Effects of the regulatory light chain phosphorylation of myosin II on mitosis and cytokinesis of mammalian cells

Smooth muscle myosin phosphorylated at single head shows sustained mechanical activity

Smooth muscle contraction is regulated by the phosphorylation of myosin. It is well known that tonic smooth muscles can maintain force with low energy consumption (latch state); however, the molecular mechanism underlying this phenomenon is unresolved. Here we show that single-head phosphorylated smooth myosin (SHPMII) exhibits fast ( approximately 24 s(-1)) and slow prolonged ( approximately 1 s(-1)) actin interactions, whereas double-head phosphorylated myosin (DHPMII) predominantly exhibits the fast ( approximately 29 s(-1)) interaction, suggesting that the phosphorylated head of SHPMII is mechanically as active as that of DHPMII. Both the fast and the slow actin interactions of SHPMII support the positive net mechanical displacement of actin. The actin translocating velocity of SHPMII was much slower than that of DHPMII, which is consistent with the slow actin interaction of SHPMII. We propose that the "latch state" can be explained by the motor characteristics of SHPMII that is present during the sustained phase of contraction.

A novel regulatory mechanism of myosin light chain phosphorylation via binding of 14-3-3 to myosin phosphatase

Myosin II phosphorylation-dependent cell motile events are regulated by myosin light-chain (MLC) kinase and MLC phosphatase (MLCP). Recent studies have revealed myosin phosphatase targeting subunit (MYPT1), a myosin-binding subunit of MLCP, plays a critical role in MLCP regulation. Here we report the new regulatory mechanism of MLCP via the interaction between 14-3-3 and MYPT1. The binding of 14-3-3beta to MYPT1 diminished the direct binding between MYPT1 and myosin II, and 14-3-3beta overexpression abolished MYPT1 localization at stress fiber. Furthermore, 14-3-3beta inhibited MLCP holoenzyme activity via the interaction with MYPT1. Consistently, 14-3-3beta overexpression increased myosin II phosphorylation in cells. We found that MYPT1 phosphorylation at Ser472 was critical for the binding to 14-3-3. Epidermal growth factor (EGF) stimulation increased both Ser472 phosphorylation and the binding of MYPT1-14-3-3. Rho-kinase inhibitor inhibited the EGF-induced Ser472 phosphorylation and the binding of MYPT1-14-3-3. Rho-kinase specific siRNA also decreased EGF-induced Ser472 phosphorylation correlated with the decrease in MLC phosphorylation. The present study revealed a new RhoA/Rho-kinase-dependent regulatory mechanism of myosin II phosphorylation by 14-3-3 that dissociates MLCP from myosin II and attenuates MLCP activity.

The phosphorylation of myosin II at the Ser1 and Ser2 is critical for normal platelet-derived growth factor induced reorganization of myosin filaments

Phosphorylation of the regulatory light chain of myosin II (MLC(20)) at the activation sites promotes both the motor activity and the filament formation of myosin II, thus playing an important role in various cell motile processes. In contrast, the physiological function of phosphorylation of MLC(20) at the inhibitory sites is unknown. Here we report for the first time the function of the inhibitory site phosphorylation in the cells. We successfully produced the antibodies specifically recognizing the phosphorylation sites of MLC(20) at Ser1, and the platelet-derived growth factor (PDGF)-induced change in the phosphorylation at the Ser1 was monitored. The phosphorylation of MLC(20) at the Ser1 significantly increased during the PDGF-induced actin cytoskeletal reorganization. PDGF disassembled the stress fibers, and this was attenuated with the expression of unphosphorylatable MLC(20) at the Ser1/Ser2 phosphorylation sites. The present results suggest that the down-regulation of myosin II activity achieved by the phosphorylation at the Ser1/Ser2 sites plays an important role in the normal reorganization of actomyosin filaments triggered by PDGF receptor stimulation.

Two distinct modes of myosin assembly and dynamics during epithelial wound closure

Actomyosin contraction powers the sealing of epithelial sheets during embryogenesis and wound closure; however, the mechanisms are poorly understood. After laser ablation wounding of Madin–Darby canine kidney cell monolayers, we observed distinct steps in wound closure from time-lapse images of myosin distribution during resealing. Immediately upon wounding, actin and myosin II regulatory light chain accumulated at two locations: (1) in a ring adjacent to the tight junction that circumscribed the wound and (2) in fibers at the base of the cell in membranes extending over the wound site. Rho-kinase activity was required for assembly of the myosin ring, and myosin II activity was required for contraction but not for basal membrane extension. As it contracted, the myosin ring moved toward the basal membrane with ZO-1 and Rho-kinase. Thus, we suggest that tight junctions serve as attachment points for the actomyosin ring during wound closure and that Rho-kinase is required for localization and activation of the contractile ring.

Calyculin A, an enhancer of myosin, speeds up anaphase chromosome movement

Actin and myosin inhibitors often blocked anaphase movements in insect spermatocytes in previous experiments. Here we treat cells with an enhancer of myosin, Calyculin A, which inhibits myosin-light-chain phosphatase from dephosphorylating myosin; myosin thus is hyperactivated. Calyculin A causes anaphase crane-fly spermatocyte chromosomes to accelerate poleward; after they reach the poles they often move back toward the equator. When added during metaphase, chromosomes at anaphase move faster than normal. Calyculin A causes prometaphase chromosomes to move rapidly up and back along the spindle axis, and to rotate. Immunofluorescence staining with an antibody against phosphorylated myosin regulatory light chain (p-squash) indicated increased phosphorylation of cleavage furrow myosin compared to control cells, indicating that calyculin A indeed increased myosin phosphorylation. To test whether the Calyculin A effects are due to myosin phosphatase or to type 2 phosphatases, we treated cells with okadaic acid, which inhibits protein phosphatase 2A at concentrations similar to Calyculin A but requires much higher concentrations to inhibit myosin phosphatase. Okadaic acid had no effect on chromosome movement. Backward movements did not require myosin or actin since they were not affected by 2,3-butanedione monoxime or LatruculinB. Calyculin A affects the distribution and organization of spindle microtubules, spindle actin, cortical actin and putative spindle matrix proteins skeletor and titin, as visualized using immunofluorescence. We discuss how accelerated and backwards movements might arise.

Re-expression of proteins involved in cytokinesis during cardiac hypertrophy

Cardiomyocytes stop dividing after birth and postnatal heart growth is only achieved by increase in cell volume. In some species, cardiomyocytes undergo an additional incomplete mitosis in the first postnatal week, where karyokinesis takes place in the absence of cytokinesis, leading to binucleation. Proteins that regulate the formation of the actomyosin ring are known to be important for cytokinesis. Here we demonstrate for the first time that small GTPases like RhoA along with their downstream effectors like ROCK I, ROCK II and Citron Kinase show a developmental stage specific expression in heart, with high levels being expressed in cardiomyocytes only at stages when cytokinesis still occurs (i.e. embryonic and perinatal). This suggests that downregulation of many regulatory and cytoskeletal components involved in the formation of the actomyosin ring may be responsible for the uncoupling of cytokinesis from karyokinesis in rodent cardiomyocytes after birth. Interestingly, when the myocardium tries to adapt to the increased workload during pathological hypertrophy a re-expression of proteins involved in DNA synthesis and cytokinesis can be detected. Nevertheless, the adult cardiomyocytes do not appear to divide despite this upregulation of the cytokinetic machinery. The inability to undergo complete division could be due to the presence of stable, highly ordered and functional sarcomeres in the adult myocardium or could be because of the inefficiency of degradation pathways, which facilitate the division of differentiated embryonic cardiomyocytes by disintegrating myofibrils.

Hyperoxic conditions inhibit airway smooth muscle myosin phosphatase in rat pups

Exposure of rat pups to 100% oxygen is a model for studying neonatal lung injury. Airway reactivity is increased in this model, in part due to impaired airway smooth muscle (ASM) relaxation. We compared biochemical determinants of ASM contractility in rat pups exposed to 100% oxygen for 7 days vs. littermates raised in room air. The baseline quantities of ASM contractile proteins, extent of phosphorylation of the 20-kDa myosin regulatory light chain (LC20), and amount of the myosin-binding subunit of smooth muscle myosin phosphatase (MYPT) were all comparable between the two groups. Bethanechol-induced contraction increased the extent of phosphorylation of both LC20 and MYPT in the hyperoxic group (45% and 70% over control, respectively). Relaxation after electrical field stimulation demonstrated greater phosphorylation of both LC20 and MYPT in the hyperoxic group compared with controls (67% and 84%, respectively). To determine if hyperoxia induced changes in the isoforms of MYPT, isoform expression was also compared but differences were not found. To determine potential mechanisms whereby MYPT phosphorylation was increased by hyperoxia, separate tracheas were treated with the Rho kinase inhibitor Y-27632. This treatment completely eliminated differences in MYPT phosphorylation between the groups. Because phosphorylation of MYPT impairs the phosphatase activity of myosin phosphatase, these data suggest that hyperoxic conditioning during early postnatal life impairs relaxation through prolonging LC20 phosphorylation. This mechanism might contribute to increased ASM reactivity seen in bronchopulmonary dysplasia.

Regulation of myosin II dynamics by phosphorylation and dephosphorylation of its light chain in epithelial cells

Nonmuscle myosin II, an actin-based motor protein, plays an essential role in actin cytoskeleton organization and cellular motility. Although phosphorylation of its regulatory light chain (MRLC) is known to be involved in myosin II filament assembly and motor activity in vitro, it remains unclear exactly how MRLC phosphorylation regulates myosin II dynamics in vivo. We established clones of Madin Darby canine kidney II epithelial cells expressing MRLC-enhanced green fluorescent protein or its mutants. Time-lapse imaging revealed that both phosphorylation and dephosphorylation are required for proper dynamics of myosin II. Inhibitors affecting myosin phosphorylation and MRLC mutants indicated that monophosphorylation of MRLC is required and sufficient for maintenance of stress fibers. Diphosphorylated MRLC stabilized myosin II filaments and was distributed locally in regions of stress fibers where contraction occurs, suggesting that diphosphorylation is involved in the spatial regulation of myosin II assembly and contraction. We further found that myosin phosphatase or Zipper-interacting protein kinase localizes to stress fibers depending on the activity of myosin II ATPase.

Molecular cloning and functional analysis of MRLC2 differential expressed in MeishanxYorkshire F1 crossbreeds and their parents, Meishan pigs

In order to detect the molecular basis of heterosis in pigs, suppression subtractive hybridization was carried out to investigate the difference in gene expression in the Longissimus dorsi muscle tissues between MeishanxYorkshire F1 crossbreeds and their parents, Meishan pigs. The swine myosin regulatory light chain 2 (MRLC2) gene differentially expressed between the crossbreeds and the purebreds was isolated and identified using semi-quantitative reverse transcriptase polymerase chain reaction and its complete cDNA sequence was obtained using the rapid amplification of cDNA ends method. The nucleotide sequence of the gene is not homologous to any of the known porcine genes. The sequence prediction analysis reveals that the open reading frame of this gene encodes a protein of 172 amino acids containing the putative conserved domain of the EF-hand superfamily. This predicted amino acid sequence of porcine MRLC2 protein exhibits 99%, 98%, 98%, 98% and 97% identity with that of cattle, human, dog, rat and mouse, respectively. The homology analysis revealed that the MRLC2 protein was very much conserved in evolution. The tissue expression analysis indicated that the swine MRLC2 gene is highly expressed in muscle, fat, heart, liver, spleen, lung, kidney, stomach, small intestine, ovary and testis, but not expressed in pancreas.

Native nonmuscle myosin II stability and light chain binding inDrosophila melanogaster

Native nonmuscle myosin IIs play essential roles in cellular and developmental processes throughout phylogeny. Individual motor molecules consist of a heterohexameric complex of three polypeptides which, when properly assembled, are capable of force generation. Here, we more completely characterize the properties, relationships and associations that each subunit has with one another in Drosophila melanogaster. All three native nonmuscle myosin II polypeptide subunits are expressed in close to constant stoichiometry to each other throughout development. We find that the stability of two subunits, the heavy chain and the regulatory light chain, depend on one another whereas the stability of the third subunit, the essential light chain, does not depend on either the heavy chain or regulatory light chain. We demonstrate that heavy chain aggregates, which form when regulatory light chain is lacking, associate with the essential light chain in vivo-thus showing that regulatory light chain association is required for heavy chain solubility. By immunodepletion we find that the majority of both light chains are associated with the nonmuscle myosin II heavy chain but pools of free light chain and/or light chain bound to other proteins are present. We identify four myosins (myosin II, myosin V, myosin VI and myosin VIIA) and a microtubule-associated protein (asp/Abnormal spindle) as binding partners for the essential light chain (but not the regulatory light chain) through mass spectrometry and co-precipitation. Using an in silico approach we identify six previously uncharacterized genes that contain IQ-motifs and may be essential light chain binding partners.

A global, myosin light chain kinase-dependent increase in myosin II contractility accompanies the metaphase-anaphase transition in sea urchin eggs

Myosin II is the force-generating motor for cytokinesis, and although it is accepted that myosin contractility is greatest at the cell equator, the temporal and spatial cues that direct equatorial contractility are not known. Dividing sea urchin eggs were placed under compression to study myosin II-based contractile dynamics, and cells manipulated in this manner underwent an abrupt, global increase in cortical contractility concomitant with the metaphase-anaphase transition, followed by a brief relaxation and the onset of furrowing. Prefurrow cortical contractility both preceded and was independent of astral microtubule elongation, suggesting that the initial activation of myosin II preceded cleavage plane specification. The initial rise in contractility required myosin light chain kinase but not Rho-kinase, but both signaling pathways were required for successful cytokinesis. Last, mobilization of intracellular calcium during metaphase induced a contractile response, suggesting that calcium transients may be partially responsible for the timing of this initial contractile event. Together, these findings suggest that myosin II-based contractility is initiated at the metaphase-anaphase transition by Ca2+-dependent myosin light chain kinase (MLCK) activity and is maintained through cytokinesis by both MLCK- and Rho-dependent signaling. Moreover, the signals that initiate myosin II contractility respond to specific cell cycle transitions independently of the microtubule-dependent cleavage stimulus.

Rho GTPases: biochemistry and biology

Approximately one percent of the human genome encodes proteins that either regulate or are regulated by direct interaction with members of the Rho family of small GTPases. Through a series of complex biochemical networks, these highly conserved molecular switches control some of the most fundamental processes of cell biology common to all eukaryotes, including morphogenesis, polarity, movement, and cell division. In the first part of this review, we present the best characterized of these biochemical pathways; in the second part, we attempt to integrate these molecular details into a biological context.

Real-time dynamics of the F-actin cytoskeleton during secretion from chromaffin cells

Transmitted light images showed an intricate and dynamic cytoplasmic structural network in cultured bovine chromaffin cells observed under high magnification. These structures were sensitive to chemicals altering F-actin-myosin and colocalised with peripheral F-actin, beta-actin and myosin II. Interestingly, secretagogues induced a Ca2+-dependent, rapid (>10 second) and transitory (60-second cycle) disassembling of these cortical structures. The simultaneous formation of channel-like structures perpendicular to the plasmalemma conducting vesicles to the cell limits and open spaces devoid of F-actin in the cytoplasm were also observed. Vesicles moved using F-actin pathways and avoided diffusion in open, empty zones. These reorganisations representing F-actin transfer from the cortical barrier to the adjacent cytoplasmic area have been also confirmed by studying fluorescence changes in cells expressing GFP-beta-actin. Thus, these data support the function of F-actin-myosin II network acting simultaneously as a barrier and carrier system during secretion, and that transmitted light images could be used as an alternative to fluorescence in the study of cytoskeleton dynamics in neuroendocrine cells.

Epigallocatechin-3-O-gallate disrupts stress fibers and the contractile ring by reducing myosin regulatory light chain phosphorylation mediated through the target molecule 67 kDa laminin receptor

Epigallocatechin-3-O-gallate (EGCG), a major polyphenol of green tea, has been shown to inhibit the growth of various cancer cell lines. We show here that EGCG induced the disruption of stress fibers and decreased the phosphorylation of the myosin II regulatory light chain (MRLC) at Thr18/Ser19, which is necessary for both contractile ring formation and cell division. Indirect immunofluorescence analysis revealed that EGCG inhibited the concentration of both F-actin and the phosphorylated MRLC in the cleavage furrow at the equator of dividing cells. In addition, EGCG increased the percentages of cells in the G(2)/M phase and inhibited cell growth. Recently, we have demonstrated that the anticancer activity of EGCG is mediated by the metastasis-associated 67kDa laminin receptor (67LR). To explore whether the effect of EGCG is mediated by the 67LR, we transfected cells with short hairpin RNA (shRNA) expression vector to downregulate 67LR expression. When the 67LR was silenced, the suppressive effect of EGCG on the MRLC phosphorylation was significantly attenuated. These results suggest that EGCG inhibits the cell growth by reducing the MRLC phosphorylation and this effect is mediated by the 67LR.

MgcRacGAP controls the assembly of the contractile ring and the initiation of cytokinesis

Initiation of cytokinesis requires the establishment of the cleavage plane, the assembly of the contractile ring, and the ingression of the cleavage furrow. MgcRacGAP, a GTPase-activating protein for RhoA, is required for cytokinesis, but the mechanism of its action remains unknown. We report here that MgcRacGAP is required for the assembly of anillin and myosin into the contractile ring. In addition, MgcRacGAP is required for the localized activation of myosin through the RhoA-mediated phosphorylation of the myosin regulatory light chain. Cells with MgcRacGAP RNA interference (RNAi) failed cytokinesis without any ingression of the cleavage furrow. Paradoxically, MgcRacGAP, a GTPase-activating protein, associates during cytokinesis with ECT2, a guanine nucleotide exchange factor for RhoA, and the localization of ECT2 to both the central spindle and the contractile ring depends on MgcRacGAP. Knockdown of ECT2 phenocopies that of MgcRacGAP. We conclude that MgcRacGAP controls the initiation of cytokinesis by regulating ECT2, which in turn induces the assembly of the contractile ring and triggers the ingression of the cleavage furrow.

Anillin is a substrate of anaphase-promoting complex/cyclosome (APC/C) that controls spatial contractility of myosin during late cytokinesis

Anillin, an actin-binding protein localized at the cleavage furrow, is required for cytokinesis. Through an in vitro expression screen, we identified anillin as a substrate of the anaphase-promoting complex/cyclosome (APC/C), a ubiquitin ligase that controls mitotic progression. We found that the levels of anillin fluctuate in the cell cycle, peaking in mitosis and dropping drastically during mitotic exit. Ubiquitination of anillin required a destruction-box and was mediated by Cdh1, an activator of APC/C. Overexpression of Cdh1 reduced the levels of anillin, whereas inactivation of APC/C(Cdh1) increased the half-life of anillin. Functionally, anillin was required for the completion of cytokinesis. In anillin knockdown cells, the cleavage furrow ingressed but failed to complete the ingression. At late cytokinesis, the cytosol and DNA in knockdown cells underwent rapid myosin-based oscillatory movement across the furrow. During this movement, RhoA and active myosin were absent from the cleavage furrow, and myosin was redistributed to cortical patches, which powers the random oscillatory movement. We concluded that anillin functions to maintain the localization of active myosin, thereby ensuring the spatial control of concerted contraction during cytokinesis.

Regulation of myosin II during cytokinesis in higher eukaryotes

Cellular myosin II is the principal motor responsible for cytokinesis. In higher eukaryotes, phosphorylation of the regulatory light chain (MLC) of myosin II is a primary means of activating myosin II and is known to be crucial for the execution of cell division. Because signals transmitted by the mitotic spindle coordinate key spatial and temporal aspects of cytokinesis, such signals should ultimately function to activate myosin II. Thus, it follows that identification of regulatory factors involved in MLC phosphorylation should elucidate the nature of spindle-derived regulatory signals and lead to a model for how they control cytokinesis. However, the identity of these upstream molecules remains elusive. This review (which is part of the Cytokinesis series) summarizes current views of the regulatory pathway controlling MLC phosphorylation and features four candidate molecules that are likely immediate upstream myosin regulators. I discuss proposed functions for MLCK, ROCK, citron kinase and myosin phosphatase during cytokinesis and consider the possibility of a link between these molecules and the signals transmitted by the mitotic spindle.

Adhesion-dependent and contractile ring-independent equatorial furrowing during cytokinesis in mammalian cells

Myosin II-dependent contraction of the contractile ring drives equatorial furrowing during cytokinesis in animal cells. Nonetheless, myosin II-null cells of the cellular slime mold Dictyostelium divide efficiently when adhering to substrates by making use of polar traction forces. Here, we show that in the presence of 30 microM blebbistatin, a potent myosin II inhibitor, normal rat kidney (NRK) cells adhering to fibronectin-coated surfaces formed equatorial furrows and divided in a manner strikingly similar to myosin II-null Dictyostelium cells. Such blebbistatin-resistant cytokinesis was absent in partially detached NRK cells and was disrupted in adherent cells if the advance of their polar lamellipodia was disturbed by neighboring cells. Y-27632 (40 microM), which inhibits Rho-kinase, was similar to 30 microM blebbistatin in that it inhibited cytokinesis of partially detached NRK cells but only prolonged furrow ingression in attached cells. In the presence of 100 microM blebbistatin, most NRK cells that initiated anaphase formed tight furrows, although scission never occurred. Adherent HT1080 fibrosarcoma cells also formed equatorial furrows efficiently in the presence of 100 microM blebbistatin. These results provide direct evidence for adhesion-dependent, contractile ring-independent equatorial furrowing in mammalian cells and demonstrate the importance of substrate adhesion for cytokinesis.

p116Rip Decreases Myosin II Phosphorylation by Activating Myosin Light Chain Phosphatase and by Inactivating RhoA

p116Rip was originally found to be a RhoA-binding protein, but its function has been unknown. Here, we clarify the function of p116Rip. Two critical findings were made. First, we found that p116Rip activated the GTPase activity of RhoA in vitro and that p116Rip overexpression in cells consistently diminished the epidermal growth factor-induced increase in GTP-bound RhoA. Second, p116Rip activated the myosin light chain phosphatase (MLCP) activity of the holoenzyme. p116Rip did not activate the catalytic subunit alone, indicating that the activation is due to the binding of p116Rip to the myosin phosphatase targeting subunit MYPT1. Interestingly, the activation of phosphatase was specific to myosin as substrate, and p116Rip directly bound to myosin, thus facilitating myosin/MLCP interaction. The gene silencing of p116Rip consistently and significantly increased myosin phosphorylation as well as stress fiber formation in cells. Based upon these findings, we propose that p116Rip is an important regulatory component that controls the RhoA signaling pathway, thus regulating MLCP activity and myosin phosphorylation in cells.

Myosin phosphatase targeting subunit 1 affects cell migration by regulating myosin phosphorylation and actin assembly

Myosin II plays important roles in many contractile-like cell functions, including cell migration, adhesion, and retraction. Myosin II is activated by regulatory light chain (RLC) phosphorylation whereas RLC dephosphorylation by myosin light chain phosphatase containing a myosin phosphatase targeting subunit (MYPT1) leads to myosin inactivation. HeLa cells contain MYPT1 in addition to a newly identified human variant 2 containing an internal deletion. RLC dephosphorylation, cell migration, and adhesion were inhibited when either or both MYPT1 isoforms were knocked down by RNA interference. RLC was highly phosphorylated (60%) when both isoforms were suppressed by siRNA treatment relative to control cells (10%) with serum-starvation and ROCK inhibition. Prominent stress fibers and focal adhesions were associated with the enhanced RLC phosphorylation. The reintroduction of MYPT1 or variant 2 in siRNA-treated cells decreased stress fibers and focal adhesions. MYPT1 knockdown also led to an increase of F-actin relative to G-actin in HeLa cells. The myosin inhibitor blebbistatin did not inhibit this effect, indicating MYPT1 likely affects actin assembly independent of RLC phosphorylation. Proper expression of MYPT1 or variant 2 is critical for RLC phosphorylation and actin assembly, thus maintaining normal cellular functions by simultaneously controlling cytoskeletal architecture and actomyosin activation.

The N-terminus of the long MLCK induces a disruption in normal spindle morphology and metaphase arrest

We have shown previously that only the long myosin light chain kinase (MLCK), which is the predominant MLCK isoform expressed in nonmuscle cells, localizes to the cleavage furrow. To further examine the in vivo localization of the long MLCK in HeLa cells and the mechanisms responsible for kinase targeting during the cell cycle, we examined the distribution of the endogenous kinase and constructed green fluorescent protein (GFP) fusions of long HeLa MLCK truncations. A GFP fusion containing the N-terminal IgG domain and the five DXR motifs localized to stress fibers during interphase and the cleavage furrow during mitosis. Although individual fusions of the five DXRs and IgG domain both independently localized to stress fibers, only the five DXRs demonstrated a cortical localization in mitotic cells. Thus, robust targeting of the long MLCK to the cleavage furrow required the five DXRs and additional sequences from the IgG domain. Expression of the IgG domain alone or with five DXRs increased the number of multinucleate cells tenfold, whereas expression of the five DXRs or GFP had no effect. Furthermore, expression of the IgG domain alone or with five DXRs disrupted normal spindle morphology during mitosis. Extended astral microtubules and increased bundling of kinetochore microtubules, and spindle pole fragmentation were detected in mitotic cells. These microtubule defects were associated with abnormalities in metaphase chromosome alignment and a subsequent metaphase arrest caused by activation of the spindle assembly checkpoint at the kinetochores of mono-oriented chromosomes. Together, these results suggest that MLCK has an unexpected regulatory function during mitosis.

DAP-like kinase, a member of the death-associated protein kinase family, associates with centrosomes, centromers, and the contractile ring during mitosis

DAP-like kinase (Dlk) is a nuclear serine/threonine-specific kinase which has been implicated in apoptosis. However, induction of apoptosis by Dlk requires its relocation to the cytoplasm, particularly association with the actin cytoskeleton, which is achieved through interaction with pro-apoptotic protein Par-4. On the other hand, nuclear Dlk does not induce apoptosis and has rather been implicated in transcription. To further explore the biological functions of Dlk, we established a cell clone of MCF-7 cells stably expressing a GFP-Dlk fusion protein at low level. Ectopic expression of GFP-Dlk did not affect the growth properties of the cells. During interphase, GFP-Dlk showed a diffuse nuclear distribution with punctate staining in a subpopulation of cells. During mitosis, however, Dlk was associated with centrosomes, centromeres, and the contractile ring, but not with the mitotic spindle. Association with centrosomes, as confirmed by colocalization with gamma-tubulin and pericentrin persisted throughout mitosis but was also seen in interphase cells. Interestingly, GFP-Dlk and gamma-tubulin could be co-immunoprecipitated indicating that they are present in the same protein complex. Association of Dlk with centromeres, as verified by confocal fluorescence microscopy with centromere-specific antibodies was more restricted and discernable from prophase to early anaphase. Centromere association of Dlk coincides with H3 phosphorylation at Thr11 that is specifically phosphorylated by Dlk in vitro (U. Preuss, G. Landsberg, K. H. Scheidtmann, Nucleic Acids Res. 31, 878-885, 2003). During cytokinesis, Dlk was enriched in the contractile acto-myosin ring and colocalized with Ser19-phosphorylated myosin light chain, which is an in vitro substrate of Dlk. Strikingly, a C-terminal truncation mutant of Dlk generated multi-nucleated cells. Together, these data suggest that Dlk participates in regulation and, perhaps, coordination of mitotis and cytokinesis.

Myosin II-dependent cortical movement is required for centrosome separation and positioning during mitotic spindle assembly

The role of myosin II in mitosis is generally thought to be restricted to cytokinesis. We present surprising new evidence that cortical myosin II is also required for spindle assembly in cells. Drug- or RNAi-mediated disruption of myosin II in cells interferes with normal spindle assembly and positioning. Time-lapse movies reveal that these treatments block the separation and positioning of duplicated centrosomes after nuclear envelope breakdown (NEBD), thereby preventing the migration of the microtubule asters to opposite sides of chromosomes. Immobilization of cortical movement with tetravalent lectins produces similar spindle defects to myosin II disruption and suggests that myosin II activity is required within the cortex. Latex beads bound to the cell surface move in a myosin II-dependent manner in the direction of the separating asters. We propose that after NEBD, completion of centrosome separation and positioning around chromosomes depends on astral microtubule connections to a moving cell cortex.

ZIP kinase is responsible for the phosphorylation of myosin II and necessary for cell motility in mammalian fibroblasts

Reorganization of actomyosin is an essential process for cell migration and myosin regulatory light chain (MLC₂₀) phosphorylation plays a key role in this process. Here, we found that zipper-interacting protein (ZIP) kinase plays a predominant role in myosin II phosphorylation in mammalian fibroblasts. Using two phosphorylation site-specific antibodies, we demonstrated that a significant portion of the phosphorylated MLC₂₀ is diphosphorylated and that the localization of mono- and diphosphorylated myosin is different from each other. The kinase responsible for the phosphorylation was ZIP kinase because (a) the kinase in the cell extracts phosphorylated Ser19 and Thr18 of MLC₂₀ with similar potency; (b) immunodepletion of ZIP kinase from the cell extracts markedly diminished its myosin II kinase activity; and (c) disruption of ZIP kinase expression by RNA interference diminished myosin phosphorylation, and resulted in the defect of cell polarity and migration efficiency. These results suggest that ZIP kinase is critical for myosin phosphorylation and necessary for cell motile processes in mammalian fibroblasts.

New roles of myosin II during vesicle transport and fusion in chromaffin cells

Modified herpes virus (amplicons) were used to express myosin regulatory light chain (RLC) chimeras with green fluorescent protein (GFP) in cultured bovine chromaffin cells to study myosin II implication in secretion. After infection, RLC-GFP constructs were clearly identified in the cytoplasm and accumulated in the cortical region, forming a complex network that co-localized with cortical F-actin. Cells expressing wild type RLC-GFP maintained normal vesicle mobility, whereas cells expressing an unphosphorylatable form (T18A/S19A RLC-GFP) presented severe restrictions in granule movement as measured by individual tracking in dynamic confocal microscopy studies. Interestingly, the overexpression of this mutant form of RLC also affected the initial secretory burst elicited by either high K(+) or BaCl(2), as well as the secretion induced by fast release of calcium from caged compounds in individual cells. Moreover, T18A/S19A RLC-GFP-infected cells presented slower fusion kinetics of individual granules compared with controls as measured by analysis of amperometric spikes. Taken together, our results demonstrate the implication of myosin II in the transport of vesicles, and, surprisingly, in the final phases of exocytosis involving transitions affecting the activity of docked granules, and therefore uncovering a new role for this cytoskeletal element.

Variations on a theme: the many modes of cytokinesis

Animal cell division is believed to be mediated primarily by the 'purse-string' mechanism, which entails furrowing of the equatorial region, driven by the interaction of actin and myosin II filaments within contractile rings. However, myosin II-null Dictyostelium cells on substrates divide efficiently in a cell cycle-coupled manner. This process, termed cytokinesis B, appears to be driven by polar traction forces. Data in the literature can be interpreted as suggesting that adherent higher animal cells also use a cytokinesis B-like mechanism for cytokinesis. An additional chemotaxis-based cytokinesis that involves a 'midwife' cell has also been reported. Collectively, these findings demonstrate an unexpected diversity of mechanisms by which animal cells carry out cytokinesis.

C-type natriuretic peptide induces human colonic myofibroblast relaxation

Intestinal response to injury requires coordinated regulation of the tension exerted by subepithelial myofibroblasts (SEM). However, the signals governing relaxation of intestinal SEM have not been investigated. Our aim was to test the hypothesis that signal transduction pathways initiated by C-type natriuretic peptide (CNP) induce intestinal SEM relaxation. We directly quantified the effects of CNP on isometric tension exerted by cultured human colonic SEM. We also measured the effects of CNP on cGMP content, myosin regulatory light chain (MLC) phosphorylation, and cytosolic Ca2+ concentration. CNP induced relaxation of SEM within 10 s. By 10 min, relaxation reached a plateau that was sustained for 2 h. CNP-induced relaxation was saturable, with a maximal decrease in tension (51.7 +/- 3.8 dyn) observed at 250 nM. SEM relaxation in response to CNP constituted approximately 23% of total basal tension. CNP increased intracellular cGMP content and reduced MLC phosphorylation. Effects of CNP on cGMP and MLC exhibited the same dose dependence as CNP-induced relaxation. MLC phosphorylation decreased within 2 min of CNP exposure and was sustained for at least 45 min. CNP also stimulated a large transient increase in cytosolic Ca2+ concentration that occurred within 30 s and was nearly complete by 1 min. We also observed that calyculin-A, a potent inhibitor of MLC phosphatase, completely abolished the reduction in MLC phosphorylation induced by CNP. These results suggest that CNP induces intestinal SEM relaxation through cGMP-associated reductions in MLC phosphorylation. Moreover, these findings raise the possibility that CNP plays a role in intestinal wound healing.

Multiple Parallelisms in Animal Cytokinesis

The process of cytokinesis in animal cells is usually presented as a relatively simple picture: A cleavage plane is first positioned in the equatorial region by the astral microtubules of the anaphase mitotic apparatus, and a contractile ring made up of parallel filaments of actin and myosin II is formed and encircles the cortex at the division site. Active sliding between the two filament systems constricts the perimeter of the cortex, leading to separation of two daughter cells. However, recent studies in both animal cells and lower eukaryotic model organisms have demonstrated that cytokinesis is actually far more complex. It is now obvious that the three key processes of cytokinesis, cleavage plane determination, equatorial furrowing, and scission, are driven by different mechanisms in different types of cells. In some cases, moreover, multiple pathways appear to have redundant functions in a single cell type. In this review, we present a novel hypothesis that incorporates recent observations on the activities of mitotic microtubules and the biochemistry of Rho-type GTPase proteins and postulates that two different sets of microtubules are responsible for the two known mechanisms of cleavage plane determination and also for two distinct mechanisms of equatorial furrowing.

Uncoordinated regulation of stress fibers and focal adhesions by DAP kinase

Death-associated protein kinase (DAP kinase) is a proapoptotic, calcium/calmodulin-dependent serine/threonine kinase. Here, we report that DAP kinase phosphorylates the regulatory light chain of myosin II (MLC) both in vitro and in vivo, and that this phosphorylation occurs preferentially at residue Ser19. In quiescent fibroblasts, DAP kinase stabilizes stress fibers through phosphorylation of MLC, but it is dispensable for the formation of peripheral microfilament bundles. This cytoskeletal effect of DAP kinase occurs before the onset of apoptosis and does not require an intact death domain. In addition, DAP kinase is required for serum-induced stress-fiber formation, which is associated with the upregulation of its catalytic activity. Despite being both sufficient and necessary for the assembly or maintenance of stress fibers, DAP kinase is incapable of stimulating the formation of focal adhesions in quiescent cells. Moreover, it promotes the disassembly of focal adhesions but not stress fibers in cells receiving serum factors. Together, our results identify a novel and unique function of DAP kinase in the uncoupling of stress fibers and focal adhesions. Such uncoupling would lead to a perturbation of the balance between contractile and adhesion forces and subsequent cell detachment, which might contribute to its pro-apoptotic activity.

Structure of kinetochore fibres in crane-fly spermatocytes after irradiation with an ultraviolet microbeam: Neither microtubules nor actin filaments remain in the irradiated region

We studied chromosome movement after kinetochore microtubules were severed. Severing a kinetochore fibre in living crane-fly spermatocytes with an ultraviolet microbeam creates a kinetochore stub, a birefringent remnant of the spindle fibre connected to the kinetochore and extending only to the edge of the irradiated region. After the irradiation, anaphase chromosomes either move poleward led by their stubs or temporarily stop moving. We examined actin and/or microtubules in irradiated cells by means of confocal fluorescence microscopy or serial-section reconstructions from electron microscopy. For each cell thus examined, chromosome movement had been recorded continuously until the moment of fixation. Kinetochore microtubules were completely severed by the ultraviolet microbeam in cells in which chromosomes continued to move poleward after the irradiation: none were seen in the irradiated regions. Similarly, actin filaments normally present in kinetochore fibres were severed by the ultraviolet microbeam irradiations: the irradiated regions contained no actin filaments and only local spots of non-filamentous actin. There was no difference in irradiated regions when the associated chromosomes continued to move versus when they stopped moving. Thus, one cannot explain motion with severed kinetochore microtubules in terms of either microtubules or actin-filaments bridging the irradiated region. The data seem to negate current models for anaphase chromosome movement and support a model in which poleward chromosome movement results from forces generated within the spindle matrix that propel kinetochore fibres or kinetochore stubs poleward.

Myosin localization during meiosis I of crane-fly spermatocytes gives indications about its role in division

We showed previously that in crane-fly spermatocytes myosin is required for tubulin flux [Silverman-Gavrila and Forer, 2000a: J Cell Sci 113:597-609], and for normal anaphase chromosome movement and contractile ring contraction [Silverman-Gavrila and Forer, 2001: Cell Motil Cytoskeleton 50:180-197]. Neither the identity nor the distribution of myosin(s) were known. In the present work, we used immunofluorescence and confocal microscopy to study myosin during meiosis-I of crane-fly spermatocytes compared to tubulin, actin, and skeletor, a spindle matrix protein, in order to further understand how myosin might function during cell division. Antibodies to myosin II regulatory light chain and myosin II heavy chain gave similar staining patterns, both dependent on stage: myosin is associated with nuclei, asters, centrosomes, chromosomes, spindle microtubules, midbody microtubules, and contractile rings. Myosin and actin colocalization along kinetochore fibers from prometaphase to anaphase are consistent with suggestions that acto-myosin forces in these stages propel kinetochore fibres poleward and trigger tubulin flux in kinetochore fibres, contributing in this way to poleward chromosome movement. Myosin and actin colocalization at the cell equator in cytokinesis, similar to studies in other cells [e.g., Fujiwara and Pollard, 1978: J Cell Biol 77:182-195], supports a role of actin-myosin interactions in contractile ring function. Myosin and skeletor colocalization in prometaphase spindles is consistent with a role of these proteins in spindle formation. After microtubules or actin were disrupted, myosin remained in spindles and contractile rings, suggesting that the presence of myosin in these structures does not require the continued presence of microtubules or actin. BDM (2,3 butanedione, 2 monoxime) treatment that inhibits chromosome movement and cytokinesis also altered myosin distributions in anaphase spindles and contractile rings, consistent with the physiological effects, suggesting also that myosin needs to be active in order to be properly distributed.

Phosphorylation of myosin II regulatory light chain is necessary for migration of HeLa cells but not for localization of myosin II at the leading edge

To investigate the role of phosphorylated myosin II regulatory light chain (MRLC) in living cell migration, these mutant MRLCs were engineered and introduced into HeLa cells. The mutant MRLCs include an unphosphorylatable form, in which both Thr-18 and Ser-19 were substituted with Ala (AA-MRLC), and pseudophosphorylated forms, in which Thr-18 and Ser-19 were replaced with Ala and Asp, respectively (AD-MRLC), and both Thr-18 and Ser-19 were replaced with Asp (DD-MRLC). Mutant MRLC-expressing cell monolayers were mechanically stimulated by scratching, and the cells were forced to migrate in a given direction. In this wound-healing assay, the AA-MRLC-expressing cells migrated much more slowly than the wild-type MRLC-expressing cells. In the case of DD-MRLC- and AD-MRLC-expressing cells, no significant differences compared with wild-type MRLC-expressing cells were observed in their migration speed. Indirect immunofluorescence staining showed that the accumulation of endogenous diphosphorylated MRLC at the leading edge was not observed in AA-MRLC-expressing cells, although AA-MRLC was incorporated into myosin heavy chain and localized at the leading edge. In conclusion, we propose that the phosphorylation of MRLC is required to generate the driving force in the migration of the cells but not necessary for localization of myosin II at the leading edge.

Agonist-induced changes in the phosphorylation of the myosin- binding subunit of myosin light chain phosphatase and CPI17, two regulatory factors of myosin light chain phosphatase, in smooth muscle

The inhibition of myosin light chain phosphatase (MLCP) enhances smooth muscle contraction at a constant [Ca2+]. There are two components, myosin-binding subunit of MLCP (MBS) and CPI17, thought to be responsible for the inhibition of MLCP by external stimuli. The phosphorylation of MBS at Thr-641 and of CPI17 at Thr-38 inhibits the MLCP activity in vitro. Here we determined the changes in the phosphorylation of MBS and CPI17 after agonist stimulation in intact as well as permeabilized smooth muscle strips using phosphorylation-site-specific antibodies as probes. The CPI17 phosphorylation transiently increased after agonist stimulation in both alpha-toxin skinned and intact fibres. The time course of the increase in CPI17 phosphorylation after stimulation correlated with the increase in myosin regulatory light chain (MLC) phosphorylation. The increase in CPI17 phosphorylation was significantly diminished by Y27632, a Rho kinase inhibitor, and GF109203x, a protein kinase C inhibitor, suggesting that both the protein kinase C and Rho kinase pathways influence the change in CPI17 phosphorylation. On the other hand, a significant level of MBS phosphorylation at Thr-641, an inhibitory site, was observed in the resting state for both skinned and intact fibres and the agonist stimulation did not significantly alter the MBS phosphorylation level at Thr-641. While the removal of the agonist markedly decreased MLC phosphorylation and induced relaxation, the phosphorylation of MBS was unchanged, while CPI17 phosphorylation markedly diminished. These results strongly suggest that the phosphorylation of CPI17 plays a more significant role in the agonist-induced increase in myosin phosphorylation and contraction of smooth muscle than MBS phosphorylation in the Ca2+-independent activation mechanism of smooth muscle contraction.

Roles of Rho-associated kinase and myosin light chain kinase in morphological and migratory defects of focal adhesion kinase-null cells

Fibroblasts derived from focal adhesion kinase (FAK)-null mouse embryos have a reduced migration rate and an increase in the number and size of peripherally localized adhesions (Ilic, D., Furuta, Y., Kanazawa, S., Takeda, N., Sobue, K., Nakatsuji, N., Nomura, S., Fujimoto, J., Okada, M., and Yamamoto, T. (1995) Nature 377, 539-544). In this study, we have found that Y27632, a specific inhibitor for Rho-associated kinase (Rho-kinase), dramatically reversed the round cell morphology of FAK(-/-) cells to a spread fibroblast-like shape in 30 min and significantly enhanced their motility. The effects of Y27632 on the FAK(-/-) cell morphology and motility were concomitant with reorganization of the actin cytoskeleton and redistribution of focal adhesions. Conversely, the expression of the constitutively active Rho-kinase in FAK(+/+) cells led to round cell shape and inhibition of cell motility. Furthermore, coincident with the formation of cortical actin filaments, myosin light chain (MLC), Ser-19-phosphorylated MLC, and MLC kinase mainly accumulated at the FAK(-/-) cell periphery. We found that the disruption of actin filaments by cytochalasin D prevented the peripheral accumulation of MLC kinase and that inhibition of myosin-mediated contractility by 2,3-butanedione monoxime induced FAK(-/-) cells to spread. Taken together, our results suggest that Rho-kinase may mediate the formation of cortical actomyosin filaments at the FAK(-/-) cell periphery, which further recruits MLC kinase to the cell periphery and generates a non-polar contractile force surrounding the cell, leading to cell rounding and decreased motility.

Translocation of telokin by cGMP signaling in smooth muscle cells

Telokin is an acidic protein with a sequence identical to the COOH-terminal domain of myosin light chain kinase (MLCK) produced by an alternate promoter of the MLCK gene. Although it is abundantly expressed in smooth muscle, its physiological function is not understood. In the present study, we attempted to clarify the function of telokin by analyzing its spatial and temporal localization in living single smooth muscle cells. Primary cultured smooth muscle cells were transfected with green fluorescent protein (GFP)-tagged telokin. The telokin-GFP localized mostly diffusely in cytosol. Stimulation with both sodium nitroprusside (SNP) and 8-bromo-cyclic GMP induced translocation of GFP-tagged telokin to near plasma membrane in living single smooth muscle cells. The translocation was slow, and it took more than 10 min at room temperature. Mutation of the phosphorylation sites of telokin (S13A, S19A, and S13A/S19A) significantly attenuated SNP-induced translocation. Both KT-5823 (cGMP-dependent protein kinase inhibitor) and PD-98059 (mitogen-activated protein kinase inhibitor) diminished the telokin-GFP translocation. These results suggest that telokin changes its intracellular localization because of phosphorylation at Ser13 and/or Ser19 via the cGMP-signaling pathway.

Cytokinesis in eukaryotes

Cytokinesis is the final event of the cell division cycle, and its completion results in irreversible partition of a mother cell into two daughter cells. Cytokinesis was one of the first cell cycle events observed by simple cell biological techniques; however, molecular characterization of cytokinesis has been slowed by its particular resistance to in vitro biochemical approaches. In recent years, the use of genetic model organisms has greatly advanced our molecular understanding of cytokinesis. While the outcome of cytokinesis is conserved in all dividing organisms, the mechanism of division varies across the major eukaryotic kingdoms. Yeasts and animals, for instance, use a contractile ring that ingresses to the cell middle in order to divide, while plant cells build new cell wall outward to the cortex. As would be expected, there is considerable conservation of molecules involved in cytokinesis between yeast and animal cells, while at first glance, plant cells seem quite different. However, in recent years, it has become clear that some aspects of division are conserved between plant, yeast, and animal cells. In this review we discuss the major recent advances in defining cytokinesis, focusing on deciding where to divide, building the division apparatus, and dividing. In addition, we discuss the complex problem of coordinating the division cycle with the nuclear cycle, which has recently become an area of intense research. In conclusion, we discuss how certain cells have utilized cytokinesis to direct development.

Dephosphorylation of the two regulatory components of myosin phosphatase, MBS and CPI17

Dephosphorylation of the two key regulatory factors of myosin light chain phosphatase (MLCP), CPI17 and MBS (myosin binding subunit) of MLCP was studied. While Thr38 phosphorylated CPI17 is quite susceptible to protein phosphatases, phosphorylated MBS was highly resistant to dephosphorylation. Type 2A, 2B and 2C protein phosphatases (PP2A, PP2B and PP2C), but not type 1 (PP1), dephosphorylated CPI17. The majority of the CPI17 phosphatase activity in smooth muscle was attributed to PP2A and PP2C. Phospholipids inhibited dephosphorylation of MBS and arachidonic acid (AA) inhibited PP2A activity against both MBS and CPI17, raising the possibility that AA favors the preservation of active MLCP. Consistently, while the phosphorylation of CPI17 was promptly decreased when the agonist was removed, the phosphorylation of MBS was unchanged in intact smooth muscle fiber. The results suggest that MBS phosphorylation mediated regulation of MLCP is not suitable for regulating rapid change in myosin phosphorylation. On the other hand, phosphorylated CPI17 is readily dephosphorylated thus likely to play a role in regulating fast phosphorylation-dephosphorylation cycle in cells.

Animal cell cytokinesis

Cytokinesis creates two daughter cells endowed with a complete set of chromosomes and cytoplasmic organelles. This conceptually simple event is mediated by a complex and dynamic interplay between the microtubules of the mitotic spindle, the actomyosin cytoskeleton, and membrane fusion events. For many decades the study of cytokinesis was driven by morphological studies on specimens amenable to physical manipulation. The studies led to great insights into the cellular structures that orchestrate cell division, but the underlying molecular machinery was largely unknown. Molecular and genetic approaches have now allowed the initial steps in the development of a molecular understanding of this fundamental event in the life of a cell. This review provides an overview of the literature on cytokinesis with a particular emphasis on the molecular pathways involved in the division of animal cells.

Rho-dependent agonist-induced spatio-temporal change in myosin phosphorylation in smooth muscle cells

Agonist-induced translocation of RhoA and the spatio-temporal change in myosin regulatory light chain (MLC20) phosphorylation in smooth muscle was clarified at the single cell level. We expressed green fluorescent protein-tagged RhoA in the differentiated tracheal smooth muscle cells and visualized the translocation of RhoA in a living cell with three-dimensional digital imaging analysis. The stimulation of the cells by carbachol initiated the translocation of green fluorescent protein-tagged wild type RhoA to the plasma membrane within a minute. The change in MLC20 phosphorylation level after carbachol stimulation was monitored by using phospho-Ser-19-specific antibody recognizing the phosphorylated MLC20 in single cells. Cells expressing the dominant negative form (T19N) of RhoA significantly suppressed sustained MLC20 phosphorylation during the prolonged phase (>300 s), whereas the maximum phosphorylation level (reached at 10 s after stimulation) of these cells was not significantly different from the control cells. The kinetics of RhoA translocation was consistent with that of sustained myosin phosphorylation, suggesting the involvement of a RhoA pathway. Carbachol stimulation increased myosin phosphorylation within a minute both at the cortical and the central region. On the other hand, during prolonged phase, myosin phosphorylation was sustained at the cortical region of the cells but not at the central fibers. A myosin light chain kinase-specific inhibitor, ML-9, diminished myosin phosphorylation at the central region of the cells after the stimulation but not at the cortical area. On the other hand, Y-27632, a Rho kinase-specific inhibitor, diminished myosin phosphorylation at the cortical region but not the central region. The results clearly show that the myosin light chain kinase pathway and the Rho pathway distinctly change myosin phosphorylation in smooth muscle cells in both a temporal and spatial manner.

The pebble GTP exchange factor and the control of cytokinesis

Several G proteins of the Rho family have been shown to be required for cytokinesis. The activity of these proteins is regulated by GTP exchange factors (GEFs), which stimulate GDP/GTP exchange, and by GTPase activating proteins (GAPs), which suppress activity by stimulating the intrinsic GTPase activity. The role of Rho family members during cytokinesis is likely to be determined by their spatial and temporal interactions with these factors. Here we focus on the role of the pebble (pbl) gene of Drosophila melanogaster, a RhoGEF that is required for cytokinesis. We summarise the evidence that the primary target of PBL is Rho1 and describe genetic approaches to elucidating the function of PBL and identifying other components of the PBL-activated Rho signalling pathway.

Effects of anti-myosin drugs on anaphase chromosome movement and cytokinesis in crane-fly primary spermatocytes

To investigate whether myosin is involved in crane-fly primary spermatocyte division, we studied the effects of myosin inhibitors on chromosome movement and on cytokinesis. With respect to chromosome movement, the myosin ATPase inhibitor 2,3-butanedione 2-monoxime (BDM) added during autosomal anaphase reversibly perturbed the movements of all autosomes: autosomes stopped, slowed, or moved backwards during treatment. BDM added before anaphase onset altered chromosome movement less than when BDM was added during anaphase: chromosome movements only rarely were stopped. They often were normal initially and then, if altered at all, were slowed. To confirm that the effects of BDM were due to myosin inhibition, we treated cells with ML-7, a drug that inhibits myosin light chain kinase (MLCK), an enzyme necessary to activate myosin. ML-7 affected anaphase movement only when added in early prometaphase: this treatment prevented chromosome attachment to the spindle. We treated cells with H-7 as a control for possible non-myosin effects of ML-7. H-7, which has a lower affinity than ML-7 for MLCK but a higher affinity than ML-7 for other potential targets, had no effect. These data confirm that the BDM effect is on myosin and indicate that the myosin used for chromosome movement is activated near the start of prometaphase. With respect to cytokinesis, BDM did not block furrow initiation but did block subsequent contraction of the contractile ring. When BDM was added after initiation of the furrow, the contractile ring either stalled or relaxed. ML-7 blocked contractile ring contraction when added at all stages after autosomal anaphase onset, including when added during cytokinesis. H-7 had no effect. These results confirm that the effects of BDM are on myosin and indicate that the myosin used for cytokinesis is activated starting from autosomal anaphase and continuing throughout cytokinesis.

Zipper-interacting protein kinase induces Ca(2+)-free smooth muscle contraction via myosin light chain phosphorylation

The inhibition of myosin phosphatase evokes smooth muscle contraction in the absence of Ca(2+), yet the underlying mechanisms are not understood. To this end, we have cloned smooth muscle zipper-interacting protein (ZIP) kinase cDNA. ZIP kinase is present in various smooth muscle tissues including arteries. Triton X-100 skinning did not diminish ZIP kinase content, suggesting that ZIP kinase associates with the filamentous component in smooth muscle. Smooth muscle ZIP kinase phosphorylated smooth muscle myosin as well as the isolated 20-kDa myosin light chain in a Ca(2+)/calmodulin-independent manner. ZIP kinase phosphorylated myosin light chain at both Ser(19) and Thr(18) residues with the same rate constant. The actin-activated ATPase activity of myosin increased significantly following ZIP kinase-induced phosphorylation. Introduction of ZIP kinase into Triton X-100-permeabilized rabbit mesenteric artery provoked a Ca(2+)-free contraction. A protein phosphatase inhibitor, microcystin LR, also induced contraction in the absence of Ca(2+), which was accompanied by an increase in both mono- and diphosphorylation of myosin light chain. The observed sensitivity of the microcystin-induced contraction to various protein kinase inhibitors was identical to the sensitivity of isolated ZIP kinase to these inhibitors. These results suggest that ZIP kinase is responsible for Ca(2+) independent myosin phosphorylation and contraction in smooth muscle.

RhoA/rho-associated kinase mediates fibroblast contractile force generation

The intracellular signals governing contractile force generation by non-muscle cells remain uncertain. Our aim was to test the hypothesis that the rhoA/rho-associated kinase signaling pathway is a principal mediator of contractile force generation in non-muscle cells. We measured myosin II regulatory light chain (MLC) phosphorylation and directly quantitated force generation by chicken embryo fibroblasts in the absence and presence of selective inhibitors of rhoA, and its downstream effector, rho-associated kinase. Inactivation of rhoA, with C3 transferase, inhibited serum-stimulated MLC phosphorylation and contractile force generation. Y-27632, an inhibitor of rho-associated kinase, reduced basal contractile tension, and inhibited both serum and endothelin-1 stimulated MLC phosphorylation and contractile force generation. The results of this study provide novel evidence indicating that the rhoA/rho-associated kinase signaling pathway is a principal mediator of MLC phosphorylation and consequent contractile force generation by non-muscle cells.