Overexpression of human fibroblast caldesmon fragment containing actin-, Ca++/calmodulin-, and tropomyosin-binding domains stabilizes endogenous tropomyosin and microfilaments

Leupaxin stimulates adhesion and migration of prostate cancer cells through modulation of the phosphorylation status of the actin-binding protein caldesmon

The focal adhesion protein leupaxin (LPXN) is overexpressed in a subset of prostate cancers (PCa) and is involved in the progression of PCa. In the present study, we analyzed the LPXN-mediated adhesive and cytoskeletal changes during PCa progression. We identified an interaction between the actin-binding protein caldesmon (CaD) and LPXN and this interaction is increased during PCa cell migration. Furthermore, knockdown of LPXN did not affect CaD expression but reduced CaD phosphorylation. This is known to destabilize the affinity of CaD to F-actin, leading to dynamic cell structures that enable cell motility. Thus, downregulation of CaD increased migration and invasion of PCa cells. To identify the kinase responsible for the LPXN-mediated phosphorylation of CaD, we used data from an antibody array, which showed decreased expression of TGF-beta-activated kinase 1 (TAK1) after LPXN knockdown in PC-3 PCa cells. Subsequent analyses of the downstream kinases revealed the extracellular signal-regulated kinase (ERK) as an interaction partner of LPXN that facilitates CaD phosphorylation during LPXN-mediated PCa cell migration. In conclusion, we demonstrate that LPXN directly influences cytoskeletal dynamics via interaction with the actin-binding protein CaD and regulates CaD phosphorylation by recruiting ERK to highly dynamic structures within PCa cells.

Sox4-mediated caldesmon expression facilitates skeletal myoblast differentiation

Caldesmon (CaD), originally identified as an actin-regulatory protein, is involved in the regulation of diverse actin-related signaling processes, including cell migration and proliferation, in various cells. The cellular function of CaD has been studied primarily in the smooth muscle system; nothing is known about its function in skeletal muscle differentiation. In this study, we found that the expression of CaD gradually increased as C2C12 myoblast differentiation progressed. Silencing of CaD inhibited cell spreading and migration, resulting in a decrease in myoblast differentiation. Promoter analysis of the caldesmon gene (CALD1) and gel mobility shift assays identified Sox4 as a major trans-acting factor for the regulation of CALD1 expression during myoblast differentiation. Silencing of Sox4 decreased not only CaD protein synthesis but also myoblast fusion in C2C12 cells and myofibril formation in mouse embryonic muscle. Overexpression of CaD in Sox4-silenced C2C12 cells rescued the differentiation process. These results clearly demonstrate that CaD, regulated by Sox4 transcriptional activity, contributes to skeletal muscle differentiation.

Caldesmon regulates axon extension through interaction with myosin II

To begin the process of forming neural circuits, new neurons first establish their polarity and extend their axon. Axon extension is guided and regulated by highly coordinated cytoskeletal dynamics. Here we demonstrate that in hippocampal neurons, the actin-binding protein caldesmon accumulates in distal axons, and its N-terminal interaction with myosin II enhances axon extension. In cortical neural progenitor cells, caldesmon knockdown suppresses axon extension and neuronal polarity. These results indicate that caldesmon is an important regulator of axon development.

Diversification of caldesmon-linked actin cytoskeleton in cell motility

The actin cytoskeleton plays a key role in regulating cell motility. Caldesmon (CaD) is an actin-linked regulatory protein found in smooth muscle and non-muscle cells that is conserved among a variety of vertebrates. It binds and stabilizes actin filaments, as well as regulating actin-myosin interaction in a calcium (Ca2+)/calmodulin (CaM)- and/or phosphorylation-dependent manner. CaD function is regulated qualitatively by Ca2+/CaM and by its phosphorylation state and quantitatively at the mRNA level, by three different transcriptional regulation of the CALD1 gene. CaD has numerous functions in cell motility, such as migration, invasion, and proliferation, exerted via the reorganization of the actin cytoskeleton. Here we will outline recent findings regarding CaD's structural features and functions.

Proteomic approach identifies alterations in cytoskeletal remodelling proteins during decidualization of human endometrial stromal cells

Decidualization is a tissue remodelling process within the uterus in preparation for embryo implantation and pregnancy. In this study we isolated primary human endometrial stromal cells and stimulated decidualization with cAMP. We then used 2D- differential in-gel electrophoresis (DIGE) to identify proteins induced by decidualization. Eighty-eight out of 2714 spots were differentially regulated, 18 of which were assigned clear identities by mass spectrometry. Many of these are proteins known to be associated with cell structure and cytoskeletal remodelling. We validated five of these proteins by Western blot and immunohistochemistry on human endometrial tissue. The validated proteins are caldesmon 1, src substrate contactin 8, tropomyosin alpha-4 chain, protein disulfide isomerase 1A, and LIM and SH3 domain protein. With the exception of caldesmon 1, none of the identified proteins have previously been associated with decidualization. This study provides insight into our understanding of decidualization, which is important for successful embryo implantation and establishment of pregnancy.

Requirement of TCTG(G/C) Direct Repeats and Overlapping GATA Site for Maintaining the Cardiac-Specific Expression of Cardiac troponin T in Developing and Adult Mice

The cardiac-specific -497 bp promoter of rat cardiac troponin T (cTnT) contains two similar modules, D and F, each of which possesses TCTG(G/C) direct repeats and A/T-rich sites. To identify cis-elements critical for cardiac specificity, a -249 bp promoter containing only module F and its site-directed mutations were used to generate transgenic mice. Transgene expression of the -249 bp promoter remained cardiac-specific, despite low and nonuniform expression. The nonuniform expression pattern of the transgene coincided with differential expression of HMGB1, which appeared to be the predominant form of HMGB family proteins in the heart. The HMGB1 binds to the A/T-rich/MEF2-like sites of the cTnT promoter, as determined by chromatin immunoprecipitation assays. Mice carrying the -249 bp promoter with point mutations disrupting the direct repeats expressed transgene at lower levels in the heart and ectopically in the brain. Ectopic expression of transgene was also observed in developing limbs and head. These results suggest an important role for the direct repeat in determining the cardiac specificity. Furthermore, mice carrying a mutant promoter simultaneously disrupting the direct repeats and overlapping GATA site failed to express the transgene in any tissues tested. Therefore, the direct repeat and overlapping GATA site are critical for the expression level and cardiac specificity. The F module controls one level of cardiac specificity. For a uniform and high level of cardiac-specific expression, the upstream element (-497 to -250 bp) is further required, possibly through the D enhancer module and the combination of Nkx2.5 and GATA sites.

Calponin in non-muscle cells

Calponin is an actin filament-associated regulatory protein expressed in smooth muscle and non-muscle cells. Calponin is an inhibitor of the actin-activated myosin ATPase. Three isoforms of calponin have been found in the vertebrates. Whereas the role of calponin in regulating smooth muscle contractility has been extensively investigated, the function and regulation of calponin in non-muscle cells is much less understood. Based on recent progresses in the field, this review focuses on the studies of calponin in non-muscle cells, especially its regulation by cytoskeleton tension and function in cell motility. The ongoing research has demonstrated that calponin plays a regulatory role in non-muscle cell motility. Therefore, non-muscle calponin is an attractive target for the control of cell proliferation, migration and phagocytosis, and the treatment of cancer metastasis.

Glucocorticoid receptor-mediated expression of caldesmon regulates cell migration via the reorganization of the actin cytoskeleton

Glucocorticoids (GCs) play important roles in numerous cellular processes, including growth, development, homeostasis, inhibition of inflammation, and immunosuppression. Here we found that GC-treated human lung carcinoma A549 cells exhibited the enhanced formation of the thick stress fibers and focal adhesions, resulting in suppression of cell migration. In a screen for GC-responsive genes encoding actin-interacting proteins, we identified caldesmon (CaD), which is specifically up-regulated in response to GCs. CaD is a regulatory protein involved in actomyosin-based contraction and the stability of actin filaments. We further demonstrated that the up-regulation of CaD expression was controlled by glucocorticoid receptor (GR). An activated form of GR directly bound to the two glucocorticoid-response element-like sequences in the human CALD1 promoter and transactivated the CALD1 gene, thereby up-regulating the CaD protein. Forced expression of CaD, without GC treatment, also enhanced the formation of thick stress fibers and focal adhesions and suppressed cell migration. Conversely, depletion of CaD abrogated the GC-induced phenotypes. The results of this study suggest that the GR-dependent up-regulation of CaD plays a pivotal role in regulating cell migration via the reorganization of the actin cytoskeleton.

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Human tropomyosin isoforms in the regulation of cytoskeleton functions

Over the past two decades, extensive molecular studies have identified multiple tropomyosin isoforms existing in all mammalian cells and tissues. In humans, tropomyosins are encoded by TPM1 (alpha-Tm, 15q22.1), TPM2 (beta-Tm, 9p13.2-p13.1), TPM3 (gamma-Tm, 1q21.2) and TPM4 (delta-Tm, 19p13.1) genes. Through the use of different promoters, alternatively spliced exons and different sites of poly(A) addition signals, at least 22 different tropomyosin cDNAs with full-length open reading frame have been cloned. Compelling evidence suggests that these isoforms play important determinants for actin cytoskeleton functions, such as intracellular vesicle movement, cell migration, cytokinesis, cell proliferation and apoptosis. In vitro biochemical studies and in vivo localization studies suggest that different tropomyosin isoforms have differences in their actin-binding properties and their effects on other actin-binding protein functions and thus, in their specification ofactin microfilaments. In this chapter, we will review what has been learned from experimental studies on human tropomyosin isoforms about the mechanisms for differential localization and functions of tropomyosin. First, we summarize current information concerning human tropomyosin isoforms and relate this to the functions of structural homologues in rodents. We will discuss general strategies for differential localization oftropomyosin isoforms, particularly focusing on differential protein turnover and differential isoform effects on other actin binding protein functions. We will then review tropomyosin functions in regulating cell motility and in modulating the anti-angiogenic activity of cleaved high molecular weight kininogen (HKa) and discuss future directions in this area.

Caldesmon and the regulation of cytoskeletal functions

Caldesmon (CaD) is an extraordinary actin-binding protein, because in addition to actin, it also bindsmyosin, calmodulin and tropomyosin. As a component of the smoothmuscle and nonmuscle contractile apparatus CaD inhibits the actomyosin ATPase activity and its inhibitory action is modulated by both Ca2+ and phosphorylation. The multiplicity of binding partners and diverse biochemical properties suggest CaD is a potent and versatile regulatory protein both in contractility and cell motility. However, after decades ofinvestigation in numerous laboratories, hard evidence is still lacking to unequivocally identify its in vivo functions, although indirect evidence is mounting to support an important role in connection with the actin cytoskeleton. This chapter reviews the highlights of the past findings and summarizes the current views on this protein, with emphasis of its interaction with tropomyosin.

Tropomyosin and caldesmon regulate cytokinesis speed and membrane stability during cell division

The contractile ring and the cell cortex generate force to divide the cell while maintaining symmetrical shape. This requires temporal and spatial regulation of the actin cytoskeleton at these areas. We force-expressed misregulated versions of actin-binding proteins, tropomyosin and caldesmon, into cells and analyzed their effects on cell division. Cells expressing proteins that increase actomyosin ATPase, such as human tropomyosin chimera (hTM5/3), significantly speed up division, whereas cells expressing proteins that inhibit actomyosin, such as caldesmon mutants defective in Ca(2+)/calmodulin binding (CaD39-AB) and in cdk1 phosphorylation sites (CaD39-6F), divide slowly. hTM5 and hTM5/3-expressing cells lift one daughter cell off the substrate and twist. Furthermore, CaD39-AB- and CaD39-6F-expressing cells are sensitive to hypotonic swelling and show severe blebbing during division, whereas hTM5/3-expressing cells are resistant to hypotonic swelling and produce membrane bulges. These results support a model where Ca(2+)/calmodulin and cdk1 dynamically control caldesmon inhibition of tropomyosin-activated actomyosin to regulate division speed and to suppress membrane blebs.

Phosphorylation of caldesmon during smooth muscle contraction and cell migration or proliferation

The actin-binding protein caldesmon (CaD) exists both in smooth muscle (the heavy isoform, h-CaD) and non-muscle cells (the light isoform, l-CaD). In smooth muscles h-CaD binds to myosin and actin simultaneously and modulates the actomyosin interaction. In non-muscle cells l-CaD binds to actin and stabilizes the actin stress fibers; it may also mediate the interaction between actin and non-muscle myosins. Both h- and l-CaD are phosphorylated in vivo upon stimulation. The major phosphorylation sites of h-CaD when activated by phorbol ester are the Erk-specific sites, modification of which is attenuated by the MEK inhibitor PD98059. The same sites in l-CaD are also phosphorylated when cells are stimulated to migrate, whereas in dividing cells l-CaD is phosphorylated more extensively, presumably by cdc2 kinase. Both Erk and cdc2 are members of the MAPK family. Thus it appears that CaD is a downstream effector of the Ras signaling pathways. Significantly, the phosphorylatable serine residues shared by both CaD isoforms are in the C-terminal region that also contains the actin-binding sites. Biochemical and structural studies indicated that phosphorylation of CaD at the Erk sites is accompanied by a conformational change that partially dissociates CaD from actin. Such a structural change in h-CaD exposes the myosin-binding sites on the actin surface and allows actomyosin interactions in smooth muscles. In the case of non-muscle cells, the change in l-CaD weakens the stability of the actin filament and facilitates its disassembly. Indeed, the level of l-CaD modification correlates very well in a reciprocal manner with the level of actin stress fibers. Since both cell migration and cell division require dynamic remodeling of actin cytoskeleton that leads to cell shape changes, phosphorylation of CaD may therefore serve as a plausible means to regulate these processes. Thus CaD not only links the smooth muscle contractility and non-muscle motility, but also provides a common mechanism for the regulation of cell migration and cell proliferation.

Accumulation of tropomyosin isoform 5 at the infection sites of host cells during Cryptosporidium invasion

The actin cytoskeleton of host cells has been implicated in Cryptosporidium invasion. However, the underlying mechanism of how actin filaments and associated proteins modulate this process remains unclear. In this study, we use in vitro cultured cell lines, human ileocecal adenocarcinoma HCT-8 and Chinese hamster ovary (CHO), and an in vivo mouse model to investigate the roles of tropomyosin isoforms in Cryptosporidium invasion. Using isoform-specific monoclonal antibodies, we found that the major human tropomyosin (hTM) isoforms expressed in HCT-8 cells are hTM4 and hTM5. HCT-8 cells also express hTM1 at low levels but not hTM2 and hTM3. During Cryptosporidium parvum infection, hTM5 colocalized to the infection sites with a novel parasite membrane protein, CP2. Neither hTM1 nor hTM4 accumulated at infection sites. Similarly, a high level of TM5 and varying amounts of TM4 accumulated at the C. parvum infection sites in CHO cells. CHO cells overexpressing hTM5 exhibit a significantly higher percent of mature meronts early in the infection process relative to CHO cells or CHO cells overexpressing a tropomyosin mutant, chimeric isoform hTM5/3. These results suggest that functional TM5 enhances Cryptosporidium invasion of host cells. In C. parvum-infected mice, accumulation and rearrangement of TM5 and TM4 were detected throughout the infected ileum. Similarly, in the Cryptosporidium muris-infected mice, TM5 accumulated in discrete regions of the epithelial cells of gastric glands and in the oocyst-laden stomach gland lumen. Cryptosporidium infection appears to rearrange and recruit host TM isoforms in both culture cells and in the mouse. Localized accumulation of tropomyosin at the infection sites may facilitate parasite invasion.

Requirement of reversible caldesmon phosphorylation at P21-activated kinase-responsive sites for lamellipodia extensions during cell migration

Caldesmon is believed to be one of the key regulators for actin dynamics and thereby cell polarity, membrane extension, and cell motility. We have shown previously that stress fiber formation and cell movement are severely impaired in the cells expressing human fibroblast caldesmon fragment defective in Ca2+/CaM binding sites. Both Ser458 and Ser489, adjacent to the Ca2+/CaM-binding sites, are phosphorylated by p21-activated kinase (PAK) in vitro. Here we report that Ser458 is phosphorylated in response to cell movement. We substituted Ser458 and Ser489 on C-terminal caldesmon (CaD39) with alanine or glutamic acid to mimic under-phosphorylated (CaD39-PAKA) or constitutively phosphorylated (CaD39-PAKE) caldesmon. In vitro, CaD39-PAKE, but not CaD39-PAKA, fails to inhibit myosin ATPase activity and exhibits reduced binding to Ca2+/CaM. When stably expressed in Chinese Hamster Ovary cells, both CaD39-PAKA and CaD39-PAKE incorporate into stress fibers and localize to the leading edge of the migrating cell. Expression of CaD39-PAKE, but not CaD39-PAKA, fails to protect stress fibers from cytochalasin depolymerization. However, both mutations inhibit cell polarization and lead to defects in membrane extension and cell migration. We conclude that phosphorylation of caldesmon by PAK is a dynamic process required to regulate actin dynamics and membrane protrusions in wound-induced cell migration.

h2-Calponin is regulated by mechanical tension and modifies the function of actin cytoskeleton

Calponin is an extensively studied actin-binding protein, but its function is not well understood. Among three isoforms of calponin, h2-calponin is found in both smooth muscle and non-muscle cells. The present study demonstrates that epidermal keratinocytes and fibroblast cells express significant amounts of h2-calponin. The expression of h2-calponin is cell anchorage-dependent. The levels of h2-calponin decrease when cells are rounded up and remain low when cells are prevented from adherence to a culture dish. h2-calponin expression resumes after the floating cells are allowed to form a monolayer in plastic dish. Cell cultures on polyacrylamide gels of different stiffness demonstrated that h2-calponin expression is affected by the mechanical properties of the culture matrix. When cells are cultured on soft gel that applies less traction force to the cell and, therefore, lower mechanical tension in the cytoskeleton, the level of h2-calponin is significantly lower than that in cells cultured on hard gel or rigid plastic dish. Force-expression of h2-calponin enhanced the resistance of the actin filaments to cytochalasin B treatment. Keratinocyte differentiation is accompanied by a mechanical tension-related up-regulation of h2-calponin. Lowering the tension of actin cytoskeleton by inhibiting non-muscle myosin II ATPase decreased h2-calponin expression. In contrast to the mechanical tension regulation of endogenous h2-calponin, the expression of h2-calponin using a cytomegalovirus promotor was independent of the stiffness of culture matrix. The results suggest that h2-calponin represents a novel manifestation of mechanical tension responsive gene regulation that may modify cytoskeleton function.

Caldesmon phosphorylation in actin cytoskeletal remodeling

Caldesmon is an actin-binding protein that is capable of stabilizing actin filaments against actin-severing proteins, inhibiting actomyosin ATPase activity, and inhibiting Arp2/3-mediated actin polymerization in vitro. Caldesmon is a substrate of cdc2 kinase and Erk1/2 MAPK, and phosphorylation by either of these kinases reverses the inhibitory effects of caldesmon. Cdc2-mediated caldesmon phosphorylation and the resulting dissociation of caldesmon from actin filaments are essential for M-phase progression during mitosis. Cells overexpressing the actin-binding carboxyterminal fragment of caldesmon fail to release the fragment completely from actin filaments during mitosis, resulting in a higher frequency of multinucleated cells. PKC-mediated MEK/Erk/caldesmon phosphorylation is an important signaling cascade in the regulation of smooth muscle contraction. Furthermore, PKC activation has been shown to remodel actin stress fibers into F-actin-enriched podosome columns in cultured vascular smooth muscle cells. Podosomes are cytoskeletal adhesion structures associated with the release of metalloproteases and degradation of extracellular matrix during cell invasion. Interestingly, caldesmon is one of the few actin-binding proteins that is associated with podosomes but excluded from focal adhesions. Caldesmon also inhibits the function of gelsolin and Arp2/3 complex that are essential for the formation of podosomes. Thus, caldesmon appears to be well positioned for playing a modulatory role in the formation of podosomes. Defining the roles of actin filament-stabilizing proteins such as caldesmon and tropomyosin in the formation of podosomes should provide a more complete understanding of molecular systems that regulate the remodeling of the actin cytoskeleton in cell transformation and invasion.

Regulation of microfilament organization by Kaposi sarcoma-associated herpes virus-cyclin.CDK6 phosphorylation of caldesmon

Kaposi sarcoma-associated herpes virus (KSHV) encodes a D-like cyclin (K-cyclin) that is thought to contribute to the viral oncogenicity. K-cyclin activates cellular cyclin-dependent kinases (CDK) 4 and 6, generating enzymes with a substrate selectivity deviant from CDK4 and CDK6 activated by D-type cyclins, suggesting different biochemical and biological functions. Here we report the identification of the actin- and calmodulin-binding protein caldesmon (CALD1) as a novel K-cyclin.CDK substrate, which is not phosphorylated by D.CDK. CALD1 plays a central role in the regulation of microfilament organization, consequently controlling cell shape, adhesion, cytokinesis and motility. K-cyclin.CDK6 specifically phosphorylates four Ser/Thr sites in the human CALD1 carboxyl terminus, abolishing CALD1 binding to its effector protein, actin, and its regulator protein, calmodulin. CALD1 is hyperphosphorylated in cells following K-cyclin expression and in KSHV-transformed lymphoma cells. Moreover, expression of exogenous K-cyclin results in microfilament loss and changes in cell morphology; both effects are reliant on CDK catalysis and can be reversed by the expression of a phosphorylation defective CALD1. Together, these data strongly suggest that K-cyclin expression modulates the activity of caldesmon and through this the microfilament functions in cells. These results establish a novel link between KSHV infection and the regulation of the actin cytoskeleton.

The role of caldesmon in the regulation of endothelial cytoskeleton and migration

The actin- and myosin-binding protein, caldesmon (CaD) is an essential component of the cytoskeleton in smooth muscle and non-muscle cells and is involved in the regulation of cell contractility, division, and assembly of actin filaments. CaD is abundantly present in endothelial cells (EC); however, the contribution of CaD in endothelial cytoskeletal arrangement is unclear. To examine this contribution, we generated expression constructs of l-CaD cloned from bovine endothelium. Wild-type CaD (WT-CaD) and truncated mutants lacking either the N-terminal myosin-binding site or the C-terminal domain 4b (containing actin- and calmodulin-binding sites) were transfected into human pulmonary artery EC. Cell fractionation experiments and an actin overlay assay demonstrated that deleting domain 4b, but not the N-terminal myosin-binding site, resulted in decreased affinity to both the detergent-insoluble cytoskeleton and soluble actin. Recombinant WT-CaD co-localized with acto-myosin filaments in vivo, but neither of CaD mutants did. Thus both domain 4b and the myosin-binding site are essential for proper localization of CaD in EC. Overexpression of WT-CaD led to cell rounding and formation of a thick peripheral subcortical actin rim in quiescent EC, which correlated with decreased cellular migration. Pharmacological inhibition of p38 MAPK, but not ERK MAPK, caused disassembly of this peripheral actin rim in CaD-transfected cells and decreased CaD phosphorylation at Ser531 (Ser789 in human h-CaD). These results suggest that CaD is critically involved in the regulation of the actin cytoskeleton and migration in EC, and that p38 MAPK-mediated CaD phosphorylation may be involved in endothelial cytoskeletal remodeling.

Caldesmon mutant defective in Ca2+-calmodulin binding interferes with assembly of stress fibers and affects cell morphology, growth and motility

Despite intensive in vitro studies, little is known about the regulation of caldesmon (CaD) by Ca2+-calmodulin (Ca2+-CaM) in vivo. To investigate this regulation, a mutant was generated of the C-terminal fragment of human fibroblast CaD, termed CaD39-AB, in which two crucial tryptophan residues involved in Ca2+-CaM binding were each replaced with alanine. The mutation abolished most CaD39-AB binding to Ca2+-CaM in vitro but had little effect on in vitro binding to actin filaments and the ability to inhibit actin/tropomyosin-activated heavy meromyosin ATPase. To study the functional consequences of these mutations in vivo, we transfected an expression plasmid carrying CaD39-AB cDNA into Chinese hamster ovary (CHO) cells and isolated several clones expressing various amounts of CaD39-AB. Immunofluorescence microscopy revealed that mutant CaD39-AB was distributed diffusely throughout the cytoplasm but also concentrated at membrane ruffle regions. Stable expression of CaD39-AB in CHO cells disrupted assembly of stress fibers and focal adhesions, altered cell morphology, and slowed cell cycle progression. Moreover, CaD39-AB-expressing cells exhibited motility defects in a wound-healing assay, in both velocity and the persistence of translocation, suggesting a role for CaD regulation by Ca2+-CaM in cell migration. Together, these results demonstrate that CaD plays a crucial role in mediating the effects of Ca2+-CaM on the dynamics of the actin cytoskeleton during cell migration.

Postnatal changes in caldesmon expression and localization in cardiac myocytes

Caldesmon is a heat-stable protein found in both muscle and non-muscle tissue. It binds to a number of contractile and cytoskeletal proteins and may be involved in regulating acto-myosin interaction in smooth muscle cells and/or the assembly of microfilaments in muscle and non-muscle cells. We have shown previously that caldesmon is localized at the Z-lines in adult cardiac myocytes and that both the low- and high-molecular-weight forms (/-caldesmon and h-caldesmon, respectively) are present in atrial and ventricular myocytes. Here we examined the expression of caldesmon and its localization in freshly isolated cardiac myocytes during postnatal development and when these myocytes were grown in culture. We found that /-caldesmon is expressed in both neonatal and adult rat ventricular myocytes. The expression of h-caldesmon, however, was not detected in myocytes from newborn animals but increased during the first 2 weeks of postnatal development. Caldesmon was generally not co-localized with alpha-actinin at the Z-lines in neonatal myocytes but became increasingly more so during the first 2 weeks of postnatal development. When myocytes from 5- and 10-day-old rats were grown in primary culture, h-caldesmon expression decreased and caldesmon could not be detected at the Z-lines in the cultured cells. These results indicate that caldesmon plays a role at the Z-lines in adult cardiac myocytes; however, its localization at the Z-lines is not necessary for the prenatal development that occurs at these sites or for the establishment of a contractile phenotype in cultured cardiac myocytes.

Localization of the novel Xin protein to the adherens junction complex in cardiac and skeletal muscle during development

Previously, we demonstrated that chick embryos treated with antisense oligonucleotides against a striated muscle-specific Xin exhibit abnormal cardiac morphogenesis (Wang et al. [1999] Development 126:1281-1294); therefore, we surmised a role for Xin in cardiac development. Herein, we examine the developmental expression of Xin through immunofluorescent staining of whole-mount mouse embryos and frozen heart sections. Xin expression is first observed within the heart tube of embryonic day 8.0 (E8.0) mice, exhibiting a peripheral localization within the cardiomyocytes. Colocalization of Xin with both beta-catenin and N-cadherin is observed throughout embryogenesis and into adulthood. Additionally, Xin is found associated with beta-catenin within the N-cadherin complex in embryonic chick hearts by coimmunoprecipitation. Xin is detected earlier than vinculin in the developing heart and colocalizes with vinculin at the intercalated disc but not at the sarcolemma within embryonic and postnatal hearts. At E10.0, Xin is also detected in the developing somites and later in the myotendon junction of skeletal muscle but not within the costameric regions of muscle. In cultured C2C12 myotubes, the Xin protein is found in many speckled and filamentous structures, coincident with tropomyosin in the stress fibers. Additionally, Xin is enriched in the regions of cell-cell contacts. These data demonstrate that Xin is one of the components at the adherens junction of cardiac muscle, and its counterpart in skeletal muscle, the myotendon junction. Furthermore, temporal and spatial expressions of Xin in relation to intercalated disc proteins and thin filament proteins suggest roles for Xin in the formation of cell-cell contacts and possibly in myofibrillogenesis.

Actomyosin cross-linking by caldesmon in non-muscle cells

The role of myosin-binding in cytoskeletal arrangement of non-muscle low molecular weight caldesmon (l-caldesmon) was studied. The N-terminal myosin-binding domain of caldesmon N152 colocalized with myosin in transiently transfected chicken fibroblasts. When added exogenously to the Triton-insoluble cytoskeleton, N152 enhanced l-caldesmon displacement by exogenous C-terminal actin-binding fragment (H1). Thus, a significant fraction of l-caldesmon cross-links actin and myosin. In contrast, in epithelioid HeLa cells most of l-caldesmon was only actin-bound as H1 alone was enough for its displacement. Phosphorylation by mitogen-activated protein kinase reduced the capability of H1 to displace endogenous l-caldesmon, suggesting it may represent a regulatory mechanism for actin-caldesmon interaction in vivo.

Mutant Caldesmon lacking cdc2 phosphorylation sites delays M-phase entry and inhibits cytokinesis

Caldesmon is phosphorylated by cdc2 kinase during mitosis, resulting in the dissociation of caldesmon from microfilaments. To understand the physiological significance of phosphorylation, we generated a caldesmon mutant replacing all seven cdc2 phosphorylation sites with Ala, and examined effects of expression of the caldesmon mutant on M-phase progression. We found that microinjection of mutant caldesmon effectively blocked early cell division of Xenopus embryos. Similar, though less effective, inhibition of cytokinesis was observed with Chinese hamster ovary (CHO) cells microinjected with 7th mutant. When mutant caldesmon was introduced into CHO cells either by protein microinjection or by inducible expression, delay of M-phase entry was observed. Finally, we found that 7th mutant inhibited the disassembly of microfilaments during mitosis. Wild-type caldesmon, on the other hand, was much less potent in producing these three effects. Because mutant caldesmon did not inhibit cyclin B/cdc2 kinase activity, our results suggest that alterations in microfilament assembly caused by caldesmon phosphorylation are important for M-phase progression.

Tyrosine phosphorylation of caldesmon is required for binding to the Shc.Grb2 complex

S3-v-erbB is a retroviral oncogene that encodes a ligand-independent, transforming mutant of the epidermal growth factor receptor. This oncogene has been shown to be sarcomagenic in vivo and to transform fibroblasts in vitro. Our previous studies (McManus, M. J., Lingle, W. L., Salisbury, J. L., and Maihle, N. J. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 11351-11356) showed that expression of S3-v-erbB in primary fibroblasts results in the tyrosine phosphorylation of caldesmon (CaD), an actin- and calmodulin-binding protein. This phosphorylation is transformation-associated, and the phosphorylated form of CaD is associated with a signaling complex consisting of Shc, Grb2, and Sos in transformed fibroblasts. To identify the tyrosine phosphorylation site(s) in the CaD molecule and to further elucidate the functional role of CaD tyrosine phosphorylation in S3-v-ErbB oncogenic signaling, we have generated a series of mutant CaDs in which one or more tyrosine residues have been replaced with phenylalanine. Using a CaD null cell line, DF1 cells (an immortalized chicken embryo fibroblast cell line), and transient transfection assays, we demonstrated that Tyr-27 and Tyr-393 are the major sites of tyrosine phosphorylation on CaD. Interestingly, Tyr-27 is located within the myosin binding domain of CaD, and Tyr-393 is adjacent to one of the major actin binding and actomyosin ATPase inhibitory domains. Our studies also show that the tyrosine phosphorylation of CaD enhances its binding to the Shc.Grb2 complex. Specifically, replacement of Tyr-27, but not of Tyr-165 or Tyr-393, significantly reduces the ability of CaD to interact with the Shc. Grb2 complex. Together, these studies demonstrate that the major sites of tyrosine phosphorylation on CaD are located in the myosin and actin binding domains of CaD and that Tyr-27 is the major tyrosine phosphorylation site through which CaD interacts with the Shc.Grb2 complex.

A role for p21-activated kinase in endothelial cell migration

The serine/threonine p21-activated kinase (PAK) is an effector for Rac and Cdc42, but its role in regulating cytoskeletal organization has been controversial. To address this issue, we investigated the role of PAK in migration of microvascular endothelial cells. We found that a dominant negative (DN) mutant of PAK significantly inhibited cell migration and increased stress fibers and focal adhesions. The DN effect mapped to the most NH(2)-terminal proline-rich SH3-binding sequence. Observation of a green fluorescent protein-tagged alpha-actinin construct in living cells revealed that the DN construct had no effect on membrane ruffling, but dramatically inhibited stress fiber and focal contact motility and turnover. Constitutively active PAK inhibited migration equally well and also increased stress fibers and focal adhesions, but had a somewhat weaker effect on their dynamics. In contrast to their similar effects on motility, DN PAK decreased cell contractility, whereas active PAK increased contractility. Active PAK also increased myosin light chain (MLC) phosphorylation, as indicated by staining with an antibody to phosphorylated MLC, whereas DN PAK had little effect, despite the increase in actin stress fibers. These results demonstrate that although PAK is not required for extension of lamellipodia, it has substantial effects on cell adhesion and contraction. These data suggest a model in which PAK plays a role coordinating the formation of new adhesions at the leading edge with contraction and detachment at the trailing edge.

Regulation of actin binding and actin bundling activities of fascin by caldesmon coupled with tropomyosin

Human fascin is an actin-bundling protein and is thought to play a role in the formation of microfilament bundles of microspikes and stress fibers in cultured cells. To explore the regulation of fascin-actin interaction, we have examined the effects of culture cell caldesmon and tropomyosin (TM) on actin binding activity of human fascin. Caldesmon alone or TM alone has little or no effect on the actin binding of fascin. However, caldesmon together with TM completely inhibits actin binding of human fascin. When calmodulin is added, the inhibition of fascin-actin interaction by caldesmon and TM becomes Ca2+ dependent because Ca2+/calmodulin blocks actin binding of caldesmon. Furthermore, as phosphorylation of caldesmon by cdc2 kinase inhibits actin binding of caldesmon, phosphorylation can also control actin binding of fascin in the presence of TM. As expected by the inhibition of fascin-actin binding, caldesmon coupled with TM also inhibits actin bundling activity of fascin. Whereas smooth muscle caldesmon alone or TM alone shows no effect, caldesmon together with TM completely inhibits actin bundling activity of fascin. This inhibition is again Ca2+ dependent when calmodulin is added to the system. These results suggest important roles for caldesmon and TM in the regulation of the function of human fascin.

Tropomyosin isoform diversity and neuronal morphogenesis

Tropomyosins (Tm) are a large family of isoforms obtained from multiple genes and by extensive alternative splicing. They bind in the alpha-helical groove of the actin filament and are therefore core components of this extensive cytoskeletal system. In non-muscle cells the Tm isoforms have been implicated in a diversity of processes including cytokinesis, vesicle transport, motility, morphogenesis and cell transformation. Using immunohistochemical localization in cultured primary cortical neurons with an antibody that potentially identifies all non-muscle TM5 gene isoforms compared with one that specifically identifies a subset of isoforms, the possibility was raised that there were considerably more isoforms derived from this gene than the four previously described. Using polymerase chain reaction (PCR) analysis we have now shown that the rat brain generates at least 10 mRNA isoforms using multiple combinations of terminal exons and two internal exons. There is extensive developmental regulation of these isoforms in the brain and there appears to be a switch in the preferential use of the two internal exons 6a to 6b from the embryonic to the adult isoforms. Specific isoforms using alternate carboxyl-terminal exons are differentially localized within the adult rat cerebellum. It is suggested that the tightly regulated spatial and temporal expression of Tm isoforms plays an important role in the development and maintenance of specific neuronal compartments. This may be achieved by isoforms providing unique structural properties to actin-based filaments within functionally distinct neuronal domains.

TGF-alpha-induced breakdown of stress fibers and degradation of tropomyosin in NRK cells is blocked by a proteasome inhibitor

Treatment of NRK cells with TGF-alpha in the presence of serum initiates disassembly of cytoskeletal stress fibers and suppresses the synthesis of tropomyosin isoforms (TMs) 1, 2, and 3 but not TMs 4 and 5 (Cooper et al., Cancer Res. 47, 4493-4500, 1987). In order to determine how the loss of tropomyosin is induced and what role it plays in cytoskeletal disruption, the turnover of tropomyosin was studied in the presence of the transforming growth factor and protease inhibitors. Cells were pulse-labeled with [35S]methionine and chased in the absence or the presence of the growth factor. It was found that TMs 1, 2, and 3 are degraded at about twice the rate of TMs 4 and 5 in control cells and that the rate of degradation of TMs 1-3 is accelerated by the growth factors. Degradation of TMs in control and growth factor-treated cells is blocked by a membrane-permeable inhibitor of cysteine proteases (LLnL) that acts upon calpains and proteasomes, and the cells maintain a flattened shape with a normal complement of stress fibers. Application of inhibitors that block calpains but not proteasomes does not block TM degradation. Treatments (suspension culture or cytochalasin B) that disrupt stress fibers without application of the growth factors also accelerate TM degradation, suggesting that acceleration of TM degradation is a consequence of its release from stress fibers during their breakdown. The normally more rapid turnover of the TM isoforms 1-3 that are lost in the phenotypically transformed cells could serve to facilitate the cytoskeletal reorganization that follows the activation of signal transduction pathways by the transforming growth factors observed in this study or during other rearrangements of the cytoskeleton such as occur during cell migration or mitosis.

Phosphatidylserine liposomes can be tethered by caldesmon to actin filaments

Rotary shadowing electron microscopy revealed that attachment of caldesmon to phosphatidylserine (PS) liposomes was mainly through its C-terminal end. To determine the PS-binding sites of caldesmon, we have made use of synthetic peptides covering the two C-terminal calmodulin binding sites and a recombinant fragment corresponding to the N-terminal end of the C-terminal domain that contains an amphipathic helix. Interactions of these peptides with the PS liposomes were studied by nondenaturing gel electrophoresis and fluorescence spectroscopy. The results showed that both calmodulin-binding sites of caldesmon were able to interact with PS. The affinity (Kd) of PS for these sites was in the range of 1.8-14.3 x 10(-5) M, compared to 0.69 x 10(-5) M for the whole caldesmon molecule. Fragments located outside of calmodulin-binding sites bound PS weakly (3.85 x 10(-4) M) and thus may contain a second class of lipid-binding sites. Binding of PS induced conformational changes in regions other than the C-terminal PS-binding sites, as evidenced by the changes in the susceptibility to proteolytic cleavages. Most significantly, the presence of caldesmon greatly increased binding of PS to F-actin, suggesting that caldesmon may tether PS liposomes to actin filaments. These results raise the possibility that caldesmon-lipid interactions could play a functionally important role in the assembly of contractile filaments near the membranes.

Coordinate expression of alpha-tropomyosin and caldesmon isoforms in association with phenotypic modulation of smooth muscle cells

Isoform diversity of tropomyosin is generated from the limited genes by a combination of differential transcription and alternative splicing. In the case of the alpha-tropomyosin (alpha-TM) gene, exon 2a rather than exon 2b is specifically spliced in alpha-TM-SM mRNA, which is one of the major tropomyosin isoforms in smooth muscle cells. Here we demonstrate that expressions of alpha-tropomyosin and caldesmon isoforms are coordinately regulated in association with phenotypic modulation of smooth muscle cells. Molecular cloning and Western and Northern blottings have revealed that in addition to the down-regulation of beta-TM-SM, alpha-TM-SM converted to alpha-TM-F1 and alpha-TM-F2 by a selectional change from exon 2a to exon 2b during dedifferentiation of smooth muscle cells in culture. Simultaneously, a change of caldesmon isoforms from high Mr type to low Mr type was also observed by alternative selection between exons 3b and 4 in the caldesmon gene during this process. In contrast, cultured smooth muscle cells maintaining a differentiated phenotype continued to express alpha-TM-SM, beta-TM-SM, and high Mr caldesmon. In situ hybridization revealed specific coexpression of alpha-TM-SM and high Mr caldesmon in smooth muscle in developing embryos. These results suggest a common splicing mechanism for phenotype-dependent expression of tropomyosin and caldesmon isoforms in both visceral and vascular smooth muscle cells.

Tropomyosin localization reveals distinct populations of microfilaments in neurites and growth cones

The functional and structural differences between neurites and growth cones suggests the possibility that distinct microfilament populations may exist in each domain. Tropomyosins are integral components of the actin-based microfilament system. Using antibodies which detect three different sets of tropomyosin isoforms, we found that the vast majority of tropomyosin was found in a microfilament-enriched fraction of cultured cortical neurons, therefore enabling us to use the antisera to evaluate compositional differences in neuritic and growth cone microfilaments. An antibody which reacts with all known nonmuscle isoforms of the alpha Tms gene (Tm5NM1-4) stains both neurites and growth cones, whereas a second antibody against the isoform subset, Tm5NM1-2, reacts only with the neurite. A third antibody which reacts with the Tm5a/5b isoforms encoded by a separate gene from alpha Tms was strongly reactive with both neurites and growth cones in 16-h cultures but only with the neurite shaft in 40-h cultures. Treatment of neurons with cytochalasin B allowed neuritic Tm5NM1-2 to spread into growth cones. Removal of the drug resulted in the disappearance of Tm5NM1-2 from the growth cone, indicating that isoform segregation is an active process dependent on intact microfilaments. Treatment of 40-h cultures with nocodazole resulted in the removal of Tm5NM1-2 from the neurite whereas Tm5a/5b now spread back into the growth cone. We conclude that the organization of Tm5NM1-2 and Tm5a/5b in the neurite is at least partially dependent on microtubule integrity. These results indicate that tropomyosin isoforms Tm5NM1-2, Tm5NM3-4, and Tm5a/5b mark three distinct populations of actin filaments in neurites and growth cones. Further, the composition of microfilaments differs between neurites and growth cones and is subject to temporal regulation.

Tropomyosin isoforms in nonmuscle cells

Vertebrate nonmuscle cells, such as human and rat fibroblasts, express multiple isoforms of tropomyosin, which are generated from four different genes and a combination of alternative promoter activities and alternative splicing. The amino acid variability among these isoforms is primarily restricted to three alternatively spliced exon regions; an amino-terminal region, an internal exon, and a carboxyl-terminal exon. Recent evidence reveals that these variable exon regions encode amino acid sequences that may dictate isoform-specific functions. The differential expression of tropomyosin isoforms found in cell transformation and cell differentiation, as well as the differential localization of tropomyosin isoforms in some types of culture cells and developing neurons suggest a differential isoform function in vivo. Tropomyosin in striated muscle works together with the troponin complex to regulate muscle contraction in a Ca(2+)-dependent fashion. Both in vitro and in vivo evidence suggest that multiple isoforms of tropomyosin in nonmuscle cells may be required for regulating actin filament stability, intracellular granule movement, cell shape determination, and cytokinesis. Tropomyosin-binding proteins such as caldesmon, tropomodulin, and other unidentified proteins may be required for some of these functions. Strong evidence for the distinct functions carried out by different tropomyosin isoforms has been generated from genetic analysis of yeast and Drosophila tropomyosin mutants.

Affinity and structure of complexes of tropomyosin and caldesmon domains

The interaction of caldesmon domains with tropomyosin has been studied using x-ray crystallography and an optical biosensor. Only whole caldesmon and the carboxyl-terminal domain of caldesmon (CaD-4, chicken gizzard residues 597-756) bound to tropomyosin with greater than millimolar affinity at 100 and 150 microM salt. Under these conditions the affinities of whole caldesmon and CaD-4 were both in the micromolar range. Data from the x-ray studies showed that whole caldesmon bound to tropomyosin in several places, with the region of tightest interaction being at tropomyosin residues 70-100 and/or 230-260. Studies with CaD-4 revealed that this region corresponded to the strong binding site seen with whole caldesmon. Weaker association of other regions of caldesmon to tropomyosin residues 180-210 and 5-50 was also observed. The results suggest that the carboxyl-terminus of caldesmon binds tightly to tropomyosin and that other regions of caldesmon may interact with tropomyosin tightly only when they are held close to tropomyosin by the carboxyl-terminal domain. Four models are presented to show the possible interactions of caldesmon with tropomyosin.

A high molecular mass non-muscle tropomyosin isoform stimulates retrograde organelle transport

Although non-muscle tropomyosins (TM) have been implicated in various cellular functions, such as stabilization of actin filaments and possibly regulation of organelle transport, their physiological role is still poorly understood. We have probed the role of a high molecular mass isoform of human fibroblast TM, hTM3, in regulating organelle transport by microinjecting an excess amount of bacterially-expressed protein into normal rat kidney (NRK) epithelial cells. The microinjection induced the dramatic retrograde translocation of organelles into the perinuclear area. Microinjection of hTM5, a low molecular mass isoform had no effect on organelle distribution. Fluorescent staining indicated that hTM3 injection stimulated the retrograde movement of both mitochondria and lysosomes. Moreover, both myosin I and cytoplasmic dynein were found to redistribute with the translocated organelles to the perinuclear area, indicating that these organelles were able to move along both microtubules and actin filaments. The involvement of microtubules was further suggested by the partial inhibition of hTM3-induced organelle movement by the microtubule-depolymerizing drug nocodazole. Our results, along with previous genetic and antibody microinjection studies, suggest that hTM3 may be involved in the regulation of organelle transport.

Overexpression of microfilament-stabilizing human caldesmon fragment, CaD39, affects cell attachment, spreading, and cytokinesis

Previous studies have demonstrated that overexpression of the carboxyl-terminal fragment, CaD39, of human fibroblast caldesmon in Chinese hamster ovary cells protected endogenous tropomyosin from turnover and stabilized actin microfilament bundles [Warren et al., 1994: J. Cell Biol. 125:359-368]. To assess the consequences of having CaD39-stabilized microfilaments in living cell, we characterized the motile behaviors of stable CaD39-expressing lines. We here found that CaD39-expressing cells adhered faster to plastic, glass, fibronectin-coated glass, and collagen-coated glass than control cells. Moreover, the CaD39-expressing cells also exhibited enhanced spreading immediately after attachment. Despite these differences, overexpression of CaD39 had little effect on the velocity of intracellular granule movement, or the velocity and persistence of cellular translocation. However, CaD39-expressing cells were more elongate and encompassed less area than non-expressing cells during migration in a wound-healing assay. In interphase cells, the expressed CaD39 fragments were found associated with tropomyosin-enriched microfilaments. Like endogenous caldesmon, the CaD39 fragment was also modified at mitosis. Although a significant portion of CaD39 underwent only partial modification, the majority of the CaD39 was released from the microfilaments during mitosis. This is consistent with the finding that the CaD39-induced advantage for attachment and spreading was lost during mitosis. In CaD39-expressing cells, an incomplete release of the CaD39 from microfilaments at mitosis was found which may be responsible for the increase in the frequency of multinuclear cells in CaD39-expressing lines.

Essential role of caldesmon in the actin filament reorganization induced by glucocorticoids

Glucocorticoids induce the remodeling of the actin cytoskeleton and the formation of numerous stress fibers in a protein synthesis-dependent fashion in a variety of cell types (Castellino, F., J. Heuser, S. Marchetti, B. Bruno, and A. Luini. 1992. Proc. Natl. Acad. Sci. USA. 89:3775-3779). These cells can thus be used as models to investigate the mechanisms controlling the organization of actin filaments. Caldesmon is an almost ubiquitous actin- and calmodulin-binding protein that synergizes with tropomyosin to stabilize microfilaments in vitro (Matsumura, F., and Yamashiro, S. 1993. Current Opin. Cell Biol. 5:70-76). We now report that glucocorticoids (but not other steroids) enhanced the levels of caldesmon (both protein and mRNA) and induced the reorganization of microfilaments with similar time courses and potencies in A549 cells. A caldesmon antisense oligodeoxynucleotide targeted to the most abundant caldesmon isoform in A549 cells dramatically inhibited glucocorticoid-induced caldesmon synthesis and actin reorganization with similar potencies. Several control oligonucleotides were inactive. These results demonstrate that caldesmon has a crucial role in vivo in the organization of the actin cytoskeleton and suggest that hormone-induced changes in caldesmon levels mediate microfilament remodeling.

Location of smooth-muscle myosin and tropomyosin binding sites in the C-terminal 288 residues of human caldesmon

We have produced nine recombinant fragments, H1 to H9, from a human cDNA that codes for the C-terminal 288 residues of caldesmon. The fragment H1, encompassing the 288 residues, is equivalent to domains 3 and 4 of caldesmon (amino acids 506-793 in human, 476-737 in the chicken gizzard sequence). It has been shown [Huber, Redwood, Avent, Tanner and Marston (1993) J. Muscle Res. Cell Motil. 14, 385-391] to bind to actin, Ca(2+)-calmodulin, tropomyosin and myosin. The fragments, H2 to H9, differ in length between 60 and 176 residues and cover the whole of domains 3 and 4 with many of the fragments overlapping. We have characterized the myosin and tropomyosin binding of these fragments. The binding of both tropomyosin and myosin is highly dependent on salt concentration, indicating the ionic nature of these interactions. The location of the myosin binding is an extended region encompassing the junction of domains 3/4 and domain 4a (residues 622-714, human; 566-657, chicken gizzard). Tropomyosin binds in a smaller region within domain 4a of caldesmon (residues 663-714, human; 606-657 chicken gizzard). We confirmed predictions based on sequence similarities of a tropomyosin binding site in domain 3 of caldesmon; however, this site bound to skeletal-muscle tropomyosin and had little affinity for the smooth-muscle tropomyosin isoform. None of the protein fragments H2-H9 retained the affinity of the parent fragment H1 for either myosin or tropomyosin. This indicates the need for several interaction sites scattered over an extended region to attain higher affinity. The regions interacting with caldesmon in both tropomyosin and myosin are coiled-coil structures. This is probably the reason for their shared interaction sites on caldesmon and their similar natures of binding.

Two isoforms of Drosophila dynamin in wild-type and shibire(ts) neural tissue: different subcellular localization and association mechanisms

The temperature-sensitive mutations of the shibire (shi) gene in Drosophila cause endocytic arrest, resulting in neurotransmission block and paralysis at high temperatures. However, underlying mechanism for the defects is not yet known. We examined the subcellular distribution of dynamin, a product of the shi gene, by immunoblotting and immunocytochemical assays. Two isoforms of dynamin with apparent M(r) of 92 kD and 94 kD have been detected in wild-type and shi(n) adult neural tissue. The two isoforms were reproducibly associated with different subcellular fractions of head homogenates. The 94kD isoform is fractionated in the low speed (2.000 x g) pellet containing plasma membrane fragments, and the 92kD isoform in the high speed (130,000 x g) pellet. In this procedure, very little dynamin remained in the high speed supernatant fraction. The 94 kD isoform represents the majority (65-75%) of total dynamin and appears to be a peripheral membrane protein. It can be extracted from the low speed membrane pellet by high salt, Na2CO3 (pH 11) or Triton X-100 treatments. Extracted 94kD dynamin from both wild-type and mutant homogenates is able to reassociate with artificial phospholipid vesicles at both permissive and restrictive temperatures. Binding of the 94 kD dynamin to liposomes appears to be pH-dependent, varying most significantly within the physiological pH range, which may be functionally important. The 92 kD isoform cannot be released by high salt or Na2CO3 treatments and only a small fraction is released by Triton X-100, suggesting a different mechanism of association with cell structures. The distribution of the two isoforms is not altered by the presence of stabilized microtubules in homogenates. No apparent degradation or subcellular redistribution of mutant dynamin was detected in two shi(n) alleles after heat shock or block of the dynamin GTPase activity, suggesting that intracellular redistribution or degradation of mutant dynamin are not involved in the endocytosis arrest in these mutants. These observations resemble the effect of endocytosis arrest by GTP-gamma-S in rat brain synaptosomes (Takei et al., 1995), in which dynamin is trapped at the neck of invaginated pits but is absent in the clathrin-coated distal end undergoing internalization. Our finding that endocytosis arrest by shi(n) mutations and GTP-gamma-S do not lead to cumulation of dynamin in the low speed pellet fraction further suggests that the 94 kD isoform remains associated with the plasma membrane during coated vesicle pinch-off and that the two isoforms do not appear to correspond to different functional states of dynamin but are likely to be involved in separate cellular compartments within the membrane cycling pathway (e.g., the plasma membrane, endosomes, and endoplasmic reticulum).

Molecular dissection of radixin: distinct and interdependent functions of the amino- and carboxy-terminal domains

The ERM proteins--ezrin, radixin, and moesin--occur in particular cortical cytoskeletal structures. Several lines of evidence suggest that they interact with both cytoskeletal elements and plasma membrane components. Here we described the properties of full-length and truncated radixin polypeptides expressed in transfected cells. In stable transfectants, exogenous full-length radixin behaves much like endogenous ERM proteins, localizing to the same cortical structures. However, the presence of full-length radixin or its carboxy-terminal domain in cortical structures correlates with greatly diminished staining of endogenous moesin in those structures, suggesting that radixin and moesin compete for a limiting factor required for normal associations in the cell. The results also reveal distinct roles for the amino- and carboxy-terminal domains. At low levels relative to endogenous radixin, the carboxy-terminal polypeptide is associated with most of the correct cortical targets except cleavage furrows. In contrast, the amino-terminal polypeptide is diffusely localized throughout the cell. Low level expression of full-length radixin or either of the truncated polypeptides has no detectable effect on cell physiology. However, high level expression of the carboxy-terminal domain dramatically disrupts normal cytoskeletal structures and functions. At these high levels, the amino-terminal polypeptide does localize to cortical structures, but does not affect the cells. We conclude that the behavior of radixin in cells depends upon activities contributed by separate domains of the protein, but also requires modulating interactions between those domains.

Forced expression of chimeric human fibroblast tropomyosin mutants affects cytokinesis

Human fibroblasts generate at least eight tropomyosin (TM) isoforms (hTM1, hTM2, hTM3, hTM4, hTM5, hTM5a, hTM5b, and hTMsm alpha) from four distinct genes, and we have previously demonstrated that bacterially produced chimera hTM5/3 exhibits an unusually high affinity for actin filaments and a loss of the salt dependence typical for TM-actin binding (Novy, R.E., J. R. Sellers, L.-F. Liu, and J.J.-C. Lin, 1993. Cell Motil. & Cytoskeleton. 26: 248-261). To examine the functional consequences of expressing this mutant TM isoform in vivo, we have transfected CHO cells with the full-length cDNA for hTM5/3 and compared them to cells transfected with hTM3 and hTM5. Immunofluorescence microscopy reveals that stably transfected CHO cells incorporate force-expressed hTM3 and hTM5 into stress fibers with no significant effect on general cell morphology, microfilament organization or cytokinesis. In stable lines expressing hTM5/3, however, cell division is slow and sometimes incomplete. The doubling time and the incidence of multinucleate cells in the stable hTM5/3 lines roughly parallel expression levels. A closely related chimeric isoform hTM5/2, which differs only in the internal, alternatively spliced exon also produces defects in cytokinesis, suggesting that normal TM function may involve coordination between the amino and carboxy terminal regions. This coordination may be prevented in the chimeric mutants. As bacterially produced hTM5/3 and hTM5/2 can displace hTM3 and hTM5 from actin filaments in vitro, it is likely that CHO-expressed hTM5/3 and hTM5/2 can displace endogenous TMs to act dominantly in vivo. These results support a role for nonmuscle TM isoforms in the fine tuning of microfilament organization during cytokinesis. Additionally, we find that overexpression of TM does not stabilize endogenous microfilaments, rather, the hTM-expressing cells are actually more sensitive to cytochalasin B. This suggests that regulation of microfilament integrity in vivo requires stabilizing factors other than, or in addition to, TM.

Over-expression of smooth muscle caldesmon in mouse fibroblasts

Caldesmon is an actin, calmodulin, tropomyosin, and myosin binding protein implicated in the regulation of actomyosin interactions. We have investigated the effect of overexpression of the higher molecular weight smooth muscle isoform of caldesmon on mouse L cell physiology. Mouse L(TK-) cells were transfected stably with plasmids carrying the TK+ gene and a full length human smooth muscle caldesmon cDNA under control of the adenovirus major late promoter. Two clones displaying four and eight times the level of the endogenous mouse high molecular weight caldesmon were isolated. These cells acquire a distinct phenotype characterized by an altered morphology, including an increased number of processes and larger area due to enhanced cell spreading, and a significantly slower growth rate than that of untransfected control cells, or cells transfected with the TK+ gene alone. The majority of the overexpressed caldesmon appears to be active and localized on cytoskeleton structures as determined by detergent lysis. Immunofluorescence analysis of the clones revealed that the caldesmon is localized as punctate staining on stress-fibers and in membrane ruffles. The immunofluorescence images suggest that caldesmon overexpressing cells have more total filaments than control cells. The effects of excess caldesmon on cell mobility are ambiguous: one clone displayed increased motility compared to the control, while the motility of the second clone was decreased relative to the control.