we have examined the effects of these isoforms on the interactions of actin with two structurally distinct classes of myosin: striated muscle myosin-II and brush border (BB) myosin-I. All of the bacterially produced nonmuscle tropomyosins bind to F-actin with the expected stoichiometry and with affinities comparable to that of a tissue produced alpha-tropomyosin, although the striated muscle tropomyosin CTm7 has a lower affinity for F-actin than a tissue-purified striated muscle alpha tropomyosin. The bacterially produced isoforms also protect F-actin from severing by villin as effectively as tissue-purified striated muscle alpha-tropomyosin. The bacterially produced 284 amino acid striated muscle tropomyosin isoform CTm7, the 284 amino acid nonmuscle tropomyosin isoform CTm4, and two chimeric tropomyosins (CTm47 and CTm74) all inhibit the actin-activated MgATPase activity of muscle myosin S1 by approximately 70-85%, comparable to the inhibition seen with tissue-purified striated muscle alpha tropomyosin. The 248 amino acid tropomyosin XTm4 stimulated the actin-activated MgATPase activity of muscle myosin S1 approximately two- to threefold. The in vitro sliding of actin filaments translocated by muscle myosin-II (2.4 microns/sec at 19 degrees C, 5.0 microns/s at 24 degrees C) increased 25-65% in the presence of XTm4. Tropomyosins CTm4, CTm7, CTm47, and CTm74 had no detectable effect on myosin-II motility. The actin-activated MgATPase activity of BB myosin-I was inhibited 75-90% by all of the tropomyosin isoforms tested, including the 248 amino acid tropomyosin XTm4. BB myosin-I motility (50 nm/s) was completely inhibited by both the 248 and 284 amino acid tropomyosins. These results demonstrate that bacterially produced tropomyosins can differentially regulate myosin enzymology and mechanochemistry, and suggest a role for tropomyosin in the coordinated regulation of myosin isoforms in vivo.

Novel 130-kDa rat liver myosin-1 will translocate actin filaments

We have recently purified and characterized from rat liver, polypeptides of 110-kDa and 130-kDa which possess several characteristics of myosin-1 [Coluccio and Conaty: Cell Motil. Cytoskeleton 24:189-199, 1993]. What roles these myosin-1 molecules play in hepatocytes is not yet defined. One hypothesis is that they are involved in either intracellular transport or locomotion. As a first step in establishing their function, we have investigated whether these molecules are capable of supporting motility in vitro. Our results clearly demonstrate that the isolated 130-kDa-calmodulin complex will translocate filaments at a rate of 0.03-0.05 microns/sec; motility is inhibited in free calcium ion concentrations above 0.1 microM. This inhibition is reversed with the addition of exogenous calmodulin. These results provide supporting evidence of a motile role for the 130-kDa-calmodulin complex in vivo. This is the first demonstration that in higher eukaryotes, myosin-1 from a tissue other than intestine will support motility. Partial peptide sequence analysis indicates that the 130-kDa polypeptide resembles the recently described myr 1 [Ruppert et al.: J. Cell Biol. 120:1393-1403, 1993] or MM1 alpha [Sherr et al.: J. Cell Biol. 1405-1416, 1993] gene product.