The distribution of troponin-like proteins on thin filaments of the bay scallop, aequipecten irradians

Crenomytilus grayanus 40kDa calponin-like protein: cDNA cloning, sequence analysis, tissue expression, and post-translational modifications

Calponin-like protein (CaP-40), a third major protein after actin and tropomyosin, has recently been identified by us in the Ca2+-regulated thin filaments of mussel Crenomytilus grayanus. It contains calponin homology domain, five calponin family repeats and possesses similar biochemical properties as vertebrate smooth muscle calponin. In this paper, we report a full-length cDNA sequence of CaP-40, study its expression pattern on mRNA and protein levels, evaluate CaP-40 post-translational modifications and perform protein-protein interaction analysis. The full-length sequence of CaP-40 consists of 398 amino acids and has high similarity to calponins among molluscan species. CaP-40 gene is widely expressed in mussel tissues, with the highest expression in adductor and mantle. Comparison of these data with protein content established by mass-spectrometry analysis revealed that the high mRNA content is mirrored by high protein levels for adductor smooth muscles. To provide unbiased insight into the function of CaP-40 and effect of its over-expression in adductor smooth muscle, we built protein-protein interaction network of identified Crenomytilus grayanus proteome. In addition, we showed that CaP-40 is subjected to post-translational N- and C-terminal acetylation at N127, G229 and G349 sites which potentially regulates its function in vivo.

Ca2+ -induced tropomyosin movement in scallop striated muscle thin filaments

Striated muscle contraction in most animals is regulated at least in part by the troponin-tropomyosin (Tn-Tm) switch on the thin (actin-containing) filaments. The only group that has been suggested to lack actin-linked regulation is the mollusks, where contraction is regulated through the myosin heads on the thick filaments. However, molluscan gene sequence data suggest the presence of troponin (Tn) components, consistent with actin-linked regulation, and some biochemical and immunological data also support this idea. The presence of actin-linked (in addition to myosin-linked) regulation in mollusks would simplify our general picture of muscle regulation by extending actin-linked regulation to this phylum as well. We have investigated this question structurally by determining the effect of Ca(2+) on the position of Tm in native thin filaments from scallop striated adductor muscle. Three-dimensional reconstructions of negatively stained filaments were determined by electron microscopy and single-particle image analysis. At low Ca(2+), Tm appeared to occupy the "blocking" position, on the outer domain of actin, identified in earlier studies of regulated thin filaments in the low-Ca(2+) state. In this position, Tm would sterically block myosin binding, switching off filament activity. At high Ca(2+), Tm appeared to move toward a position on the inner domain, similar to that induced by Ca(2+) in regulated thin filaments. This Ca(2+)-induced movement of Tm is consistent with the hypothesis that scallop thin filaments are Ca(2+) regulated.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Functional characterization of the two isoforms of troponin C from the arthropod Balanus nubilus

Two isoforms of troponin C (BTnC1 and BTnC2) from the striated muscle of the arthropod Balanus nubilus Darwin (giant barnacle) have been purified (Potter et al., 1987; Collins et al., 1991). Both isoforms were present in all of the white striated muscle fibres studied but not in the red fibres. The ratio of BTnC2 to BTnC1 in different fibre types varied between 3:1 and 1:1. Both forms of TnC could be readily extracted from myofibrillar bundles of barnacle muscle in low ionic strength EDTA solutions, reducing force activation to less than 10%. Both forms either separately or together reassociated with the TnC-depleted fibres in a relaxing (LR) solution (pCa greater than 8.0, [Mg2+] free = 1 mM, I = 0.15 M), and the reconstituted fibres could be subsequently activated in contraction (LA) solution (pCa = less than 3.8, [Mg2+] free = 1 mM, I = 0.15 M). The dissociation of BTnC 1 + 2 is blocked in low ionic strength solutions containing Mg2+ (greater than or equal to 10 mM). The two isoforms of crayfish TnC (CrTnC1 and CrTnC2) were also found to be equivalent to the barnacle TnCs in their ability to reactivate TnC-depleted barnacle myofibrillar bundles. Similar experiments using rabbit skeletal muscle TnC (STnC) (I = 0.15 M) in BTnC-depleted myofibrillar bundles of barnacle showed considerable variability. STnC could associate, although weakly, with the depleted bundles in either LR or LA, and force could be partially restored. In neither situation was it as effective as either BTnC or CrTnC. Interestingly, bovine cardiac TnC (CTnC), although it did not associate at pCa greater than 7.0, did associate and effectively activate force at pCa less than 3.8, but dissociated on return to pCa greater than 7.0 (LR). Neither barnacle TnC isoform associated with TnC-depleted skinned fibres from rabbit skeletal muscle at pCa greater than 7.0, but did associate and activate these fibres at pCa less than 3.8. Once these fibres were returned to LR and then placed in LA at pCa 3.8 all BTnC-restored force was lost, indicating a dissociation of BTnC once the Ca2+ is lowered, as observed with CTnC in barnacle myofibrillar bundles. Finally, the inhibitory effect of BTnI on force and the absence of an effect of calmodulin, trifluoperazine or ATP-gamma-S on force were all taken as evidence for a thin filament regulated Ca2+ control system.

Calcium regulated thin filaments from molluscan catch muscles contain a caldesmon-like regulatory protein

The thin filaments of the anterior byssus retractor muscle of the edible mussel Mytilus and the transluscent and opaque adductors of the oyster Crassostrea have been isolated and their properties investigated. We find that the thin filaments from all three muscles can activate skeletal muscle myosin ATPase in the presence of calcium but that the activity is inhibited in its absence. The filaments contain a protein which interacts with antibodies to vertebrate smooth muscle caldesmon on immunoblots. The antibodies relieve the inhibition of the thin-filament-activated myosin MgATPase. They can also bundle the thin filaments. We conclude that a caldesmon-like protein is present in molluscan muscle. As in the vertebrate smooth muscle, it could act as part of a control mechanism in addition to the myosin regulatory system. Vertebrate smooth muscle caldesmon can crosslink actin and myosin and it has been suggested that it may in this way contribute to the latch state. A similar interaction may be involved in the catch mechanism in molluscan muscle.