Comparison of Ca2+-dependent effects of caldesmon-tropomyosin-calmodulin and troponin-tropomyosin complexes on the structure of F-actin in ghost fibers and its interaction with myosin heads

Myopathy-causing Q147P TPM2 mutation shifts tropomyosin strands further towards the open position and increases the proportion of strong-binding cross-bridges during the ATPase cycle

The molecular mechanisms of skeletal muscle dysfunction in congenital myopathies remain unclear. The present study examines the effect of a myopathy-causing mutation Q147P in β-tropomyosin on the position of tropomyosin on troponin-free filaments and on the actin–myosin interaction at different stages of the ATP hydrolysis cycle using the technique of polarized fluorimetry. Wild-type and Q147P recombinant tropomyosins, actin, and myosin subfragment-1 were modified by 5-IAF, 1,5-IAEDANS or FITC-phalloidin, and 1,5-IAEDANS, respectively, and incorporated into single ghost muscle fibers, containing predominantly actin filaments which were free of troponin and tropomyosin. Despite its reduced affinity for actin in co-sedimentation assay, the Q147P mutant incorporates into the muscle fiber. However, compared to wild-type tropomyosin, it locates closer to the center of the actin filament. The mutant tropomyosin increases the proportion of the strong-binding myosin heads and disrupts the co-operation of actin and myosin heads during the ATPase cycle. These changes are likely to underlie the contractile abnormalities caused by this mutation.

The interface between caldesmon domain 4b and subdomain 1 of actin studied by nuclear magnetic resonance spectroscopy

The ability of caldesmon to inhibit actomyosin ATPase activity involves the interaction of three nonsequential segments of caldesmon domain 4 (amino acids 600-756) with actin. Two of these contacts are located in the C-terminal half of this region of caldesmon which has been designated domain 4b (658-756). To investigate the spatial relationship between the two sites and to determine whether their corresponding contacts on actin are sequentially distinct, we have used NMR spectroscopy to compare the actin binding properties of the minimal inhibitory peptide LW30 comprising residues 693-722 with those of the recombinant domain 4b constructs 658C (658-756) and Cg1 (a mutant of 658C in which the sequence (691)Glu-Trp-Leu-Thr-Lys-Thr(696) is changed to Pro-Gly-His-Tyr-Asn-Asn). Cg1 retains dual-sited actin attachment but displays lowered actin affinity. In the presence of tropomyosin, domain 4b-actin contacts were stronger but not qualitatively different, indicating that tropomyosin affected the conformational equilibrium of caldesmon binding. Simultaneous dual-sited attachment of domain 4b to actin is enabled by the conformational properties of the site-spanning sequence common to 658C, Cg1, and LW30 as reflected in the corresponding NOE and other NMR spectral parameters. A backbone turn region ((713)Gly-Asp-Val-Ser(716)) preceded by an extended segment (Ser(702)-Pro-Ala-Pro-Lys-Pro) acts to constrain the relative disposition of the flanking actin contact sites of domain 4b. In tests with a library of actin peptides, only the C-terminus, 350-375, bound to 658C and LW30. The use of Cu(2+) as a paramagnetic spectral probe bound to the unique His-371 provided evidence of a well-defined geometry for the complex between LW30 and actin residues 350-375 with the N-terminal, site B of domain 4b close to the C-terminal residues of actin. The data are discussed in the context of the potentiation of inhibitory activity by tropomyosin.

Conformational changes of contractile proteins and their role in muscle contraction

The review summarizes the results of studies on conformational changes in contractile proteins that occur during muscle contraction. Polarized fluorescence of tryptophan residues in actin and of fluorescent probes bound specifically to different sites on actin, myosin, or tropomyosin in muscle fibers was measured. The results show that the transition of actomyosin complex from the weak to the strong-binding state is accompanied by a change in the orientation of F-actin subunits with the C and N termini moving opposite to a large part of the subunit. Myosin light chains and some areas in the 20-kDa domain of myosin head move in the same direction as the C- and N-terminal regions of actin. It is established that troponin, caldesmon, calponin, and myosin systems of regulation of muscle contraction modify intramolecular actomyosin rearrangements in a Ca(2+)-dependent manner. The role of intramolecular movements of contractile proteins in muscle contraction is discussed.

Effects of denervation and muscle inactivity on the organization of F-actin

To discriminate between the influences of a motoneuron and muscle activity on the conformation of actin filaments, the extrinsic polarized fluorescence [of rhodamine-phalloidin and N-(iodoacetylamine)-1-naphthylamine-5-sulfonic acid attached to F-actin] was measured in "ghost" fibers from intact rat soleus muscles and atrophying muscles after denervation, immobilization, or tenotomy. The results show that the conformation of F-actin changed in all the atrophying muscles, but differently. In the denervated muscle, the flexibility of the actin filaments decreased, whereas in the other experimental muscles it remained as in the intact muscle. In the denervated muscle, the mobility of the C-terminus of the actin polypeptide increased. Attachment of myosin subfragment-1 influenced the F-actin conformation differently in the denervated muscle than in the other muscles studied. These results suggest that changes in the conformation of the actin filament are induced by the lack of connection with the motoneuron rather than by muscle inactivity.

Both N-terminal myosin-binding and C-terminal actin-binding sites on smooth muscle caldesmon are required for caldesmon-mediated inhibition of actin filament velocity

It has been suggested that the tethering caused by binding of the N-terminal region of smooth muscle caldesmon (CaD) to myosin and its C-terminal region to actin contributes to the inhibition of actin-filament movement over myosin heads in an in vitro motility assay. However, direct evidence for this assumption has been lacking. In this study, analysis of baculovirus-generated N-terminal and C-terminal deletion mutants of chicken-gizzard CaD revealed that the major myosin-binding site on the CaD molecule resides in a 30-amino acid stretch between residues 24 and 53, based on the very low level of binding of CaDDelta24-53 lacking the residues 24-53 to myosin compared with the level of binding of CaDDelta54-85 missing the adjacent residues 54-85 or of the full-length CaD. As expected, deletion of the region between residues 24 and 53 or between residues 54 and 85 had no effect on either actin-binding or inhibition of actomyosin ATPase activity. Deletion of residues 24-53 nearly abolished the ability of CaD to inhibit actin filament velocity in the in vitro motility experiments, whereas CaDDelta54-85 strongly inhibited actin filament velocity in a manner similar to that of full-length CaD. Moreover, CaD1-597, which lacks the major actin-binding site(s), did not inhibit actin-filament velocity despite the presence of the major myosin-binding site. These data provide direct evidence for the inhibition of actin filament velocity in the in vitro motility assay caused by the tethering of myosin to actin through binding of both the CaD N-terminal region to myosin and the C-terminal region to actin.

Modulation of actin conformation and inhibition of actin filament velocity by calponin

Calponin, an actin/calmodulin-binding protein present in smooth muscle thin filaments, modulates the actin-myosin interaction and actomyosin ATPase activity of smooth muscle myosin II. Binding of myosin heads to actin under conditions that produce weak or strong binding induces conformational changes in actin. Polarized fluorimetric measurements of rhodamine-phalloidin complex and 1,5-IAEDANS specifically linked to actin in myosin-free muscle fibers (ghost fibers) and to Cys-707 in myosin head, respectively, revealed conformational changes, as determined from the changes in orientation and mobility of fluorescent probes, upon addition of calponin to ghost fibers. The effect of calponin on conformational changes produced upon binding of phosphorylated or dephosphorylated heavy meromyosin (HMM) was also determined. Subfragment-1 preparation modified with NEM (NEM-S1) or pPDM (pPDM-S1) were used as models of strong and weak binding, respectively. Calponin changed both the orientation of fluorophores on the actin and the flexibility of the actin filaments, as determined from the angle between an actin filament and the fiber axis. Changes in the flexibility of actin filaments and the orientation of fluorophores produced by phosphorylated smooth muscle HMM were similar to those seen with NEM-S1, which formed a strong-binding association with actin and caused the transition of actin monomers to the "on" state; calponin markedly inhibited this effect. In contrast, pPDM-S1 and dephosphorylated HMM induced weak binding and the transition of actin monomers to the "of" state, and these effects were enhanced by calponin. Furthermore, calponin decreased the velocity of actin filament movement over skeletal muscle myosin O gamma phosphorylated smooth muscle myosin heads in an in vitro motility assay. These results suggest that calponin induces modulation of smooth muscle contraction by inhibiting the force-producing (strong-binding) state of cross-bridges and involves changes in actin conformation.

Mutagenesis analysis of functionally important domains within the C-terminal end of smooth muscle caldesmon

The ability of chicken gizzard smooth muscle caldesmon (CaD) to inhibit actomyosin ATPase activity is due mainly to an inhibitory domain that resides within the C-terminal 67 amino acid residues of the CaD molecule. In the present study, a series of C-terminal truncation and internal deletion mutants of chicken gizzard smooth muscle CaD were systematically designed using a site-directed mutagenesis approach, and these mutant proteins were overexpressed in a baculovirus expression system. Analysis of actin binding and inhibition of actomyosin ATPase activity using these mutants identified a strong actin-binding motif of 6 amino acid residues (from Lys718 to Glu723), which also form the core sequence for CaD-induced inhibition of actomyosin ATPase. However, maximal inhibition by CaD requires the presence of residues 728-731, which are not associated with actin binding. Our data provide direct evidence for the requirement of actin binding to a specific region in CaD for CaD-induced inhibition of actin activation of smooth muscle myosin ATPase. Furthermore, our findings also show that the region between residues 690 and 717 is responsible for the weak inhibition of actomyosin ATPase and reveal that the inhibitory determinants located in the regions between residues 690 and 717 and residues 718 and 756 can function independently.

Characterization of the functional domains on the C-terminal region of caldesmon using full-length and mutant caldesmon molecules

A series of C-terminal deletion mutants of chicken gizzard smooth muscle caldesmon (CaD) were made using a polymerase chain reaction cloning strategy and a baculovirus expression system, and the precise locations of the functional domains of CaD involved in the regulation of actomyosin ATPase and the binding of actin, tropomyosin, and calmodulin were analyzed. Our results reveal a high affinity calmodulin-binding domain that consists of at least three calmodulin-binding determinants localized in residues 690-717, 658-689, and 628-657. The residues between positions 718 and 756 and positions 598 and 627 have no detectable calmodulin-binding site. A high affinity tropomyosin-binding domain is located between residues 718 and 756. The 159 residues at the C terminus of CaD contain multiple actin-binding determinants; the major ones are localized in the regions between residues 718 and 756 and residues 690 and 717. The amino acid residues between positions 718 and 756 contain the major determinant involved in the inhibition of the actin activation of smooth muscle myosin ATPase since CaD-(1-717) caused only 30% of the inhibition produced by the full-length CaD. Further deletion between residues 690 and 717 (CaD-(1-689) revealed a low level (10% of that seen for full-length CaD) of inhibition of the actomyosin ATPase. These data clearly demonstrate that the region of the last 66 amino acid residues at the CaD C terminus contains two or more major actin-binding motifs, one tropomyosin-binding domain, one high affinity calmodulin-binding determinant, and the domain that is responsible for the inhibition of the actin-activated ATPase of myosin.

Effect of unphosphorylated smooth muscle myosin on caldesmon-mediated regulation of actin filament velocity

The effect of smooth muscle myosin at different levels of light chain phosphorylation on caldesmon-mediated movement of actin filaments was investigated using an in vitro motility assay. Myosin at different levels of phosphorylation was obtained by mixing different proportions of fully phosphorylated and unphosphorylated myosin in monomeric form, while keeping the total myosin concentration constant. The average velocity of actin filaments containing tropomyosin was 1.20 +/- 0.046 microns s-1 at 30 degrees C with fully phosphorylated myosin. This velocity was not altered when the percentage of unphosphorylated myosin coated on the nitrocellulose surface was increased to 80%; further increases lowered the velocity. When the actin filaments with caldesmon bound at stoichiometric levels were used, filament velocity was unaffected until 50% of the myosin was unphosphorylated, but further increases in the percentage of unphosphorylated myosin induced a decrease in the velocity, and at 95% unphosphorylated myosin, filament movement had ceased. The decreased filament velocity in the presence of caldesmon was also observed when phosphorylated myosin was mixed with myosin rod instead of unphosphorylated myosin, but was not observed when the 38 kDa caldesmon C-terminal fragment, which lacks the myosin-binding domain, was used instead of intact caldesmon. These data indicate that the decreased filament velocity in the presence of caldesmon reflects the mechanical load produced by the tethering of actin to myosin through the interaction of the caldesmon N-terminal domain and the myosin S-2 region. The tethering effect mediated by caldesmon may play a role in smooth muscle contraction when a large number of myosin heads are dephosphorylated, as in force maintenance.

Overexpression, purification, and characterization of full-length and mutant caldesmons using a baculovirus expression system

Three recombinant chicken gizzard caldesmon (CaD) baculovirus vectors that contained the full-length CaD codon sequence (Pv1CaD), the full-length CaD codon sequence and a six-histidine tag at the 5'-end (pBlueBacHisCaD), or the full-length CaD codon sequence and an extra six-histidine codon sequence at the 3'-end (PvlHisCaD) were constructed. Spodoptera frugiperda (Sf9) cells transfected with these constructs overexpressed full-length CaD, yielding 2, 20, and 50 micrograms per 10(6) cells for pBlueBacHisCaD, PvlHisCaD, and PvlCaD, respectively. Time course assays for the expressed proteins demonstrated that the optimum harvest time was 36 h postinfection. Immunofluorescence microscopy revealed PvlCaD localized on the plasma membrane of Sf9 cells at 24 h postinfection and distributed throughout the cytoplasm at 36-48 h postinfection. Analysis of the purified recombinant full-length CaD revealed most of the characteristics of the authentic CaD, including (a) an electrophoretic mobility corresponding to 125 kDa, (b) heat stability, (c) binding to actin, tropomyosin-actin, myosin, and calmodulin, (d) ability to inhibit actin-activated ATP hydrolysis by smooth muscle myosin, and (e) ability of Ca(2+)-calmodulin to reverse the inhibition. A CaD mutant with a deletion of 159 amino acids from the carboxyl terminus of the full-length CaD was also expressed at high levels in Sf9 cells. However, this mutant showed a decreased ability to bind to actin, tropomyosin-actin, and calmodulin, whereas the myosin binding was unaffected; actin-activated ATP hydrolysis by smooth muscle myosin was not inhibited by this mutant.

The effect of Ca2+ on the conformation of tropomyosin and actin in regulated actin filaments with or without bound myosin subfragment 1

The effects of Ca2+ and myosin subfragment 1 on the conformation of tropomyosin and actin in regulated actin filaments in ghost fibers were investigated by means of the polarized fluorescence technique. Regulated thin filaments were reconstituted in skeletal muscle ghost fibers by incorporation into the fibers of either skeletal muscle troponin-tropomyosin or smooth-muscle caldesmon-calmodulin-tropomyosin complexes. Tropomyosin and actin were specifically labeled with fluorescent probes, 1,5-IAEDANS and phalloidin-rhodamine, respectively. Analysis of the fluorescence parameters indicated that the binding of Ca2+ to regulated actin filaments induces conformational changes in tropomyosin and actin that lead to the strengthening of the interaction between these two proteins and weakening of the binding of actin monomers in the filament. These changes become larger when regulated actin forms rigor links with myosin subfragment 1. No notable alterations in the position of tropomyosin relative to actin in the frontal plane of the fiber were detected either upon binding of Ca2+ or upon the additional binding of myosin subfragment 1 to regulated actin.

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

Troponin I and caldesmon restrict alterations in actin structure occurring on binding of myosin subfragment 1

The effect of troponin I and caldesmon on phalloidin-rhodamine- and 1,5-IAEDANS-labelled actin in skeletal muscle ghost fibers was investigated by polarized fluorescence. Both these proteins inhibited the structural alterations in the actin monomer and the increase of flexibility of actin filaments occurring on binding of myosin heads, and their effects were potentiated by tropomyosin. This immobilization of the actin filament through troponin I and caldesmon seems to originate from restriction of the relative motions of the two domains within the monomer.

Phosphorylation of high-Mr caldesmon by protein kinase C modulates the regulatory function of this protein on the interaction between actin and myosin

High-Mr caldesmon, which is involved in smooth muscle contraction, was phosphorylated by protein kinase C. By chymotryptic digestion, actin- and calmodulin-binding assays and immunoprecipitation with the antibody to the C-terminal 35-kDa fragment, we have identified that all phosphate groups are incorporated exclusively into this fragment, which is the functional domain for binding actin and calmodulin. Phosphorylation of high-Mr caldesmon and its C-terminal 35-kDa fragment reduced their binding abilities to both F-actin and calmodulin. Further, their inhibitory effects on the actin-activated ATPase activity of gizzard myosin were also reversed in proportion to the degree of phosphorylation. These results suggest that phosphorylation of high-Mr caldesmon by protein kinase C, which is restricted within the C-terminal 35-kDa domain, results in the modulation of its activity in the smooth muscle actin--myosin interaction.

Caldesmon weakens the bonding between myosin heads and actin in ghost fibers

Earlier studies using polarized microphotometry have shown that caldesmon inhibits the alterations in structure and flexibility of actin in ghost fibers that take place upon the binding of myosin heads (Gałazkiewicz et al. (1987) Biochim. Biophys. Acta 916, 368-375). The present investigations, performed with an IAEDANS label attached to myosin subfragment 1 (S-1), revealed that this inhibition results from the weakening of the binding between myosin heads and actin as indicated by the caldesmon-induced increase in the random movement of S-1. Parallel experiments with actin labeled at Cys-374 demonstrated that this effect of caldesmon is transmitted to the C-terminus of the actin molecule resulting in a conformational adjustment in this region of the molecule.