The single CH domain of calponin is neither sufficient nor necessary for F-actin binding

GxcDD, a putative RacGEF, is involved in Dictyostelium development

Background

Rho subfamily GTPases are implicated in a large number of actin-related processes. They shuttle from an inactive GDP-bound form to an active GTP-bound form. This reaction is catalysed by Guanine nucleotide exchange factor (GEFs). GTPase activating proteins (GAPs) help the GTPase return to the inactive GDP-bound form. The social amoeba Dictyostelium discoideum lacks a Rho or Cdc42 ortholog but has several Rac related GTPases. Compared to our understanding of the downstream effects of Racs our understanding of upstream mechanisms that activate Rac GTPases is relatively poor.

Results

We report on GxcDD (Guanine exchange factor for Rac GTPases), a Dictyostelium RacGEF. GxcDD is a 180-kDa multidomain protein containing a type 3 CH domain, two IQ motifs, three PH domains, a RhoGEF domain and an ArfGAP domain. Inactivation of the gene results in defective streaming during development under different conditions and a delay in developmental timing. The characterization of single domains revealed that the CH domain of GxcDD functions as a membrane association domain, the RhoGEF domain can physically interact with a subset of Rac GTPases, and the ArfGAP-PH tandem accumulates in cortical regions of the cell and on phagosomes. Our results also suggest that a conformational change may be required for activation of GxcDD, which would be important for its downstream signaling.

Conclusion

The data indicate that GxcDD is involved in proper streaming and development. We propose that GxcDD is not only a component of the Rac signaling pathway in Dictyostelium, but is also involved in integrating different signals. We provide evidence for a Calponin Homology domain acting as a membrane association domain. GxcDD can bind to several Rac GTPases, but its function as a nucleotide exchange factor needs to be studied further.

Trix, a novel Rac guanine-nucleotide exchange factor from Dictyostelium discoideum is an actin-binding protein and accumulates at endosomes

Small Rho family GTPases are involved in regulation of actin cytoskeleton dynamics. These molecular switches are themselves mainly controlled by specific GTPase-activating proteins (GAPs) and guanine-nucleotide exchange factors (GEFs). We have cloned and initially characterized a novel putative RhoGEF from Dictyostelium discoideum. The predicted 135-kDa protein displays a unique domain organization in its N-terminus by harboring two type3 calponin homology (CH) domains followed by a single type1 CH domain. The C-terminal region encompasses a diffuse B-cell lymphoma homology/pleckstrin homology tandem domain that is typically found in RhoGEFs. We therefore refer to this protein as Trix (triple CH-domain array exchange factor). A recombinant N-terminal region of Trix carrying all three CH domains binds to F-actin and bundles actin filaments. Trix-null mutants are viable and display only subtle defects when compared to wild-type cells with the exception of a substantial decrease in exocytosis of a fluid-phase marker. GFP fusions with the full-length protein or the N-terminal part containing all three CH domains revealed that Trix localizes to the cortical region and strongly accumulates on late endosomes. Our results suggest that Trix is specifically involved in a Rho GTPase-signaling pathway that is required for regulation of the actin cytoskeleton during exocytosis.

The CH-domain of calponin does not determine the modes of calponin binding to F-actin

Many actin-binding proteins have been observed to have a modular architecture. One of the most abundant modules is the calponin-homology (CH) domain, found as tandem repeats in proteins that cross-link actin filaments (such as fimbrin, spectrin and alpha-actinin) or link the actin cytoskeleton to intermediate filaments (such as plectin). In proteins such as the eponymous calponin, IQGAP1, and Scp1, a single CH-domain exists, but there has been some controversy over whether this domain binds to actin filaments. A previous three-dimensional reconstruction of the calponin-F-actin complex has led to the conclusion that the visualized portion of calponin bound to actin belongs to its amino-terminal homology (CH) domain. We show, using a calponin fragment lacking the CH-domain, that this domain is not bound to F-actin, and cannot be positioning calponin on F-actin as hypothesized. Further, using classification methods, we show a multiplicity in cooperative modes of binding of calponin to F-actin, similar to what has been observed for other actin-binding proteins such as tropomyosin and cofilin. Our results suggest that the form and function of the structurally conserved CH-domain found in many other actin-binding proteins have diverged. This has broad implications for inferring function from the presence of structurally conserved domains.

Xin-repeats and nebulin-like repeats bind to F-actin in a similar manner

Xin and nebulette are striated muscle-specific actin-binding proteins that both contain multiple actin-binding repeats. The nature of these repeats is different: nebulette has nebulin-like repeats, while Xin contains its own unique repeats. However, the suggestion was made from biochemical data that the Xin-repeats may bind to multiple sites on the actin molecule as was found for nebulin. We have used electron microscopy and the iterative helical real space reconstruction to visualize complexes of F-actin with Xin fragments containing either three or six Xin-repeats, and with the CN5-nebulette fragment, containing five nebulin-like repeats. Our results indicate that Xin and nebulette fragments bind to F-actin in a similar manner and in two distinct modes: in one mode actin subdomain 1 is bound, while in the second mode the binding bridges between a different site on actin subdomains 1/2 of one protomer and subdomains 3/4 of an adjacent actin protomer. Taken together with published data about nebulin, tropomyosin and ADF/cofilin, our results suggest that the ability to bind in multiple modes to the actin protomer is a general property of many actin-binding proteins.

Analysis of the kinesin superfamily: insights into structure and function

Kinesin superfamily proteins (KIFs) are key players or 'hub' proteins in the intracellular transport system, which is essential for cellular function and morphology. The KIF superfamily is also the first large protein family in mammals whose constituents have been completely identified and confirmed both in silico and in vivo. Numerous studies have revealed the structures and functions of individual family members; however, the relationships between members or a perspective of the whole superfamily structure until recently remained elusive. Here, we present a comprehensive summary based on a large, systematic phylogenetic analysis of the kinesin superfamily. All available sequences in public databases, including genomic information from all model organisms, were analyzed to yield the most complete phylogenetic kinesin tree thus far, comprising 14 families. This comprehensive classification builds on the recently proposed standardized nomenclature for kinesins and allows systematic analysis of the structural and functional relationships within the kinesin superfamily.

Stg1 is a novel SM22/transgelin-like actin-modulating protein in fission yeast

We identified a novel actin-modulating protein Stg 1 in the fission yeast Schizosaccharomyces pombe. Stg 1 is similar to mammalian SM22/transgelin, and biochemical experiments showed that Stg 1 crosslinked F-actin. Microscopic observation suggested that Stg 1 was a component of actin patch. Overexpression of Stg 1 caused a defect in cytokinesis by suppressing the formation of a contractile ring and formation of abnormal aggregates of F-actin in the ends and mid-region of cells. Although distribution of the actin cytoskeleton was not affected by disrupting Stg 1(+), genetic interaction suggested that Stg 1 was likely involved in controlling the organization of the actin cytoskeleton in cell morphogenesis and cytokinesis in fission yeast.

Cross-stress tolerance and expression of stress-related proteins in osmotically desiccated entomopathogenicSteinernema feltiaeIS-6

Infective juveniles (IJs) of the entomopathogenic nematode (EPN)Steinernema feltiaeIS-6 can survive exposure to 24% glycerol solution by entering an osmotically desiccated state. Exposure of osmotically desiccated nematodes to extreme temperature assays (40 °C for 10 h and −20 °C for 360 h) resulted in gradual reduction in survival, whereas non-desiccated IJs died within a short exposure to the assay conditions. Through SDS-PAGE, a stress-related protein UNC-87 was found in osmotically desiccated IJs exposed to 40 °C for 3, 6, and 8 h, whose survival rates were 98·9±1·43, 78·5±5·87 and 20·9±4·93%, respectively. The protein was not found in IJs following exposure of osmotically desiccated individuals to 40 °C for 10 h, in which none of the IJs survived. After exposure to −20 °C for 360 h, the survival of osmotically desiccated EPNs with a weak band of UNC-87 was 13·0±3·32%. To identify other responsive proteins that are required for osmotic stress, we used 2-dimensional electrophoresis to analyse the proteins in osmotically desiccated EPNs. The results revealed that 10 novel protein spots and 10 up-regulated protein spots in osmotically desiccated IJs were detected by digital image analysis. Mass spectrometry analysis of 7 significant spots indicated that osmotic stress in desiccated IJs was associated with the induction of actin, Proteasome regulatory particle (ATPase-like), GroEL chaperonin, GroES co-chaperonin and transposase family member. It seems to show actin, UNC-87 and Proteasome regulatory particle may play distinct roles in specific aspects of organization of macromolecular structures under desiccation stress. GroEL and GroES are members of the Hsp60 family of chaperons.

Identification of caspase-3 degradome by two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization-time of flight analysis

The activation of caspases is a critical event for the execution phase of programmed cell death. Caspases are highly specific in their ability to activate or inhibit many crucial proteins in the cell via site-specific cleavage. To date, more than 60 proteins have been shown to be substrates of one or more caspases in mammalian cells, and the list is still growing. In this study, to identify human caspase-3 substrates, we digested lysates obtained from a caspase-3-deficient MCF-7 cell line with purified caspase-3 and analyzed eliminated or decreased spots by 2-DE. Proteins degraded by caspase-3, termed as caspase-3 degradome, are involved in a variety of cellular functions, such as stress-responsive proteins, signaling molecules, structural proteins, and unclassified proteins. Interestingly, the cellular level of vinculin, a caspase-3 substrate, was dramatically reduced during the apoptotic process, where the expression level of caspase-3 was increased. This degradomic approach could provide a powerful tool in finding physiological substrates of many proteolytic enzymes whose functions remain to be determined.

Spectrin, alpha-actinin, and dystrophin

Spectrin family proteins represent an important group of actin-bundling and membrane-anchoring proteins found in diverse structures from yeast to man. Arising from a common ancestral alpha-actinin gene through duplications and rearrangements, the family has increased to include the spectrins and dystrophin/utrophin. The spectrin family is characterized by the presence of spectrin repeats, actin binding domains, and EF hands. With increasing divergence, new domains and functions have been added such that spectrin and dystrophin also contain specialized protein-protein interaction motifs and regions for interaction with membranes and phospholipids. The acquisition of new domains also increased the functional complexity of the family such that the proteins perform a range of tasks way beyond the simple bundling of actin filaments by alpha-actinin in S. pombe. We discuss the evolutionary, structural, functional, and regulatory roles of the spectrin family of proteins and describe some of the disease traits associated with loss of spectrin family protein function.

The Arg non-receptor tyrosine kinase modifies F-actin structure

The Arg (Abl-related gene) protein belongs to the Abl family of non-receptor tyrosine kinases that regulate cell motility and morphogenesis. It contains two actin-binding domains, one containing the talin-like I/LWEQ motif, and a C-terminal calponin homology (CH) domain. We used electron microscopy and single particle image analysis to reconstruct complexes of F-actin with full-length Arg, and fragments lacking either the I/LWEQ or CH domains. The Arg CH domain binds to actin's subdomain-1 (SD1) and induces a tilt of actin protomers. The I/LWEQ domain binds to either SD1 or SD4, closing the nucleotide binding cleft of actin. Although Arg can use either its CH or ILWEQ domains to bind an actin filament, both domains within Arg cannot bind simultaneously to adjacent protomers in the filament, consistent with its F-actin-bundling activity. The conformational changes in the filament introduced by Arg can explain the cooperative binding of Arg to F-actin and might prevent other actin binding proteins from binding to actin filaments.

Modulation of smooth muscle contractility by CHASM, a novel member of the smoothelin family of proteins

Cyclic nucleotides acting through their associated protein kinases, the cGMP- and cAMP-dependent protein kinases, can relax smooth muscles without a change in free intracellular calcium concentration ([Ca2+]i), a phenomenon referred to as Ca2+ desensitization. The molecular mechanisms by which these kinases bring about Ca2+ desensitization are unknown and an understanding of this phenomenon may lead to better therapies for treating diseases involving defects in the contractile response of smooth muscles such as hypertension, bronchospasm, sexual dysfunction, gastrointestinal disorders and glaucoma. Utilizing a combination of real-time proteomics and smooth muscle physiology, we characterized a distinct subset of protein targets for cGMP-dependent protein kinase in smooth muscle. Among those phosphoproteins identified was calponin homology-associated smooth muscle (CHASM), a novel protein that contains a calponin homology domain and shares sequence similarity with the smoothelin family of smooth muscle specific proteins. Recombinant CHASM was found to evoke relaxation in a concentration dependent manner when added to permeabilized smooth muscle. A co-sedimentation assay with actin demonstrated that CHASM does not possess actin binding activity. Our findings indicate that CHASM is a novel member of the smoothelin protein family that elicits Ca2+ desensitization in smooth muscle.

Xin repeats define a novel actin-binding motif

Xin is a protein that is expressed during early developmental stages of cardiac and skeletal muscles. Immunolocalization studies indicated a peripheral localization in embryonic mouse heart, where Xin localizes with beta-catenin and N-cadherin. In adult tissues, Xin is found primarily in the intercalated discs of cardiomyocytes and the myotendinous junctions of skeletal muscle cells, both specialized attachment sites of the myofibrillar ends to the sarcolemma. A large part of the Xin protein consists of unique 16 amino acid repeats with unknown function. We have investigated the characteristics of the Xin repeats by transfection experiments and actin-binding assays and ascertained that, upon expression in cultured cells, these repeats bind to and stabilize the actin-based cytoskeleton. In vitro co-sedimentation assays with skeletal muscle actin indicated that they not only directly bind actin filaments, but also have the capability of arranging microfilaments into networks that sediment upon low-speed centrifugation. Very similar repeats were also found in 'Xin-repeat protein 2' (XIRP2), a novel protein that seems to be expressed mainly in striated muscles. Human XIRP2 contains 28 Xin repeats with properties identical to those of Xin. We conclude that the Xin repeats define a novel, repetitive actin-binding motif present in at least two different muscle proteins. These Xin-repeat proteins therefore constitute the first two members of a novel family of actin-binding proteins.

Actin filament binding by a monomeric IQGAP1 fragment with a single calponin homology domain

IQGAP1 is a homodimeric protein that reversibly associates with F-actin, calmodulin, activated Cdc42 and Rac1, CLIP-170, beta-catenin, and E-cadherin. Its F-actin binding site includes a calponin homology domain (CHD) located near the N-terminal of each subunit. Prior studies have implied that medium- to high-affinity F-actin binding (5-50 microM K(d)) requires multiple CHDs located either on an individual polypeptide or on distinct subunits of a multimeric protein. For IQGAP1, a series of six tandem IQGAP coiled-coil repeats (IRs) located past the C-terminal of the CHD of each subunit support protein dimerization and, by extension, the IRs or an undefined subset of them were thought to be essential for F-actin binding mediated by its CHDs. Here we describe efforts to determine the minimal region of IQGAP1 capable of binding F-actin. Several truncation mutants of IQGAP1, which contain progressive deletions of the IRs and CHD, were assayed for F-actin binding in vitro. Fragments that contain both the CHD and at least one IR could bind F-actin and, as expected, removal of all six IRs and the CHD abolished binding. Unexpectedly, a fragment called IQGAP1(2-210), which contains the CHD, but lacks IRs, could bind actin filaments. IQGAP1(2-210) was found to be monomeric, to bind F-actin with a K(d) of approximately 47 microM, to saturate F-actin at a molar ratio of one IQGAP1(2-210) per actin monomer, and to co-localize with cortical actin filaments when expressed by transfection in cultured cells. These collective results identify the first known example of high-affinity actin filament binding mediated by a single CHD.

Actin-binding domain of mouse plectin. Crystal structure and binding to vimentin

Plectin, a large and widely expressed cytolinker protein, is composed of several subdomains that harbor binding sites for a variety of different interaction partners. A canonical actin-binding domain (ABD) comprising two calponin homology domains (CH1 and CH2) is located in proximity to its amino terminus. However, the ABD of plectin is unique among actin-binding proteins as it is expressed in the form of distinct, plectin isoform-specific versions. We have determined the three-dimensional structure of two distinct crystalline forms of one of its ABD versions (pleABD/2alpha) from mouse, to a resolution of 1.95 and 2.0 A. Comparison of pleABD/2alpha with the ABDs of fimbrin and utrophin revealed structural similarity between plectin and fimbrin, although the proteins share only low sequence identity. In fact, pleABD/2alpha has been found to have the same compact fold as the human plectin ABD and the fimbrin ABD, differing from the open conformation described for the ABDs of utrophin and dystrophin. Plectin harbors a specific binding site for intermediate filaments of various types within its carboxy-terminal R5 repeat domain. Our experiments revealed an additional vimentin-binding site of plectin, residing within the CH1 subdomain of its ABD. We show that vimentin binds to this site via the amino-terminal part of its rod domain. This additional amino-terminal intermediate filament protein binding site of plectin may have a function in intermediate filament dynamics and assembly, rather than in linking and stabilizing intermediate filament networks.

Tyrosine phosphorylation of calponins. Inhibition of the interaction with F-actin

The phosphorylation-dephosphorylation of serine and threonine residues of calponin is known to modulate in vitro its interaction with F-actin and is thought to regulate several biological processes in cells, involving either of the calponin isoforms. Here, we identify, for the first time, tyrosine-phosphorylated calponin h3 within COS 7 cells, before and after their transfection with the pSV vector containing cDNA encoding the cytoplasmic, Src-related, tyrosine kinase, Fyn. We then describe the specific tyrosine phosphorylation in vitro of calponin h1 and calponin h3 by this kinase. 32P-labeling of tyrosine residues was monitored by combined autoradiography, immunoblotting with a specific phosphotyrosine monoclonal antibody and dephosphorylation with the phosphotyrosine-specific protein phosphatase, YOP. PhosphorImager analyses showed the incorporation of maximally 1.4 and 2.0 mol of 32P per mol of calponin h3 and calponin h1, respectively. As a result, 75% and 68%, respectively, of binding to F-actin was lost by the phosphorylated calponins. Furthermore, F-actin, added at a two- or 10-fold molar excess, did not protect, but rather increased, the extent of 32P-labeling in both calponins. Structural analysis of the tryptic phosphopeptides from each 32P-labeled calponin revealed a single, major 32P-peptide in calponin h3, with Tyr261 as the phosphorylation site. Tyr261 was also phosphorylated in calponin h1, together with Tyr182. Collectively, the data point to the potential involvement, at least in living nonmuscle cells, of tyrosine protein kinases and the conserved Tyr261, located in the third repeat motif of the calponin molecule, in a new level of regulation of the actin-calponin interaction.

Alpha1-adrenergic signaling mechanisms in contraction of resistance arteries

Our goal in this review is to provide a comprehensive, integrated view of the numerous signaling pathways that are activated by alpha(1)-adrenoceptors and control actin-myosin interactions (i.e., crossbridge cycling and force generation) in mammalian arterial smooth muscle. These signaling pathways may be categorized broadly as leading either to thick (myosin) filament regulation or to thin (actin) filament regulation. Thick filament regulation encompasses both "Ca(2+) activation" and "Ca(2+)-sensitization" as it involves both activation of myosin light chain kinase (MLCK) by Ca(2+)-calmodulin and regulation of myosin light chain phosphatase (MLCP) activity. With respect to Ca(2+) activation, adrenergically induced Ca(2+) transients in individual smooth muscle cells of intact arteries are now being shown by high resolution imaging to be sarcoplasmic reticulum-dependent asynchronous propagating Ca(2+) waves. These waves differ from the spatially uniform increases in [Ca(2+)] previously assumed. Similarly, imaging during adrenergic activation has revealed the dynamic translocation, to membranes and other subcellular sites, of protein kinases (e.g., Ca(2+)-activated protein kinases, PKCs) that are involved in regulation of MLCP and thus in "Ca(2+) sensitization" of contraction. Thin filament regulation includes the possible disinhibition of actin-myosin interactions by phosphorylation of CaD, possibly by mitogen-activated protein (MAP) kinases that are also translocated during adrenergic activation. An hypothesis for the mechanisms of adrenergic activation of small arteries is advanced. This involves asynchronous Ca(2+) waves in individual SMC, synchronous Ca(2+) oscillations (at high levels of adrenergic activation), Ca(2+) sparks, "Ca(2+)-sensitization" by PKC and Rho-associated kinase (ROK), and thin filament mechanisms.

Neuronal protein NP25 interacts with F-actin

Neuronal protein NP25 is a neuron-specific protein present in highly differentiated neural cells, but its functional properties have not been well characterized. NP25 shows high amino acid sequence homology with the smooth muscle cell cytoskeleton-associated proteins, SM22, mp20, and calponin. To gain an insight into the biological functions of NP25, we first examined its subcellular localization in the human neuroblastoma cell line, SK-N-SH. NP25 diffusely distributed in the cytoplasm and fiber-like staining was also observed. It showed that NP25 co-localized with F-actin on stress fibers. A co-sedimentation assay demonstrated that NP25 bound to filamentous actin. Further investigations using fluorescence resonance energy transfer (FRET) technique revealed intracellular binding of NP25 and actin. The significance of the interaction between NP25 and F-actin is discussed.

Biochemical evidence for interaction between smoothelin and filamentous actin

The two major isoforms of smoothelin (A and B) contain a calponin homology (CH) domain, colocalize with alpha-smooth muscle actin (alpha-SMA) in stress fibers and are only expressed in contractile smooth muscle cells (SMCs). Based on these findings, we hypothesized that smoothelins are involved in smooth muscle cell contraction, presumably via interaction with actin. The interaction between smoothelins and three different actin isoforms (alpha- and gamma-smooth muscle and alpha-skeletal actin [alpha-SKA]) was investigated using several in vitro assays. Smoothelin-B co-immunoprecipitated with alpha-smooth muscle actin from pig aorta extracts. In rat embryonic fibroblasts, transfected smoothelins-A and -B associated with stress fibers. In vitro dot blot assays, in which immobilized actin was overlaid with radio-labeled smoothelin, showed binding of smoothelin-A to actin filaments, but not to monomeric G-actin. A truncated smoothelin, containing the calponin homology domain, associated with stress fibers when transfected and bound to actin filaments in overlay, but to a lesser extent. ELISA results showed that the binding of smoothelin to actin has no significant isoform specificity. Our results indicate an interaction between smoothelin and actin filaments. Moreover, the calponin homology domain and its surrounding sequences appear to be sufficient to accomplish this interaction, although the presence of other domains is apparently necessary to facilitate and/or strengthen the binding to actin.

The Saccharomyces cerevisiae calponin/transgelin homolog Scp1 functions with fimbrin to regulate stability and organization of the actin cytoskeleton

Calponins and transgelins are members of a conserved family of actin-associated proteins widely expressed from yeast to humans. Although a role for calponin in muscle cells has been described, the biochemical activities and in vivo functions of nonmuscle calponins and transgelins are largely unknown. Herein, we have used genetic and biochemical analyses to characterize the budding yeast member of this family, Scp1, which most closely resembles transgelin and contains one calponin homology (CH) domain. We show that Scp1 is a novel component of yeast cortical actin patches and shares in vivo functions and biochemical activities with Sac6/fimbrin, the one other actin patch component that contains CH domains. Purified Scp1 binds directly to filamentous actin, cross-links actin filaments, and stabilizes filaments against disassembly. Sequences in Scp1 sufficient for actin binding and cross-linking reside in its carboxy terminus, outside the CH domain. Overexpression of SCP1 suppresses sac6Delta defects, and deletion of SCP1 enhances sac6Delta defects. Together, these data show that Scp1 and Sac6/fimbrin cooperate to stabilize and organize the yeast actin cytoskeleton.

Calponin repeats regulate actin filament stability and formation of podosomes in smooth muscle cells

Phorbol ester induces actin cytoskeleton rearrangements in cultured vascular smooth muscle cells. Calponin and SM22 alpha are major components of differentiated smooth muscle and potential regulators of actin cytoskeleton interactions. Here we show that actin fibers decorated with h1 CaP remain stable, whereas SM22 alpha-decorated actin bundles undergo rapid reorganization into podosomes within 30 min of PDBu exposure. Ectopic expression of GFP alpha-actinin had no effect on the stability of the actin cytoskeleton and alpha-actinin was transported rapidly into PDBu-induced podosomes. Our results demonstrate the involvement of CaP and SM22 alpha in coordinating the balance between stabilization and dynamics of the actin cytoskeleton in mammalian smooth muscle. We provide evidence for the existence of two functionally distinct actin filament populations and introduce a molecular mechanism for the stabilization of the actin cytoskeleton by the unique actin-binding interface formed by calponin family-specific CLIK23 repeats.

Actin binding of a minispectrin

A "minispectrin" has been constructed from the tail end of the alpha/beta heterodimer, and its actin-binding properties have been characterised. It is a complex of the N-terminal fragment of the beta-subunit consisting of the actin-binding domain plus the two first triple-helical repeats beta 1 and beta 2, and the C-terminal fragment of the alpha-subunit containing the repeats alpha 19 and alpha 20 plus the calmodulin-like domain. This minispectrin exists in a dimeric form that contains one copy of each polypeptide and binds to actin in a cooperative manner with an apparent K(d) of 2.5 microM. Calcium seems not to have any effect on its binding to actin. Electron microscopic analysis shows that the minispectrin decorates actin filaments as clusters, and induces formation of actin bundles. This study shows that the actin-binding region of the spectrin alpha/beta heterodimer retains its functional properties in a truncated form and establishes basis for further research on spectrin's structure and function.

Mapping the microtubule binding regions of calponin

The smooth muscle basic calponin interacts with F-actin and inhibits the actomyosin ATPase in a calmodulin or phosphorylation modulated manner. It also binds in vitro to microtubules and its acidic isoform, present in nonmuscle cells, and co-localizes with microfilaments and microtubules in cultured neurons. To assess the physiological significance and the molecular basis of the calponin-microtubule interaction, we have first studied the solution binding of recombinant acidic calponin to microtubules using quantitative cosedimentation analyses. We have also characterized, for the first time, the ability of both calponin isoforms to induce the inhibition of the microtubule-stimulated ATPase activity of the cytoskeletal, kinesin-related nonclaret dysjunctional motor protein (ncd) and the abolition of this effect by calcium calmodulin. This property makes calponin a potent inhibitor of all filament-activated motor ATPases and, therefore, a potential regulatory factor of many motor-based biological events. By combining the enzymatic measurements of the ncd-microtubules system with various in vitro binding assays employing proteolytic, recombinant and synthetic fragments of basic calponin, we further unambiguously identified the interaction of microtubules at two distinct calponin sites. One is inhibitory and resides in the segment 145-182, which also binds F-actin and calmodulin. The other one is noninhibitory, specific for microtubules, and is located on the COOH-terminal repeat-containing region 183-292. Finally, quantitative fluorescence studies of the binding of basic calponin to the skeletal pyrenyl F-actin in the presence of microtubules did not reveal a noticeable competition between the two sets of filaments for calponin. This result implies that calponin undergoes a concomitant binding to both F-actin and microtubules by interaction at the former site with actin and at the second site with microtubules. Thus, in the living cells, calponin could potentially behave as a cross-linking protein between the two major cytoskeletal filaments.

A CH domain-containing N terminus in NuMA?

Nuclear mitotic apparatus protein (NuMA) is an essential vertebrate component in organizing microtubule ends at spindle poles. The NuMA-dynactin/dynein motor multiprotein complex not only explains the transport of NuMA along spindle fibers but also is linked to the process of microtubule focusing. The interaction sites of NuMA to dynein/dynactin have not been mapped. In the yet functionally uncharacterized N terminus of NuMA, we predict a calponin-homology (CH) domain, a motif with binding activity for actin-like molecules. We substantiate the primary sequence analysis-based prediction with secondary structure and fold recognition analysis, and we propose the N-terminal CH domain of NuMA as a likely interaction site for actin-related protein 1 (Arp1) protein of the dynactin/dynein complex.

Tensile stress stimulates microtubule outgrowth in living cells

Cell motility is driven by the sum of asymmetric traction forces exerted on the substrate through adhesion foci that interface with the actin cytoskeleton. Establishment of this asymmetry involves microtubules, which exert a destabilising effect on adhesion foci via targeting events. Here, we demonstrate the existence of a mechano-sensing mechanism that signals microtubule polymerisation and guidance of the microtubules towards adhesion sites under increased stress. Stress was applied either by manipulating the body of cells moving on glass with a microneedle or by stretching a flexible substrate that cells were migrating on. We propose a model for this mechano-sensing phenomenon whereby microtubule polymerisation is stimulated and guided through the interaction of a microtubule tip complex with actin filaments under tension.

The molecular basis for the autoregulation of calponin by isoform-specific C-terminal tail sequences

The three genetic isoforms of calponin (CaP), h1, h2 and acidic, are distinguished mostly by their individual C-terminal tail sequences. Deletion of these sequences beyond the last homologous residue Cys273 increases actin filament association for all three isoforms, indicating a negative regulatory role for the unique tail regions. We have tested this hypothesis by constructing a series of deletion and substitution mutants for all three CaP isoforms. Here we demonstrate that the C-terminal sequences regulate actin association by altering the function of the second actin-binding site, ABS2, in CaP comprised of the three 29-residue calponin repeats. Removal of the inhibitory tail resulted in an increased binding and bundling activity, and caused a prominent re-localization of h2 CaP from the peripheral actin network to the central actin stress fibers in transfected A7r5 smooth muscle cells. Domain-swap experiments demonstrated that the tail sequence of h2 CaP can downregulate cytoskeletal association efficiently in all three CaP isoforms, whereas the tail of the smooth-muscle-specific h1 CaP variant had little effect. Site-directed mutagenesis further revealed that the negatively charged residues within the tail region are essential for this regulatory function. Finally we demonstrate that the tail sequences regulate the second actin-binding site (ABS2) and not the strong actin-binding ABS1 region in CaP.

MICAL, a novel CasL interacting molecule, associates with vimentin

CasL/HEF1 belongs to the p130(Cas) family. It is tyrosine-phosphorylated following beta(1) integrin and/or T cell receptor stimulation and is thus considered to be important for immunological reactions. CasL has several structural motifs such as an SH3 domain and a substrate domain and interacts with many molecules through these motifs. To obtain more insights on the CasL-mediated signal transduction, we sought proteins that interact with the CasL SH3 domain by far Western screening, and we identified a novel human molecule, MICAL (a Molecule Interacting with CasL). MICAL is a protein of 118 kDa and is expressed in the thymus, lung, spleen, kidney, testis, and hematopoietic cells. MICAL has a calponin homology domain, a LIM domain, a putative leucine zipper motif, and a proline-rich PPKPP sequence. MICAL associates with CasL through this PPKPP sequence. MICAL is a cytoplasmic protein and colocalizes with CasL at the perinuclear area. Through the COOH-terminal region, MICAL also associates with vimentin that is a major component of intermediate filaments. Immunostaining revealed that MICAL localizes along with vimentin intermediate filaments. These results suggest that MICAL may be a cytoskeletal regulator that connects CasL to intermediate filaments.

Functional plasticity of CH domains

With the refinement of algorithms for the identification of distinct motifs from sequence databases, especially those using secondary structure predictions, new protein modules have been determined in recent years. Calponin homology (CH) domains were identified in a variety of proteins ranging from actin cross-linking to signaling and have been proposed to function either as autonomous actin binding motifs or serve a regulatory function. Despite the overall structural conservation of the unique CH domain fold, the individual modules display a quite striking functional variability. Analysis of the actopaxin/parvin protein family suggests the existence of novel (type 4 and type 5) CH domain families which require special attention, as they appear to be a good example for how CH domains may function as scaffolds for other functional motifs of different properties.

Solution structure of the calponin CH domain and fitting to the 3D-helical reconstruction of F-actin:calponin

Calponin is involved in the regulation of contractility and organization of the actin cytoskeleton in smooth muscle cells. It is the archetypal member of the calponin homology (CH) domain family of actin binding proteins that includes cytoskeletal linkers such as alpha-actinin, spectrin, and dystrophin, and regulatory proteins including VAV, IQGAP, and calponin. We have determined the first structure of a CH domain from a single CH domain-containing protein, that of calponin, and have fitted the NMR-derived coordinates to the 3D-helical reconstruction of the F-actin:calponin complex using cryo-electron microscopy. The tertiary fold of this single CH domain is typical of, yet significantly different from, those of the CH domains that occur in tandem pairs to form high-affinity ABDs in other proteins. We thus provide a structural insight into the mode of interaction between F-actin and CH domain-containing proteins.

β1PIX, the PAK-interacting exchange factor, requires localization via a coiled-coil region to promote microvillus-like structures and membrane ruffles

PIX is a Rho-family guanine nucleotide exchange factor that binds PAK. We previously described two isoforms of PIX that differ in their N termini. Here, we report the identification of a new splice variant of βPIX, designated β2PIX, that is the dominant species in brain and that lacks the region of ∼120 residues with predicted coiled-coil structure at the C terminus of β1PIX. Instead, β2PIX contains a serine-rich C terminus. To determine whether these splice variants differ in their cellular function, we studied the effect of expressing these proteins in HeLa cells. We found that the coiled-coil region plays a key role in the localization of β1PIX to the cell periphery and is also responsible for PIX dimerization. Overexpression of β1, but not β2PIX, drives formation of membrane ruffles and microvillus-like structures (via activation of Rac1 and Cdc42, respectively), indicating that its function requires localized activation of these GTPases. Thus, β1PIX, like other RhoGEFs, exerts specific morphological functions that are dependent on its intracellular location and are mediated by its C-terminal dimerization domain.

Invited review: cross-bridge regulation by thin filament-associated proteins

This minireview will cover current concepts on the identity and mechanistic function of smooth muscle actin binding proteins that may regulate actin-myosin interactions. The potential roles of tropomyosin, caldesmon, calponin, and SM22 will be discussed. The review, for purposes of brevity, will be nonexhaustive but will give an overview of available information on the in vitro biochemistry and potential in vivo function of these proteins. Preterm labor is discussed as a possible example of where thin filament regulation may be relevant. Considerable controversy surrounds the putative physiological significance of these proteins, and emphasis will be placed on the need for more experimental work to determine the degree to which tissue- and species-specific effects have clouded the interpretation of functional data.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

The interaction of plectin with actin: evidence for cross-linking of actin filaments by dimerization of the actin-binding domain of plectin

Plectin is a major component of the cytoskeleton and is expressed in a wide variety of cell types. It plays an important role in the integrity of the cytoskeleton by cross-linking the three filamentous networks and stabilizing cell-matrix and cell-cell contacts. Sequence analysis showed that plectin contains a highly conserved actin-binding domain, consisting of a pair of calponin-like subdomains. Using yeast two-hybrid assays in combination with in vitro binding experiments, we demonstrate that the actin-binding domain of plectin is fully functional and preferentially binds to polymeric actin. The sequences required for actin binding were identified at the C-terminal end of the first calponin homology domain within the actin-binding domain of plectin. We found that the actin-binding domain of plectin is able to bundle actin filaments and we present evidence that this is mediated by the dimerization of this domain. In addition we also show that plectin and another member of the plakin family, dystonin, can heterodimerize by their actin-binding domains. We propose a new mechanism by which plectin and possibly also other actin-binding proteins can regulate the organization of the F-actin network in the cell.

UNC-87 is an actin-bundling protein

The Caenorhabditis elegans unc-87 gene product is essential for the maintenance of the nematode body wall muscle where it is found colocalized with actin in the I band. The molecular domain structure of the protein reveals similarity to the C-terminal repeat region of the smooth muscle actin-binding protein calponin. In this study we investigated the in vitro function of UNC-87 using both the full-length recombinant molecule and several truncated mutants. According to analytical ultracentrifugation UNC-87 occurs as a monomer in solution. UNC-87 cosedimented with both smooth and skeletal muscle F-actin, but not with monomeric G-actin, and exhibited potent actin filament bundling activity. Actin binding was independent of the presence of tropomyosin and the actin cross-linking proteins filamin and alpha-actinin. Consistent with its actin bundling activity in vitro, UNC-87 tagged with green fluorescent protein associated with and promoted the formation of actin stress fiber bundles in living cells. These data identify UNC-87 as an actin-bundling protein and highlight the calponin-like repeats as a novel actin-binding module.

Parvin, a 42 kDa focal adhesion protein, related to the alpha-actinin superfamily

We have identified and cloned a novel 42-kDa protein termed alpha-parvin, which has a single alpha-actinin-like actin-binding domain. Unlike other members of the alpha-actinin superfamily, which are large multidomain proteins, alpha-parvin lacks a rod domain or any other C-terminal structural modules and therefore represents the smallest known protein of the superfamily. We demonstrate that mouse alpha-parvin is widely expressed as two mRNA species generated by alternative use of two polyadenylation signals. We analyzed the actin-binding properties of mouse alpha-parvin and determined the K(d) with muscle F-actin to be 8.4+/-2.1 microM. The GFP-tagged alpha-parvin co-localizes with actin filaments at membrane ruffles, focal contacts and tensin-rich fibers in the central area of fibroblasts. Domain analysis identifies the second calponin homology domain of parvin as a module sufficient for targeting the focal contacts. In man and mouse, a closely related paralogue beta-parvin and a more distant relative gamma-parvin have also been identified and cloned. The availability of the genomic sequences of different organisms enabled us to recognize closely related parvin-like proteins in flies and worms, but not in yeast and Dictyostelium. Phylogenetic analysis of alpha-parvin and its para- and orthologues suggests, that the parvins represent a new family of alpha-actinin-related proteins that mediate cell-matrix adhesion.

Mutual effects of alpha-actinin, calponin and filamin on actin binding

The mutual effect of three actin-binding proteins (alpha-actinin, calponin and filamin) on the binding to actin was analyzed by means of differential centrifugation and electron microscopy. In the absence of actin alpha-actinin, calponin and filamin do not interact with each other. Calponin and filamin do not interfere with each other in the binding to actin bundles. Slight interference was observed in the binding of alpha-actinin and calponin to actin bundles. Higher ability of calponin to depress alpha-actinin binding can be due to the higher stoichiometry calponin/actin in the complexes formed. The largest interference was observed in the pair filamin-alpha-actinin. These proteins interfere with each other in the binding to the bundled actin filaments; however, neither of them completely displaced another protein from its complexes with actin. The structure of actin bundles formed in the presence of any one actin-binding protein was different from that observed in the presence of binary mixtures of two actin-binding proteins. In the case of calponin or its binary mixtures with alpha-actinin or filamin the total stoichiometry actin-binding protein/actin was larger than 0.5. This means that alpha-actinin, calponin and filamin may coexist on actin filaments and more than mol of any actin-binding protein is bound per two actin monomers. This may be important for formation of different elements of cytoskeleton.

Regulation of protein kinase C by the cytoskeletal protein calponin

Previous studies from this laboratory have shown that, upon agonist activation, calponin co-immunoprecipitates and co-localizes with protein kinase Cepsilon (PKCepsilon) in vascular smooth muscle cells. In the present study we demonstrate that calponin binds directly to the regulatory domain of PKC both in overlay assays and, under native conditions, by sedimentation with lipid vesicles. Calponin was found to bind to the C2 region of both PKCepsilon and PKCalpha with possible involvement of C1B. The C2 region of PKCepsilon binds to the calponin repeats with a requirement for the region between amino acids 160 and 182. We have also found that calponin can directly activate PKC autophosphorylation. By using anti-phosphoantibodies to residue Ser-660 of PKCbetaII, we found that calponin, in a lipid-independent manner, increased auto-phosphorylation of PKCalpha, -epsilon, and -betaII severalfold compared with control conditions. Similarly, calponin was found to increase the amount of (32)P-labeled phosphate incorporated into PKC from [gamma-(32)P]ATP. We also observed that calponin addition strongly increased the incorporation of radiolabeled phosphate into an exogenous PKC peptide substrate, suggesting an activation of enzyme activity. Thus, these results raise the possibility that calponin may function in smooth muscle to regulate PKC activity by facilitating the phosphorylation of PKC.

Mutagenesis analysis of human SM22: characterization of actin binding

SM22 is a 201-amino acid actin-binding protein expressed at high levels in smooth muscle cells. It has structural homology to calponin, but how SM22 binds to actin remains unknown. We performed site-directed mutagenesis to generate a series of NH(2)-terminal histidine (His)-tagged mutants of human SM22 in Escherichia coli and used these to analyze the functional importance of potential actin binding domains. Purified full-length recombinant SM22 bound to actin in vitro, as demonstrated by cosedimentation assay. Binding did not vary with calcium concentration. The COOH-terminal domain of SM22 is required for actin affinity, because COOH terminally truncated mutants [SM22-(1-186) and SM22-(1-166)] exhibited markedly reduced cosedimentation with actin, and no actin binding of SM22-(1-151) could be detected. Internal deletion of a putative actin binding site (154-KKAQEHKR-161) partially prevented actin binding, as did point mutation to neutralize either or both pairs of positively charged residues at the ends of this region (KK154LL and/or KR160LL). Internal deletion of amino acids 170-180 or 170-186 also partially or almost completely inhibited actin cosedimentation, respectively. Of the three consensus protein kinase C or casein kinase II phosphorylation sites in SM22, only Ser-181 was readily phosphorylated by protein kinase C in vitro, and such phosphorylation greatly decreased actin binding. Substitution of Ser-181 to aspartic acid (to mimic serine phosphorylation) also reduced actin binding. Immunostains of transiently transfected airway myocytes revealed that full-length NH(2)-terminal FLAG-tagged SM22 colocalizes with actin filaments, whereas FLAG-SM22-(1-151) does not. These data confirm that SM22 binds to actin in vitro and in vivo and, for the first time, demonstrate that multiple regions within the COOH-terminal domain are required for full actin affinity.

Live dynamics of GFP-calponin: isoform-specific modulation of the actin cytoskeleton and autoregulation by C-terminal sequences

The calponin family of F-actin-, tropomyosin- and calmodulin-binding proteins currently comprises three genetic variants. Their functional roles implicated from in vitro studies include the regulation of actomyosin interactions in smooth muscle cells (h1 calponin), cytoskeletal organisation in non-muscle cells (h2 calponin) and the control of neurite outgrowth (acidic calponin). We have now investigated the effects of calponin (CaP) isoforms and their C-terminal deletion mutants on the actin cytoskeleton by time lapse video microscopy of GFP fusion proteins in living smooth muscle cells and fibroblasts. It is shown that h1 CaP associates with the actin stress fibers in the more central part of the cell, whereas h2 CaP localizes to the ends of stress fibres and in the motile lamellipodial protrusions of spreading cells. Cells expressing h2 CaP spread more efficiently than those expressing h1 CaP and expression of GFP h1 CaP resulted in reduced cell motility in wound healing experiments. Notably, expression of GFP h1 CaP, but not GFP h2 CaP, conferred increased resistance of the actin cytoskeleton to the actin polymerization antagonists cytochalasin B and latrunculin B, as well as to the protein kinase inhibitors H7-dihydrochloride and rho-kinase inhibitor Y-27632. These data point towards a dual role of CaP in the stabilization and regulation of the actin cytoskeleton in vivo. Deletion studies further identify an autoregulatory role for the unique C-terminal tail sequences in the respective CaP isoforms.

Specific subdomains of Vav differentially affect T cell and NK cell activation

The Vav protooncogene is a multidomain protein involved in the regulation of IL-2 gene transcription in T cells and the development of cell-mediated killing by cytotoxic lymphocytes. We have investigated the differential roles that specific protein subdomains within the Vav protooncogene have in the development of these two distinct cellular processes. Interestingly, a calponin homology (CH) domain mutant of Vav (CH-) fails to enhance NF-AT/AP-1-mediated gene transcription but is still able to regulate the development of cell-mediated killing. The inability of the CH- mutant to enhance NF-AT/AP-1-mediated transcription appears to be secondary to defective intracellular calcium, because 1) the CH- mutant has significantly reduced TCR-initiated calcium signaling, and 2) treatment with the calcium ionophore ionomycin or cotransfection with activated calcineurin restores NF-AT/AP-1-mediated gene transcription. The pleckstrin homology (PH) domain of Vav has also been implicated in regulating Vav activation. We found that deletion of the PH domain of Vav yields a protein that can neither enhance gene transcription from the NF-AT/AP-1 reporter nor enhance TCR- or FcR-mediated killing. In contrast, the PH deletion mutant of Vav is able to regulate the development of natural cytotoxicity, indicating a functional dichotomy for the PH domain in the regulation of these two distinct forms of killing. Lastly, mutation of three tyrosines (Y142, Y160, and Y174) within the acidic domain of Vav has revealed a potential negative regulatory site. Replacement of all three tyrosines with phenylalanine results in a hyperactive protein that increases NF-AT/AP-1-mediated gene transcription and enhances cell-mediated cytotoxicity. Taken together, these data highlight the differential roles that specific subdomains of Vav have in controlling distinct cellular functions. More broadly, the data suggest that separate lymphocyte functions can potentially be modulated by domain-specific targeting of Vav and other critical intracellular signaling molecules.

Heat shock protein (hsp90) interacts with smooth muscle calponin and affects calponin-binding to actin

Interaction of smooth muscle calponin with 90 kDa heat shock protein (hsp90) was analyzed by means of native gel electrophoresis and affinity chromatography. Under conditions used, calponin and hsp90 form a complex with an apparent dissociation constant in the micromolar range. The major hsp90-binding site is located in the N-terminal (residues 7-144) part of calponin. Addition of calponin to actin-tropomyosin complex results in formation of actin bundles. Hsp90 partially prevents bundle formation without affecting the molar ratio calponin/actin in single actin filaments or actin bundles. At low ionic strength, calponin induces polymerization of G-actin. Hsp90 decreases calponin-induced polymerization of G-actin. It is supposed that hsp90 may be involved in the assembly of actin filaments.

Crystal structure of the actin-binding region of utrophin reveals a head-to-tail dimer

BACKGROUND

Utrophin is a large multidomain protein that belongs to a superfamily of actin-binding proteins, which includes dystrophin, alpha-actinin, beta-spectrin, fimbrin, filamin and plectin. All the members of this family contain a common actin-binding region at their N termini and perform a wide variety of roles associated with the actin cytoskeleton. Utrophin is the autosomal homologue of dystrophin, the protein defective in the X-linked Duchenne and Becker muscular dystrophies, and upregulation of utrophin has been suggested as a potential therapy for muscular dystrophy patients.

RESULTS

The structure of the actin-binding region of utrophin, consisting of two calponin-homology (CH) domains, has been solved at 3.0 A resolution. It is composed of an antiparallel dimer with each of the monomers being present in an extended dumbell shape and the two CH domains being separated by a long central helix. This extended conformation is in sharp contrast to the compact monomer structure of the N-terminal actin-binding region of fimbrin.

CONCLUSIONS

The crystal structure of the actin-binding region of utrophin suggests that these actin-binding domains may be more flexible than was previously thought and that this flexibility may allow domain reorganisation and play a role in the actin-binding mechanism. Thus utrophin could possibly bind to actin in an extended conformation so that the sites previously identified as being important for actin binding may be directly involved in this interaction.

Calponin interaction with alpha-actinin-actin: evidence for a structural role for calponin

The purpose of this study was to address the paradox of calponin localization with alpha-actinin and filamin, two proteins with tandem calponin homology (CH) domains, by determining the effect of these proteins on the binding of calponin to actin. The results show that actin can accommodate near-saturating concentrations of either calponin and alpha-actinin or calponin and filamin with little change or no change in ligand affinity. Little direct interaction occurred between alpha-actinin and calponin in the absence of actin, so this effect is not likely to explain the co-distribution of these proteins. Calponin, like alpha-actinin, induced elastic gel formation when added to actin. When alpha-actinin was added to newly formed calponin/actin gels, no change was seen in the mechanical properties of the gel compared to calponin and actin alone. However, when calponin was added to newly formed alpha-actinin/actin gels, the resulting gel was much stronger than the gels formed by either ligand alone. Furthermore, gels formed by the addition of calponin to alpha-actinin/actin exhibited a phenomenon known as strain hardening, a characteristic of mechanically resilient gels. These results add weight to the concept that one of the functions of calponin is to stabilize the actin cytoskeleton.

Acidic calponin immunoreactivity in postnatal rat brain and cultures: subcellular localization in growth cones, under the plasma membrane and along actin and glial filaments

Acidic calponin, an F-actin-binding protein, is particularly enriched in brain, where calponin protein and mRNA are mainly expressed by neurons. The presence of calponin immunoreactivity in cultured astroglial cells has been reported, but the presence of acidic calponin in astrocytes in vivo appears equivocal. For the present study, we raised a specific polyclonal antibody against the 16-residue synthetic peptide covering the sequence E311-Q326 (EYPDEYPREYQYGDDQ) situated at the carboxy-terminal end of rat acidic calponin, and we investigated the cellular and subcellular localization of the protein in the developing central nervous system. Our results show that acidic calponin is particularly enriched in: (i) growth cones and submembranous fields of maturing cerebellar and cortical cells, where it codistributes with microfilaments and (ii) glial cells in vivo, including radial glia, glia limitans, Bergmann glia and mature astrocytes, and ex vivo, where acidic calponin strongly colocalizes with intermediate glial fibrillary acidic protein (GFAP) and vimentin filaments. Finally, up to four acidic calponin subtypes with different isoelectric point (pI) values were identified by two-dimensional gel electrophoresis of cerebellar and hippocampal extracts. The more acidic isoforms were developmentally regulated. As only one single mRNA for acidic calponin has been identified, these isoforms must reflect postsynthesis changes probably related to the particular functions of acidic calponin in maturing cells. Although brain acidic calponin's exact role remains uncertain, the present data suggest that it is involved in neuronal and glial plasticity.

N-terminally truncated Vav induces the formation of depolymerization-resistant actin filaments in NIH 3T3 cells

The Dbl family proto-oncogene vav is a guanine nucleotide exchange factor (GEF) for Rho family GTPases. Deletion of the N-terminus of Vav, harboring the single calponin homology (CH) domain, activates Vav's transforming potential, suggesting an important role of the CH domain in influencing Vav function. Since calponin binds actin, it has been suggested that the CH domain may mediate association with the actin cytoskeleton. In this study we have analyzed the subcellular localization and investigated the putative actin association of the Vav protein using enhanced green fluorescent protein (EGFP) fusion constructs. Our data show that both EGFP-tagged full length Vav and the CH domain-depleted EGFPvav 143-845 construct localize throughout the cytoplasm but fail to colocalize with F-actin. However, the latter construct of Vav was more strongly retained in the Triton-insoluble cytoskeleton fraction than full length Vav. Whereas removal of the CH domain had no apparent influence on the subcellular localization of Vav, deletion of the SH domains caused nuclear localization, indicating that Vav contains a functional nuclear localization signal. Expression of N-terminally truncated Vav constructs caused depolarization of fibroblasts and triggered the bundling of actin stress fibers into parallel arrays in NIH 3T3 cells. Notably, the parallel actin bundles showed prolonged resistance to the actin polymerization antagonists cytochalasin B and latrunculin B. These data point towards a regulatory role for the CH domain in Vav and suggest an actin cross-linking or bundling protein as a downstream effector molecule of vav-mediated signalling pathways.

Fluorescence studies of the carboxyl-terminal domain of smooth muscle calponin effects of F-actin and salts

The fluorescence parameters of the environment-sensitive acrylodan, selectively attached to Cys273 in the C-terminal domain of smooth muscle calponin, were studied in the presence of F-actin and using varying salt concentrations. The formation of the F-actin acrylodan labeled calponin complex at 75 mm NaCl resulted in a 21-nm blue shift of the maximum emission wavelength from 496 nm to 474 nm and a twofold increase of the fluorescent quantum yield at 460 nm. These spectral changes were observed at the low ionic strengths (< 110 mm) where the calponin : F-actin stoichiometry is 1 : 1 as well as at the high ionic strengths (> 110 mm) where the binding stoichiometry is a 1 : 2 ratio of calponin : actin monomers. On the basis of previous three-dimensional reconstruction and chemical crosslinking of the F-actin-calponin complex, the actin effect is shown to derive from the low ionic strength interaction of calponin with the bottom of subdomain-1 of an upper actin monomer in F-actin and not from its further association with the subdomain-1 of the adjacent lower monomer which occurs at the high ionic strength. Remarkably, the F-actin-dependent fluorescence change of acrylodan is qualitatively but not quantitatively similar to that earlier reported for the complexes of calponin and Ca2+-calmodulin or Ca2+-caltropin. As the three calponin ligands bind to the same segment of the protein, encompassing residues 145-182, the acrylodan can be considered as a sensitive probe of the functioning of this critical region. A distance of 29 A was measured by fluorescence resonance energy transfer between Cys273 of calponin and Cys374 of actin in the 1 : 1 F-actin-calponin complex suggesting that the F-actin effect was allosteric reflecting a global conformational change in the C-terminal domain of calponin.

Characterization of katD, a kinesin-like protein gene specifically expressed in floral tissues of Arabidopsis thaliana

Kinesin and kinesin-like proteins (KLPs) are microtubule-based motor proteins that play important roles in organelle transport. Based on the homology to these proteins, a katD cDNA has now been isolated from a library prepared from flowers of Arabidopsis thaliana ecotype Columbia. Sequence analysis of the katD cDNA revealed an open reading frame of 2691bp [corrected], encoding a protein of 987 amino acids. Comparison of the nucleotide sequences of katD genomic and cDNA clones revealed the presence of 18 introns, 17 of which conform to the GU-AG rule. The central region of the KatD polypeptide exhibits substantial amino acid sequence homology to the motor domain of kinesin heavy chains, although the motor domain of KatD appears to be phylogenetically distant from those of other KLPs in plants. The amino-terminal region of KatD shares marked sequence similarity with the calponin homology domain, whereas the approximately 240-residue carboxyl-terminal region shows no significant homology to other known proteins. The predicted secondary structure of KatD revealed the lack of an alpha-helical coiled coil structure typical of kinesin heavy chains, suggesting that KatD may function as a monomeric motor. A recombinant truncated KatD protein containing the putative motor domain was shown both to bind to mammalian microtubules in a manner dependent on a non-hydrolyzable ATP analog, and to possess microtubule-dependent ATPase activity. Immunoblot and Northern blot analyses showed that both KatD protein and mRNA are expressed specifically in floral tissues. These results suggest that the structurally distinct KatD protein functions as a floral tissue-specific motor protein.

The multiple roles of Cyk1p in the assembly and function of the actomyosin ring in budding yeast

The budding yeast IQGAP-like protein Cyk1p/Iqg1p localizes to the mother-bud junction during anaphase and has been shown to be required for the completion of cytokinesis. In this study, video microscopy analysis of cells expressing green fluorescent protein-tagged Cyk1p/Iqg1p demonstrates that Cyk1p/Iqg1p is a dynamic component of the contractile ring during cytokinesis. Furthermore, in the absence of Cyk1p/Iqg1p, myosin II fails to undergo the contraction-like size change at the end of mitosis. To understand the mechanistic role of Cyk1p/Iqg1p in actomyosin ring assembly and dynamics, we have investigated the role of the structural domains that Cyk1p/Iqg1p shares with IQGAPs. An amino terminal portion containing the calponin homology domain binds to actin filaments and is required for the assembly of actin filaments to the ring. This result supports the hypothesis that Cyk1p/Iqg1p plays a direct role in F-actin recruitment. Deletion of the domain harboring the eight IQ motifs abolishes the localization of Cyk1p/Iqg1p to the bud neck, suggesting that Cyk1p/Iqg1p may be localized through interactions with a calmodulin-like protein. Interestingly, deletion of the COOH-terminal GTPase-activating protein-related domain does not affect Cyk1p/Iqg1p localization or actin recruitment to the ring but prevents actomyosin ring contraction. In vitro binding experiments show that Cyk1p/Iqg1p binds to calmodulin, Cmd1p, in a calcium-dependent manner, and to Tem1p, a small GTP-binding protein previously found to be required for the completion of anaphase. These results demonstrate the critical function of Cyk1p/Iqg1p in regulating various steps of actomyosin ring assembly and cytokinesis.

Structural modules in actin-binding proteins: towards a new classification

The number of actin binding proteins for which (part of) the three-dimensional structure is known, is steadily increasing. This has led to a picture in which defined structural modules with actin binding capacity are shared between different actin binding proteins. A classification of these based on their common three-dimensional modules appears a logical future step and in this review we provide an initial list starting from the currently known structures. The discussed cases illustrate that a comparison of the similarities and variations within the common structural actin binding unit of different members of a particular class may ultimately provide shortcuts for defining their actin target site and for understanding their effect on actin dynamics. Within this concept, the multitude of possible interactions by an extensive, and still increasing, list of actin binding proteins becomes manageable because they can be presented as variations upon a limited number of structural themes. We discuss the possible evolutionary routes that may have produced the present array of actin binding modules.