Pack a STRIPAK with hubs inside a hub

The highly conserved striatin-interacting phosphatase and kinase (STrIPAK) multimeric complex regulates the hippo signaling pathway through phosphatase activity. A recent structure of the core STrIPAK hub reveals how striatins tetramerize to serve as a scaffolding platform for the assembly of an intricate architecture, which is distinct from that of all other protein phosphatase 2A (PP2A) complexes.

Caldesmon inhibits the actin–myosin interaction by changing its spatial orientation and mobility during the ATPase activity cycle

Orientation and mobility of acrylodan fluorescent probe specifically bound to caldesmon Cys580 incorporated into muscle ghost fibers decorated with myosin S1 and containing tropomyosin was studied in the presence or absence of MgADP, MgAMP-PNP, MgATPgammaS or MgATP. Modeling of various intermediate states of actomyosin has shown discrete changes in orientation and mobility of the dye dipoles which is the evidence for multistep changes in the structural changes of caldesmon during the ATPase hydrolysis cycle. It is suggested that S1 interaction with actin results in nucleotide-dependent displacement of the C-terminal part of caldesmon molecule and changes in its mobility. Thus inhibition of the actomyosin ATPase activity may be due to changes in caldesmon position on the thin filament and its interaction with actin. Our new findings described in the present paper as well as those published recently elsewhere might conciliate the two existing models of molecular mechanism of inhibition of the actomyosin ATPase by caldesmon.

A two-photon FRAP analysis of the cytoskeleton dynamics in the microvilli of intestinal cells

The molecular structure of the brush-border of enterocytes has been investigated since the 1980s, but the dynamics of this highly specialized subcellular domain have been difficult to study due to its small size. To perform a detailed analysis of the dynamics of cytoskeleton proteins in this domain, we developed two-photon fluorescence recovery after photobleaching and a theoretical framework for data analysis. With this method, fast dynamics of proteins in the microvilli of the brush border of epithelial intestinal cells can be measured on the millisecond timescale in volumes smaller than 1 microm3. Two major proteins of the cytoskeleton of the microvilli, actin and myosin 1a (Myo1a; formerly named brush border myosin I), are mobile in the brush-border of Caco-2 cells, an enterocyte-like cellular model. However, the mobility of actin is very different from that of Myo1a and they appear to be unrelated (diffusion coefficient of 15 microm2 s(-1) with a mobile fraction of 60% for actin, and 4 microm2 s(-1) with a mobile fraction of 90% for Myo1a). Furthermore, we show for the first time, in vivo, that the dynamics of Myo1a in microvilli reflect its motor activity.

Distribution and role of CD34-positive stromal cells and myofibroblasts in human normal testicular stroma

CD34-positive stromal cells are distributed in various organs including breast, Fallopian tubes, thyroid gland, colon, pancreas, and uterine cervix. To elucidate the distribution of CD34-positive stromal cells, smooth muscle cells, and myofibroblasts in normal human testis, we examined 48 testes obtained by autopsy and operation, including five fetal, one neonatal, and 42 adult cases without evident testicular lesions, using a streptavidin-biotin immunoperoxidase technique. The expression of alpha-smooth muscle actin (ASMA), h-caldesmon, CD34, and CD31 were immunohistochemically examined in all cases. The tunica albuginea and the inner layer of seminiferous tubules in adult testis were predominantly composed of myofibroblasts. Smooth muscle cells were also scattered throughout these sites in some cases. CD34-positive stromal cells were abundant, and they formed a reticular network around the seminiferous tubules and Leydig cells as well as the outer layer of seminiferous tubules. Moreover, myofibroblasts and the CD34 reticular network were already present in the testicular stroma during fetal or neonatal development. Double immunostaining of fetal, neonatal and adult testes using ASMA and CD34 confirmed that myofibroblasts and CD34-positive stromal cells were present in the inner and outer layers of peritubular tissue, respectively. This distribution and cytological identification was also confirmed by an ultrastructural study of four cases. Finally, CD34-positive stromal cells and myofibroblasts are major components of human testicular stroma.

Visualization of caldesmon binding to synthetic filaments of smooth muscle myosin

We have used synthetic filaments of unphosphorylated chicken gizzard myosin with a compact, highly ordered structure under relaxing conditions (in the absence of Ca2+ and in the presence of ATP) to visualize the mode of caldesmon binding to myosin filaments by negative staining and immunogold electron microscopy. We demonstrate that the addition of caldesmon to preformed myosin filaments leads to the appearance of numerous smooth projections curving out from the filament surface. The addition of caldesmon or its N-terminal fragment resulted in the partial masking of myosin filament periodicity. However, it did not change the inner structure of the filaments. It is demonstrated that most caldesmon molecules bind to myosin filaments through the N-terminal part, while the C-terminal parts protrude from the filament surface, as confirmed by immunoelectron microscopy visualization. Together with the available biochemical data on caldesmon binding to both actin and myosin and electron microscopic observations on the mode of caldesmon attachment to actin filaments with the C-termini of the molecules curving out from the filaments, the visualization of caldesmon attachment to myosin filaments completes the scenario of actin to myosin tethering by caldesmon.

Quaternary structure of the neuronal protein NAP-22 in aqueous solution

NAP-22, a myristoylated, anionic protein, is a major protein component of the detergent-insoluble fraction of neurons. After extraction from the membrane, it is readily soluble in water. NAP-22 will partition only into membranes with specific lipid compositions. The lipid specificity is not expected for a monomeric myristoylated protein. We have studied the self-association of NAP-22 in solution. Sedimentation velocity experiments indicated that the protein is largely associated. The low concentration limiting s value is approximately 1.3 S, indicating a highly asymmetric monomer. In contrast, a nonmyristoylated form of the protein shows no evidence of oligomerization by velocity sedimentation and has an s value corresponding to the smallest component of NAP-22, but without the presence of higher oligomers. Sedimentation equilibrium runs indicate that there is a rapidly reversible equilibrium between monomeric and oligomeric forms of the protein followed by a slower, more irreversible association into larger aggregates. In situ atomic force microscopy of the protein deposited on mica from freshly prepared dilute solution revealed dimers on the mica surface. The values of the association constants obtained from the sedimentation equilibrium data suggest that the weight concentration of the monomer exceeds that of the dimer below a total protein concentration of 0.04 mg/ml. Since the concentration of NAP-22 in the neurons of the developing brain is approximately 0.6 mg/ml, if the protein were in solution, it would be in oligomeric form and bind specifically to cholesterol-rich domains. We demonstrate, using fluorescence resonance energy transfer, that at low concentrations, NAP-22 labeled with Texas Red binds equally well to liposomes of phosphatidylcholine either with or without the addition of 40 mol% cholesterol. Thus, oligomerization of NAP-22 contributes to its lipid selectivity during membrane binding.

Brush border myosin-I structure and ADP-dependent conformational changes revealed by cryoelectron microscopy and image analysis

Brush border myosin-I (BBM-I) is a single-headed myosin found in the microvilli of intestinal epithelial cells, where it forms lateral bridges connecting the core bundle of actin filaments to the plasma membrane. Extending previous observations (Jontes, J.D., E.M. Wilson-Kubalek, and R.A. Milligan. 1995. Nature [Lond.]. 378:751-753), we have used cryoelectron microscopy and helical image analysis to generate three-dimensional (3D) maps of actin filaments decorated with BBM-I in both the presence and absence of 1 mM MgADP. In the improved 3D maps, we are able to see the entire light chain-binding domain, containing density for all three calmodulin light chains. This has enabled us to model a high resolution structure of BBM-I using the crystal structures of the chicken skeletal muscle myosin catalytic domain and essential light chain. Thus, we are able to directly measure the full magnitude of the ADP-dependent tail swing. The approximately 31 degrees swing corresponds to approximately 63 A at the end of the rigid light chain-binding domain. Comparison of the behavior of BBM-I with skeletal and smooth muscle subfragments-1 suggests that there are substantial differences in the structure and energetics of the biochemical transitions in the actomyosin ATPase cycle.

Phosphatidylserine liposomes can be tethered by caldesmon to actin filaments

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

Ryanodine receptors: structure and macromolecular interactions

Ryanodine receptors (RyRs), a class of intracellular calcium release channels, are the largest ion channels known. Recently, cryoelectron microscopy and image reconstructions of isolated receptors have shown that most of the protein mass forms a porous, multidomain cytoplasmic assembly. Evidence is mounting that suggests that the cytoplasmic assembly communicates with the transmembrane regions over distances of 100 or greater. RyRs are centrally important in excitation-contraction coupling, which occurs at specialized regions where the sarcoplasmic reticulum, containing the RyRs, and the plasma membrane/transverse-tubule system form junctions. Numerous proteins are present at these junctions, some of which interact directly with the RyR.

Calcineurin regulation of synaptic function: from ion channels to transmitter release and gene transcription

Calcineurin is a calcium (Ca2+)/calmodulin (CaM)-dependent protein phosphatase that has been shown to regulate the activity of ion channels, neurotransmitter and hormone release, synaptic plasticity and gene transcription. At glutamatergic synapses, the inhibition of calcineurin with immunosuppressant drugs has been reported to enhance both the presynaptic release of glutamate and postsynaptic responsiveness. Several other ligand- and voltage-gated ion channels are negatively regulated by calcineurin. Hormone release in insulin-secreting pancreatic beta cells and pituitary corticotrope tumour (AtT20) cells is also negatively regulated by calcineurin. In this article, Jerrel Yakel discusses the evidence that calcineurin plays a vital role in regulating neuronal excitability and hormone release.

Foot structure and foot protein in the cross striated muscle of a pecten

The foot-like structure of pecten (mollusk) cross striated muscle cells was studied from structural and biochemical standpoints, and compared with foot structures of vertebrate skeletal muscle cells. In vertebrate muscles, foot structures have been observed at the interspace between T-tubules and sarcoplasmic reticula (SR). In pecten muscles, T-tubules were not observed, but SR were found situated in the outer portions of the cell contacting the cell membrane, and foot-like structures were recognized at the interspace between the SR and cell membrane. We could isolate the SR fraction from these muscles in which vesicles of SR/cell membranes were included. In the SR fraction, foot-like structures were observed ultrastructurally by thin sectioning. The size and shape of the foot-like structure, whether observed in intact cells or SR fractions, appear smaller than foot structures of vertebrates. However, when calculated by SDS-PAGE, the molecular weight of the structure is similar to that of vertebrates. These findings are discussed and compared to characteristics of foot structures and foot proteins of vertebrate skeletal muscles reported in previous studies.

Immunocytochemical electron microscopic study and Western blot analysis of caldesmon and calponin in striated muscle of the fruit fly Drosophila melanogaster and in several muscle cell types of the earthworm Eisenia foetida

Caldesmon and calponin are two proteins that are characteristic of vertebrate smooth muscle. In invertebrates, caldesmon has only been studied in some molluscan muscles, and no previous references to calponin have been found. The aim of this paper was to investigate the presence and distribution of caldesmon and calponin in several invertebrate muscle cell types, classified according to their ultrastructural pattern: transversely striated muscle (flight muscle from Drosophila melanogaster), obliquely striated muscle (muscular body wall and inner muscular layer of the pseudoheart from the earthworm Eisenia foetida), and a muscle of doubtful classification which seems to be intermediate between smooth muscle and obliquely striated muscle (outer muscular layer of the pseudoheart, from E. foetida), using electron microscopy immunocytochemistry and Western blot analysis. Immunoreactions to both caldesmon and calponin were observed in the outer muscular layer cells from the earthworm pseudoheart but neither in the transversely striated muscle of D. melanogaster nor in the obliquely striated muscle from the earthworm. Present findings suggest that caldesmon- and calponin-like proteins are also present in invertebrate muscle cells, but only in those that are ultrastructurally similar to the vertebrate smooth muscle cells. Since discrepancies in the classification of some invertebrate muscles are common in the literature, the use of distinctive markers, such as troponin, caldesmon and calponin may improve our understanding of the nature and properties of many invertebrate muscles showing an ultrastructural pattern that does not resemble any of the classic muscle types.

Native structure and arrangement of inositol-1,4,5-trisphosphate receptor molecules in bovine cerebellar Purkinje cells as studied by quick-freeze deep-etch electron microscopy

We used quick-freeze deep-etch replica electron microscopy to visualize the native structure of inositol-1,4,5-trisphosphate receptor (IP3R) in the cell. In the dendrites of Purkinje neurons of bovine cerebellum there were many vesicular organelles whose surfaces were covered with a two-dimensional crystalline array of molecules. Detailed examination of the cytoplasmic true surface of such vesicles in replica revealed that the structural unit, identified as IP3R by immunocytochemistry and subsequent Fourier analysis, is a square-shaped assembly and is aligned so that the side of the square is inclined by approximately 20 degrees from the row-line of the lattice. Comparison with the ryanodine receptor (RyaR), another intracellular Ca2+ channel on the endoplasmic reticulum, suggested that IP3R, unlike RyaR, has a very compact structure, potentially reflecting the crucial difference in the function of the cytoplasmic portion of the molecule.

In vivo calcineurin crystals formed using the baculovirus expression system

Calcineurin is a heterodimeric phosphatase involved in the signal transduction of antigen-activated T cells. Coexpression of its two subunits, the regulatory subunit from human and the catalytic subunit from Neurospora crassa in cultured insect cells using the baculovirus expression system results in the formation of very large crystals in the cytoplasm. The crystals are formed initially in vesicles, but their subsequent growth appears to be uninhibited and continues without the need of an enclosing membrane until the host cell lyses. Although these in vivo crystals are low in population, ranging only 0-3 per cell, they are extremely large, over 10 mu m in some cases. Biochemical assays confirm their calcineurin origin, with the regulatory subunit incorporated being myristoylated, although both the myristoylated and unmyristoylated forms are expressed. The lattice structure of the in vivo crystals, with a spacing of 5.5 nm, is preserved with the regular electron microscopic (EM) specimen preparation procedure.

Myosin-V: a class of unconventional molecular motors

In this review we focus on the biochemical and structural properties of the myosin-V class of unconventional myosins as an example of the diversity of molecular motors within the myosin superfamily. A member of this class was first identified as a novel calmodulin-binding protein in mammalian brain (Larson RE, Pitta DE and Ferro JA (1988). Brazilian Journal of Medical and Biological Research, 21: 213-217). To date, the myosin-V class is represented by two molecules from yeast, one from nematodes, several from vertebrates (chickens, rats, mice and humans) and possibly one from plants. The domain structure of these myosins features a highly conserved head containing the ATP-hydrolysis and actin-binding sites, an extended neck composed of six tandem IQ-motifs which are sites for calmodulin binding and a large tail which has coiled-coil segments intercalated with globular regions of as yet unknown function. Biochemical studies on purified myosin-V from vertebrate brains and the description of myosin-V mutants in yeast and mice have made myosin-V one of the best characterized, unconventional myosin classes at the present time, surpassed only by the well-studied myosin-I class.

Crystal structures of human calcineurin and the human FKBP12-FK506-calcineurin complex

Calcineurin (CaN) is a calcium- and calmodulin-dependent protein serine/threonine phosphate which is critical for several important cellular processes, including T-cell activation. CaN is the target of the immunosuppressive drugs cyclosporin A and FK506, which inhibit CaN after forming complexes with cytoplasmic binding proteins (cyclophilin and FKBP12, respectively). We report here the crystal structures of full-length human CaN at 2.1 A resolution and of the complex of human CaN with FKBP12-FK506 at 3.5 A resolution. In the native CaN structure, an auto-inhibitory element binds at the Zn/Fe-containing active site. The metal-site geometry and active-site water structure suggest a catalytic mechanism involving nucleophilic attack on the substrate phosphate by a metal-activated water molecule. In the FKBP12-FK506-CaN complex, the auto-inhibitory element is displaced from the active site. The site of binding of FKBP12-FK506 appears to be shared by other non-competitive inhibitors of calcineurin, including a natural anchoring protein.

Three-dimensional architecture of the skeletal muscle ryanodine receptor

Recent advances in determining the three-dimensional architecture of the skeletal muscle ryanodine receptor/calcium release channel (RyR) by cryo-electron microscopy and three-dimensional reconstruction are discussed. The tetrameric receptor is characterized by a large 4-fold symmetric cytoplasmic assembly that consists of many domains separated by solvent-containing crevices and holes. Experimental evidence suggests that at least one regulatory ligand, calmodulin, binds to sites on the cytoplasmic assembly that are at least 10 nanometers from the transmembrane channel.

Mode of caldesmon binding to smooth muscle thin filament: possible projection of the amino-terminal of caldesmon from native thin filament

The structure of smooth muscle thin filament was examined by various electron microscopy techniques, with special attention to the mode of caldesmon binding. Chemical cross-linking was positively used to avoid the dissociation of accessory proteins upon dilution. Caldesmon in reconstituted thin filament was observed as fine filamentous projections from thin filament. Native thin filament isolated from smooth muscle showed similarly numerous fine whisker-like projections by all the techniques employed here. Antibody against the amino-terminus of caldesmon labeled the end of such projections indicating the possibility that the amino-terminal myosin binding moiety might stick out from the shaft of the thin filament. Such whiskers are often projected out as a cluster to the same side of native thin filament. Further, we could visualize the assembly of dephosphorylated heavy meromyosin (HMM) with native or reconstituted thin filament forming "nonproductive" complex in the presence of ATP. The association of HMM to the shaft of thin filament was through subfragment-2 moiety, in accordance with biochemical studies. Some HMM particles bound closer to the thin filament shaft, possibly suggesting the presence of the second myosin-binding site on caldesmon. Occasionally two kinds of HMM association as such coexisted at a single site on this filament in tandem. Thus, we constructed a structural model of thin filament. The proposed molecular arrangement is not only compatible with all the biochemical results but also provides additional support for our recent findings (E. Katayoma, G. C. Scott-Woo, and M. Ikebe (1995) J. Biol. Chem. 270, 3919-3925) regarding the capability of caldesmon to induce dephosphorylated myosin filament, which explains the existence of thick filaments in relaxed smooth muscle cells.

Brain myosin-V is a two-headed unconventional myosin with motor activity

Chicken myosin-V is a member of a recently recognized class of myosins distinct from both the myosins-I and the myosins-II. We report here the purification, electron microscopic visualization, and motor properties of a protein of this class. Myosin-V molecules consist of two heads attached to an approximately 30 nm stalk that ends in a globular region of unknown function. Myosin-V binds to and decorates F-actin, has actin-activated magnesium-ATPase activity, and is a barbed-end-directed motor capable of moving actin filaments at rates of up to 400 nm/s. Myosin-V does not form filaments. Each myosin-V heavy chain is associated with approximately four calmodulin light chains as well as two less abundant proteins of 23 and 17 kd.

Tertiary structure of calcineurin B by homology modeling

The crystal structure of the calcium-binding protein calmodulin is used to model the immunologically important calcineurin subunit B. The rough structure is produced by computer-aided homology modeling. Refinement of this using molecular dynamics leads to a suggested structure which appears to satisfy reasonable hydrophilicity and hydrogen-bonding criteria. In the absence of a crystal structure, the model may prove useful in modeling of its interactions with the phosphatase catalytic subunit calcineurin A, and help to explain the calcium modulation of this protein.

Electron microscopic images suggest both ends of caldesmon interact with actin filaments

An improved rotary shadowing technique enabled us to visualize chicken gizzard caldesmon (CaD) and its complexes with one or two covalently linked calmodulin (CaM) molecules by electron microscopy. Using a monoclonal antibody against an epitope in the N-terminal region of CaD (anti-N), we can now identify the end of the molecule that is involved in binding to another protein molecule. Thus in the 1:1 complex of CaD and CaM, the CaM molecule was almost always associated with the C-terminus of CaD, indicating preferential CaM-binding to the C-terminal region. We have also studied binding of CaD to filamentous actin (F-actin), using an EM technique that avoids spraying or freeze drying and thereby preserves the structure of F-actin. Only one end of CaD appeared to bind to F-actin, leaving the rest of the molecule projecting away from the filament. While the majority of anti-N bound at the free end of CaD, some antibody molecules were found on F-actin. These findings suggest that either end of CaD can bind to F-actin. Experiments using a monoclonal antibody against the C-terminus of CaD (anti-C) supported this idea. When the native thin filaments that contain endogenous CaD were incubated with anti-N, almost all the bound antibodies were found on the filaments, indicating that the N-terminal regions of CaD interact with actin, and that the binding affinity of the N-terminal region of CaD for actin is higher in vivo than that in vitro, either because the properties of CaD have been altered during purification, or because of the presence of some other component(s) associated with the native filaments.

Regulation of intrasteric inhibition of the multifunctional calcium/calmodulin-dependent protein kinase

A regulatory region involved in both autoinhibition and calmodulin (CaM) binding has previously been identified in the multifunctional Ca2+/CaM-dependent protein kinase (CaM kinase II). We have tested the role of various segments of the regulatory region in autoinhibition by the analysis of a series of truncation, substitution, and deletion mutants of the CaM kinase II alpha subunit (CaM kinase II alpha). Unexpectedly, the sequence Lys-Lys-Phe-Asn at positions 291-294, adjacent to the CaM binding domain, was found to be sufficient to maintain an inhibited state in a truncated form of the kinase. However, these residues are not essential in the context of the full-length protein, indicating the importance of additional residues from the overlapping CaM binding domain. We propose here a molecular model for CaM kinase II alpha based on the three-dimensional structure of the cAPK-PKI-(5-24) (protein kinase inhibitor fragment) complex. It is predicted from this model that autoinhibition is of the pseudosubstrate variety and that autophosphorylation of Thr-286 could occur by an intersubunit reaction in the holoenzyme complex.

Modification of acidic residues normalizes sodium dodecyl sulfate-polyacrylamide gel electrophoresis of caldesmon and other proteins that migrate anomalously

Caldesmon migrates as a 140-kDa protein during polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS), although its true molecular mass is close to 90 kDa. Since caldesmon's high acidic residue content may be responsible for this anomaly, it was reasoned that modification of these residues, with a loss of negative charge, might restore normal electrophoretic migration. Therefore caldesmon was reacted with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide in the presence of excess ethanolamine, which results in negatively charged carboxylates being converted to neutral amides without protein cross-linking. The absence of cross-linking was shown by rotary shadow electron microscopy. In accord with expectations, modified caldesmon migrated as a 94-kDa protein when compared to standards, which were much less affected by modification. The anomalous migration of caldesmon might be due to the repulsion of negatively charged SDS by caldesmon's acidic residues. Low binding of SDS to caldesmon is consistent with the fact that SDS, up to 1%, had little or no effect on the secondary structure of caldesmon, as monitored by circular dichroism. However, other mechanisms can also explain these observations. The abnormal migration of tropomyosin and calsequestrin, both of which have a high percentage of acidic amino acids, was also "normalized" by this treatment. Thus this method might have general application for the electrophoresis of proteins which have a high acidic residue content and migrate anomalously.

Characterization of EGTA-washed synaptosomal membrane with emphasis on its calmodulin-binding proteins. Demonstration of possible reconstitution with added calcium/calmodulin

Endogenous calmodulin (CaM) in the EGTA-washed cerebral-cortical synaptosomal membrane (SM) preparation was estimated below 3 micrograms/ml protein by the semiquantitative immunoblot analysis (Natsukari, N., Ohta, H. and Fujita, M. (1989) J. Immunol. Methods 125, 159-166). Membrane-bound CaM was immunoelectron-microscopically demonstrated in EGTA-washed, non-treated (control), and Ca(2+)-treated cerebral-cortical synaptosomal membranes (SM) as well as for the SM enriched with added CaM. The density of CaM increased in the above order. CaM-dependent adenylate cyclase and CaM-dependent protein kinase II (CaM-kinase II) activities were restored, whereas the phosphodiesterase (PDE) activity was not affected by exogenous CaM over all the Ca2+ concentrations tested. Adenylate cyclase at pCa 6.2 was synergistically activated either by GTP and CaM or by CaM and beta-adrenergic agonist, (+/-)-isoproterenol, reflecting the intactness of signal transduction pathway in the SM. Also demonstrated were the presence of protein kinase A, CaM-kinase II, and their endogenous substrates in the SM. Based on 32P-autoradiography and 125I-CaM overlay data certain CaM-binding proteins such as CaM-kinase II and synapsin I were identified on SDS-PAGE. Ca(2+)-dependent and -independent CaMBPs were distinguished by 125I-CaM gel overlay with and without Ca2+. The former had bigger molecular size (greater than or equal to 49 kDa) than the latter (less than or equal to 34 kDa). Yield of Ca(2+)-dependent CaMBPs was not affected by Ca2+ concentration during preparation of the SM while that of Ca(2+)-independent CaMBPs was reduced by exposure to 100 microM Ca2+. In contrast with the CaMBPs of brain SM, those of enterocyte and eyrthrocyte plasma membranes especially, microvillous membrane of the enterocyte, showed quite distinct CaMBP profiles. The present findings suggested that the EGTA-washed SM preparation made a useful system for studying the role of CaM in the brain SM.

The molecular anatomy of caldesmon

Images

A long helix from the central region of smooth muscle caldesmon

The central region of smooth muscle caldesmon is predicted to form alpha-helices on the basis of its primary structure. We have isolated a fragment (CT54) that contains this region. The hydrodynamic properties and the electron microscopic images suggest that CT54 is an elongated (35 nm), monomeric molecule. The circular dichroic spectrum yields an overall alpha-helical content of 55-58%. These results are consistent with the model that the middle portion of CT54 forms a long stretch of single-stranded alpha-helix. Such a structure, if it in fact exists, is thought to be stabilized by numerous salt bridges between charged residues at positions i and i + 4. The structural characteristics of this fragment not only represent an unusual protein configuration but also provide information about the functional role of caldesmon in smooth muscle contraction.

Localization of the calmodulin- and the actin-binding sites of caldesmon

Expression of the C-terminal third of chicken gizzard caldesmon in Escherichia coli, using the Nagai vector (Nagai, K., and Thøgersen, H.V. (1987) Methods Enzmol. 153, 461-481), produces a cII-caldesmon fusion protein (27 kDa) with caldesmon sequence beginning at Lys579. Degradation during purification yields five peptides with molecular masses of 24, 22, 19 (two peptides), and 15 kDa. The 24-kDa peptide begins at Phe581; the 22-kDa peptide begins at Leu597, the two 19-kDa peptides begin at Phe581 and Val629, respectively; the 15-kDa peptide also begins at Val629. We estimate that the 15-kDa and one of the 19-kDa peptides end near Leu710. Site-directed mutagenesis was used to produce truncated peptides with known C termini; one peptide (17 kDa) terminates at Asn675. Digestion of the fragments with chymotrypsin generates a second 15-kDa fragment that begins at Ser666 (15K'). All of the peptides, with the exception of 15K', bind Ca(2+)-calmodulin-Sepharose and share a common 37-amino acid peptide between Val629 and Ser666, suggesting this contains the calmodulin binding site. Comparison with published sequences (Takagi, T., Yazawa, M., Ueno, T., Suzuki, S., and Yagi, K. (1989) J. Biochem. (Tokyo) 106, 778-783 and Bartegi, A., Fattoum, A., Derancourt, J., and Kassab, R. (1990) J. Biol. Chem. 265, 15231-15238) for other calmodulin-binding fragments further restricts the binding site to 7 residues, Trp-Glu-Lys-Gly-Asn-Val-Phe, between Trp659 and Ser666. All of the fragments, except the two 15-kDa peptides, co-sediment with F-actin, indicating that there are two segments in the C-terminal third of caldesmon that can interact with F-actin: one between Leu597 and Val629, the other between Arg711 and Pro756. Although separated in the primary sequence, these domains may interact with the calmodulin-binding region in the folded structure.

Electron microscopic studies of chicken gizzard caldesmon and its complex with calmodulin

Caldesmon samples mounted on a stage rotating about a horizontal axis were shadowed keeping the shadow angle at about 3 degrees. This technique minimizes background metal deposits compared with the conventional method. The identity of caldesmon was confirmed by comparing the images of caldesmon alone with those of the caldesmon-calmodulin complex. In these samples the caldesmon molecules appeared to be elongated; most were between 30 and 80 nm in length. The maximum length was in good agreement with the earlier estimate of 74 nm based on hydrodynamic studies. Our observations also suggested the presence of a rather rigid 30-40 nm stretch in the middle of the caldesmon molecule, which was always visible under rotary shadowing, and a flexible structure of about 20 nm in length at each end of the molecule, which may or may not be visible depending on their orientation on the mica surface. In the samples of caldesmon crosslinked with calmodulin, we noticed the existence of complexes containing two calmodulin molecules per caldesmon molecule, separated by a distance of 60 nm, consistent with the suggestion that each end of caldesmon can interact with calmodulin.

Complexes between 15 kDa caldesmon fragment and actin investigated by immuno-electron microscopy

The regulatory system of smooth muscle thin filaments is thought to involve a major calcium-calmodulin and actin binding protein: caldesmon. A dissective approach was used to isolate a 35 kDa C-terminal fragment of the molecule and to produce antibodies reacting against both the intact and the 15 kDa N-terminal end of this parental fragment. While this purified 15 kDa caldesmon fragment demonstrates a weak actin association, we observed that it cross-links actin filaments into loose bundles. These structures were labelled with a selective antibody and showed regular periodic striation with repeats of approximately 40 nm. This work brings additional information to previous reports using an actin and calmodulin binding 25 kDa C-terminal fragment of the caldesmon molecule [(1989) J. Biol. Chem. 264, 2869-2875]. We demonstrate that a purified fragment corresponding to a sequence smaller than 96 amino acids, which contains no cystein residue, is able to interact with actin at a single site which is not the calmodulin modulated.

Caldesmon, calmodulin and tropomyosin interactions

Binary complex interactions between caldesmon and tropomyosin, and calmodulin and tropomyosin, and ternary complex interaction involving the three proteins were studied using viscosity, electron microscopy, fluorescence and affinity chromatography techniques. In 10 mM NaCl, caldesmon decreased the viscosity of chicken gizzard tropomyosin by 7-8 fold with a concomitant increase in turbidity (A330nm). Electron micrographs showed spindle-shaped particles in the tropomyosin-caldesmon samples. These results suggest side-by-side aggregation of tropomyosin polymers induced by caldesmon. Binding studies in 10 mM NaCl between caldesmon and chicken gizzard tropomyosin labelled with the fluorescent probe N-(1-anilinonaphthyl-4)maleimide (ANM) gave association constants from 5.3.10(6) to 7.9.10(6) M-1 and stoichiometry from 1.0 to 1.4 tropomyosin per caldesmon. Similar binding was observed for rabbit cardiac tropomyosin and caldesmon. Removal of 18 and 11 residues from the COOH ends of the gizzard and cardiac tropomyosin by carboxypeptidase A, respectively, had no significant effect on their binding to caldesmon. In the presence of Ca2+, chicken gizzard tropomyosin bound to a calmodulin-Sepharose-4B column and was eluted with a salt concentration of 140 mM. This interaction was weakened in the absence of Ca2+, and the bound tropomyosin was eluted by 65 mM KCl. ANM-labelled tropomyosin bound calmodulin in the presence of Ca2+ with a binding constant of 3.5.10(6) M-1 and a binding stoichiometry of 1 to 1.4 tropomyosin per calmodulin. In 10 mM NaCl, calmodulin reduced the specific viscosity of chicken gizzard tropomyosin in the presence of Ca2+ by 5 fold, while a 1.5-fold reduction in viscosity was observed in the absence of Ca2+. In either case, no significant increase in turbidity was observed suggesting that calmodulin reduced head-to-tail polymerization of tropomyosin. The interaction of caldesmon with the calmodulin-ANM-tropomyosin complex in the presence and absence of Ca2+ was also examined. The result is consistent with a model that in the absence of Ca2+, calmodulin binds weakly to either caldesmon or tropomyosin and has little effect on the tropomyosin-caldesmon interaction; whereas, Ca2(+)-calmodulin interacts with caldesmon and reduces its affinity to tropomyosin.

Caldesmon and the structure of smooth muscle thin filaments: electron microscopy of isolated thin filaments

Native and synthetic vertebrate smooth muscle thin filaments have been examined by electron microscopy in order to determine the arrangement of the regulatory protein caldesmon. In synthetic filaments of actin-caldesmon, long slender molecules were sometimes seen running along the thin filament, suggesting that caldesmon can associate with actin along its length, while at other times lateral projections were observed. In native filaments, containing actin, caldesmon and tropomyosin, we found no evidence for lateral projections extending from the filaments, suggesting that caldesmon does not act as a crosslinking protein in vivo. In contrast, elongated molecules were clearly seen following the long pitch actin helices. We suggest that these may represent an association of caldesmon and tropomyosin. Antibodies developed against an N-terminal fragment of caldesmon caused thin filaments to aggregate laterally into arrays displaying approximately 35-38 nm repeats; thin filament aggregates with this periodicity were obtained previously (Lehman et al., 1989) using antibodies to the C-terminal segment of caldesmon. These results suggest that both ends of caldesmon are closely associated with the shaft of the thin filament, supporting a model in which the elongated caldesmon molecule runs along the filament, possibly interacting with tropomyosin, following the long pitch actin helices.

Caldesmon and the structure of smooth muscle thin filaments: immunolocalization of caldesmon on thin filaments

Antibodies reacting with chicken gizzard caldesmon were used to determine the distribution of caldesmon on smooth muscle thin filaments. Antibodies developed against both the intact caldesmon molecule and a 40 kilodalton proteolytic fragment cause thin filaments to aggregate laterally. Aggregates produced with the latter antibody display regular periodic labelling with a repeat of approximately 38 nm, a distribution characteristic of proteins associated with tropomyosin on thin filaments. The stoichiometry of caldesmon on thin filaments has been critically reevaluated and alternative models of caldesmon distribution on thin filaments are proposed.