Structure-activity studies of the regulatory interaction of the 10 kilodalton C-terminal fragment of caldesmon with actin and the effect of mutation of caldesmon residues 691-696

HIV-1 drug discovery: targeting folded RNA structures with branched peptides

Human immunodeficiency virus type 1 (HIV-1) is an RNA virus that is prone to high rates of mutation. While the disease is managed with current antiretroviral therapies, drugs with a new mode of action are needed. A strategy towards this goal is aimed at targeting the native three-dimensional fold of conserved RNA structures. This perspective highlights medium-sized peptides and peptidomimetics used to target two conserved RNA structures of HIV-1. In particular, branched peptides have the capacity to bind in a multivalent fashion, utilizing a large surface area to achieve the necessary affinity and selectivity toward the target RNA.

Proteome analysis of proteins related to aggressive periodontitis combined with neutrophil chemotaxis dysfunction

AIM

Some patients suffering from aggressive periodontitis (Ag-P) also display neutrophil chemotaxis dysfunction. In this study, we attempted to identify the proteins involved in Ag-P associated with neutrophil chemotaxis dysfunction using proteome analysis.

MATERIAL AND METHODS

A two-dimensional fluorescence difference gel electrophoresis system was used to detect differences in protein expression between neutrophils from four patients suffering from Ag-P combined with neutrophil chemotaxis dysfunction and those from four controls. Moreover, the mRNA levels of the proteins identified by the above method were examined in neutrophils from four types of subjects using the real-time polymerase chain reaction: twenty patients suffering from Ag-P with or without the dysfunction, 15 patients with chronic periodontitis, and 15 controls.

RESULTS

Four proteins, lactoferrin, caldesmon, heat shock protein 70, and stac, displayed a higher protein expression level in the neutrophils from the patients suffering from Ag-P combined with the neutrophil dysfunction than in those from the control group. The caldesmon mRNA levels in the neutrophils from the patients suffering from Ag-P combined with the neutrophil dysfunction were high compared with those in the neutrophils from the patients suffering from the other two types of periodontitis and those from the control group.

CONCLUSION

Caldesmon may be a marker of Ag-P combined with neutrophil chemotaxis dysfunction.

Caldesmon is necessary for maintaining the actin and intermediate filaments in cultured bladder smooth muscle cells

Caldesmon (CaD), a component of microfilaments in all cells and thin filaments in smooth muscle cells, is known to bind to actin, tropomyosin, calmodulin, and myosin and to inhibit actin-activated ATP hydrolysis by smooth muscle myosin. Thus, it is believed to regulate smooth muscle contraction, cell motility and the cytoskeletal structure. Using bladder smooth muscle cell cultures and RNA interference (RNAi) technique, we show that the organization of actin into microfilaments in the cytoskeleton is diminished by siRNA-mediated CaD silencing. CaD silencing significantly decreased the amount of polymerized actin (F-actin), but the expression of actin was not altered. Additionally, we find that CaD is associated with 10 nm intermediate-sized filaments (IF) and in vitro binding assay reveals that it binds to vimentin and desmin proteins. Assembly of vimentin and desmin into IF is also affected by CaD silencing, although their expression is not significantly altered when CaD is silenced. Electronmicroscopic analyses of the siRNA-treated cells showed the presence of myosin filaments and a few surrounding actin filaments, but the distribution of microfilament bundles was sparse. Interestingly, the decrease in CaD expression had no effect on tubulin expression and distribution of microtubules in these cells. These results demonstrate that CaD is necessary for the maintenance of actin microfilaments and intermediate-sized filaments in the cytoskeletal structure. This finding raises the possibility that the cytoskeletal structure in smooth muscle is affected when CaD expression is altered, as in smooth muscle de-differentiation and hypertrophy seen in certain pathological conditions.

Role of tropomyosin in the regulation of contraction in smooth muscle

Smooth muscle contraction is due to the interaction ofmyosin filaments with thin filaments. Thin filaments are composed of actin, tropomyosin, caldesmon and calmodulin in ratios 14:2:1:1. Tissue specific isoforms of act and beta tropomyosin are expressed in smooth muscle. Compared with skeletal muscle tropomyosin, the cooperative activation of actomyosin is enhanced by smooth muscle tropomyosin: cooperative unit size is 10 and the equilibrium between on and off states is shifted towards the on state. The smooth muscle-specific actin-bindingprotein caldesmon, together with calmodulin regulates the activity of the thin filament in response to Ca2+. Caldesmon and calmodulin control the tropomyosin-mediated transition between on and offactivity states.

Role of caldesmon in the Ca2+ regulation of smooth muscle thin filaments: evidence for a cooperative switching mechanism

Smooth muscle thin filaments are made up of actin, tropomyosin, caldesmon, and a Ca(2+)-binding protein and their interaction with myosin is Ca(2+)-regulated. We suggested that Ca(2+) regulation by caldesmon and Ca(2+)-calmodulin is achieved by controlling the state of thin filament through a cooperative-allosteric mechanism homologous to troponin-tropomyosin in striated muscles. In the present work, we have tested this hypothesis. We monitored directly the thin filament transition between the ON and OFF state using the excimer fluorescence of pyrene iodoacetamide (PIA)-labeled smooth muscle alphaalpha-tropomyosin homodimers. In steady state fluorescence measurements, myosin subfragment 1 (S1) cooperatively switches the thin filaments to the ON state, and this is exhibited as an increase in the excimer fluorescence. In contrast, caldesmon decreases the excimer fluorescence, indicating a switch of the thin filament to the OFF state. Addition of Ca(2+)-calmodulin increases the excimer fluorescence, indicating a switch of the thin filament to the ON state. The excimer fluorescence was also used to monitor the kinetics of the ON-OFF transition in a stopped-flow apparatus. When ATP induces S1 dissociation from actin-PIA-tropomyosin, the transition to the OFF state is delayed until all S1 molecules are dissociated actin. In contrast, caldesmon switches the thin filament to the OFF state in a cooperative way, and no lag is displayed in the time course of the caldesmon-induced fluorescence decrease. We have also studied caldesmon and Ca(2+)-calmodulin-caldesmon binding to actin-tropomyosin in the ON and OFF states. The results are used to discuss both caldesmon inhibition and Ca(2+)-calmodulin-caldesmon activation of actin-tropomyosin.

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.

Ontogeny of brush border carbohydrate digestion and uptake in the chick

Ingestion of carbohydrates from the small intestine is the major route of energy supply in animals. In mammals these functions develop both pre- and postnatally and are coordinated for the sucking period. In birds, the physiological requirements are different and hatchlings ingest diets rich in complex carbohydrates soon after hatching. The present study examined the ontogeny of intestinal carbohydrate uptake in the chicken. The expression of mRNA for a brush border enzyme, sucrase-isomaltase (SI), which is critical in disaccharide digestion, was determined, together with that of the Na-glucose transporter (SGLT)-1, which is the major apical glucose transporter, In addition, the homeobox gene cdx, which is involved in inducing SI expression in mammals was examined. It was found that the expression of cdxA mRNA and cdxA protein increased from day 15 of incubation until hatch, after which further changes were small. CdxA protein was shown to bind to the promoter region of SI in the chick indicating that cdxA is similar to the mammalian cdx2. The mRNA of SI was observed at 15 d incubation, increased from 17 d of incubation to a peak on day 19, decreased at hatch and had a further peak of expression 2 d post-hatch. In contrast, the mRNA of SGLT-1 was not detected until 19 d of incubation when a major peak of expression was observed followed by a decrease to low levels at hatch and small increases post-hatch. It appears that both SI and SGLT-1 mRNA are expressed before hatch in the chick, but the ontogeny of expression is controlled by different mechanisms.

The inhibitory region of troponin-I alters the ability of F-actin to interact with different segments of myosin

Peptides corresponding to the N-terminus of skeletal myosin light chain 1 (rsMLC1 1-37) and the short loop of human cardiac beta-myosin (hcM398-414) have been shown to interact with skeletal F-actin by NMR and fluorescence measurements. Skeletal tropomyosin strengthens the binding of the myosin peptides to actin but does not interact with the peptides. The binding of peptides corresponding to the inhibitory region of cardiac troponin I (e.g. hcTnI128-153) to F-actin to form a 1 : 1 molar complex is also strengthened in the presence of tropomyosin. In the presence of inhibitory peptide at relatively lower concentrations the myosin peptides and a troponin I peptide C-terminal to the inhibitory region, rcTnI161-181, all dissociate from F-actin. Structural and fluorescence evidence indicate that the troponin I inhibitory region and the myosin peptides do not bind in an identical manner to F-actin. It is concluded that the binding of the inhibitory region of troponin I to F-actin produces a conformational change in the actin monomer with the result that interaction at different locations of F-actin is impeded. These observations are interpreted to indicate that a major conformational change occurs in actin on binding to troponin I that is fundamental to the regulatory process in muscle. The data are discussed in the context of tropomyosin's ability to stabilize the actin filament and facilitate the transmission of the conformational change to actin monomers not in direct contact with troponin I.

Phosphorylation of the minimal inhibitory region at the C-terminus of caldesmon alters its structural and actin binding properties

Caldesmon is an inhibitory protein believed to be involved in the regulation of thin filament activity in smooth muscles and is a major cytoplasmic substrate for MAP kinase. NMR spectroscopy shows that the actin binding properties of the minimal inhibitory region of caldesmon, residues 750-779, alter upon MAP kinase phosphorylation of Ser-759, a residue not involved in actin binding. This phosphorylation leads to markedly diminished actin affinity as a result of the loss of interaction at one of the two sites that bind to F-actin. The structural basis for the altered interaction is identified from the observation that phosphorylation destabilises a turn segment linking the two actin binding sites and thereby results in the randomisation of their relative disposition. This modulatory influence of Ser-759 phosphorylation is not merely a function of the bulkiness of the covalent modification since the stability of the turn region is observed to be sensitive to the ionisation state of the phosphate group. The data are discussed in the context of the inhibitory association of the C-terminal domain of caldesmon with F-actin.

Replacement of Lys-75 of calmodulin affects its interaction with smooth muscle caldesmon

The interaction of smooth muscle caldesmon with synthetic calmodulin (SynCam) and its five mutants with replacement of Lys-75 was analyzed by means of intrinsic Trp fluorescence, zero-length crosslinking and by caldesmon-induced inhibition of actomyosin ATPase activity. SynCam and its double mutant with replacement K75P and simultaneous insertion of KGK between residues 80 and 81 have a comparably low affinity to caldesmon and the probability of crosslinking of this mutant to caldesmon was the lowest among all mutants analyzed. SynCam and its double mutant (K75P+KGK) induced nearly complete reversion of caldesmon inhibition of actomyosin ATPase activity with half-maximal reversion achieved at about 1 microM. Two mutants, K75A and K75V, with partially stabilized less positive central domain have higher affinity to caldesmon. These mutants induce 80-85% reversion of caldesmon inhibition of actomyosin ATPase and the half-maximal reversion was achieved at about 0.3-0.4 microM. Two last mutants, K75P and K75E, with distorted central domain have high affinity to caldesmon and the probability of crosslinking of K75P to caldesmon was the highest among calmodulin mutants tested. These mutants induced complete reversion of caldesmon inhibition with half-maximal effect observed at 0.3-0.4 microM. We suggest that the length, flexibility and charge of the central domain affect binding of calmodulin mutants and their ability to reverse caldesmon-induced inhibition of actomyosin ATPase activity.

Actin and the smooth muscle regulatory proteins: a structural perspective

The structural details of the smooth muscle acto-myosin interaction and its functional implications have been much discussed in recent years, however other, smooth muscle specific, actin-binding proteins have received much less attention. With increasing technical advances in structural biology a great deal of structural information is now coming to light, information that can provide useful insight into the mechanism of action for many important nonmotor actin-binding proteins. The purpose of the review is to instill the current knowledge on the structure, and interaction sites on F-actin, of the major, non-motor actin-binding proteins from smooth muscle, proposed to have a role in regulation. In the light of the recent structural studies the probable roles of the various actin-binding proteins will be discussed with particular reference to structure function relationships.

Survey of the 1998 optical biosensor literature

The utilization of optical biosensors to study molecular interactions continues to expand. In 1998, 384 articles relating to the use of commercial biosensors were published in 130 different journals. While significant strides in new applications and methodology were made, a majority of the biosensor literature is of rather poor quality. Basic information about experimental conditions is often not presented and many publications fail to display the experimental data, bringing into question the credibility of the results. This review provides suggestions on how to collect, analyze and report biosensor data.

Structural interactions between actin, tropomyosin, caldesmon and calcium binding protein and the regulation of smooth muscle thin filaments

The basic structure and functional properties of smooth muscle thin filaments were established about 10 years ago. Since then we and others have been working on the details of how tropomyosin, caldesmon and the Ca(2+)-binding protein regulate actin interaction with myosin. Our work has tended to emphasize the similarities between caldesmon and troponin function whilst others have been more concerned with the differences. The need to resolve the resulting differences has stimulated us to find new and more direct ways of investigating the mechanism of thin filament regulation. In recent years an apparent divergence has opened up between functional measurements, which indicate an allosteric-cooperative regulatory mechanism in which caldesmon and Ca(2+)-binding protein control actin-tropomyosin state in the same way as troponin, and structural measurements which show thin filament structures unlike striated muscle thin filaments. The challenge is to interpret function in terms of structure. We have combined functional studies with expression and mutagenesis of caldesmon and with structural methods including X-ray crystalography of tropomyosin-caldesmon crystals, electron microscopy and helical reconstruction of actin-tropomyosin-caldesmon complexes and high resolution nuclear magnetic resonance spectroscopy of the C-terminus of caldesmon in interaction with actin and calmodulin. We have used this information to propose a structural mechanism for caldesmon regulation of the smooth muscle thin filament.

Characterization of the functional properties of smooth muscle caldesmon domain 4a: evidence for an independent inhibitory actin-tropomyosin binding domain

Recent analysis has shown the presence of three sequences in the C-terminal 170 amino acids of human caldesmon (domain 4) which are involved in actin binding and tropomyosin-dependent inhibition of actomyosin ATPase. Two are in domain 4b (amino acids 715-793) and one is in domain 4a (amino acids 636-714). In the present work we have compared recombinant peptides containing either domain 4a or domain 4b to address the question as to whether domain 4a alone has any inhibitory activity. We have produced three new recombinant fragments containing domain 4a: H10 [622-708], H12 [506-708] and H13 [622-726] and we have characterized their functional properties. All three fragments bound to actin and tropomyosin. Caldesmon, but not domain 4b, was able to displace the fragments H10, H12 and H13 from actin. Thus the isolated caldesmon domain 4a peptides bind to the same region on actin as in the whole molecule while domains 4a and 4b occupy different sites on the actin molecule. Unlike domain 4b, none of the domain 4a fragments inhibited the actomyosin ATPase in the absence of tropomyosin. However both domain 4a and 4b fragments displayed an inhibitory activity in the presence of tropomyosin. H13 and H12 were more potent inhibitors than H10. Ca2+-calmodulin bound to H13 and reversed the inhibitory activity of this fragment but did not bind to H10 and H12. We conclude that domain 4a can act as an independent inhibitory actin-tropomyosin binding domain, but its properties are very different from the extreme C-terminal domain 4b.