A Preparative Method for the Isolation of Calponin from Molluscan Catch Muscle.

We describe the development of a preparative method to isolate molluscan catch muscle, calponin. This method is based on the ability of calponin to interact with actin in a temperature-dependent manner. After extracting thin filaments, as previously described, the extract was ultracentrifuged at 2 °C. While other surface proteins of thin filaments co-precipitated with actin, calponin, along with some minor contaminants, remained in the supernatant. Calponin was purified through cation-exchange chromatography. The yield of pure protein was four-fold higher than that achieved through high-temperature extraction. To evaluate functionally isolated proteins, we determined the effect of calponin on Mg2+-ATPase activity of hybrid and non-hybrid actomyosin. The degree of ATPase inhibition was consistent with previously published data but strongly dependent on the environmental conditions and source of actin and myosin used. Furthermore, at low concentrations, calponin could induce the ATPase activity of hybrid actomyosin. This result was consistent with data indicating that calponin can modulate actin conformation to increase the relative content of "switched on" actin monomers in thin filaments. We assume that calponin obtained by the isolation method proposed herein is a fully functional protein that can both inhibit and induce the ATPase activity.

Troponin-like regulation in muscle thin filaments of the mussel Crenomytilus grayanus (Bivalvia: Mytiloida).

Muscles of bivalve molluscs have double calcium regulation--myosin-linked and actin-linked. While the mechanism of myosin-linked regulation is sufficiently studied, there is still no consensus on the mechanism of actin-linked regulation. Earlier we showed a high degree of Ca2+-sensitivity of thin filaments from the adductor muscle of the mussel Crenomytilus grayanus (Mytiloida). In order to elucidate the nature of this regulation, we isolated the fraction of minor proteins from the mussel thin filaments, which confers Ca2+-sensitivity to reconstituted actomyosin-tropomyosin. Proteins of this fraction, ABP-19, ABP-20, and ABP-28, were chromatographically purified and identified. According to the results of mass spectrometry and Western blot analysis, as well as by their functional properties, these mussel actin-binding proteins appeared to correspond to the troponin components from the skeletal muscles of vertebrates (TnC, TnI and TnT). The reconstituted mussel troponin complex confers to actomyosin-tropomyosin more than 80% Ca2+-sensitivity. The in vivo molar ratio of actin/tropomyosin/troponin was calculated to be 7:1:0.5, i.e., the content of troponin in mussel thin filaments is two times lower than in thin filaments of skeletal muscles of vertebrates. These data demonstrate that troponin-like regulation found in the catch muscle of the mussel C. grayanus is present at least in two suborders of bivalves: Pectinoida and Mytiloida.

Non-Straub type actin from molluscan catch muscle.

We have developed a method of obtaining natural actin from smooth muscles of the bivalves on the example of the Сrenomytilus grayanus catch muscle. The muscles were previously rigorized to prevent a loss of thin filaments during homogenization and washings. Thin filaments were isolated with a low ionic strength solution in the presence of ATP and sodium pyrophosphate. Surface proteins of thin filaments-tropomyosin, troponin, calponin and some minor actin-binding proteins-were dissociated from actin filaments by increasing the ionic strength to 0.6 M KCL. Natural fibrillar actin obtained in that way depolymerizes easily in low ionic strength solutions commonly used for the extraction of Straub-type actin from acetone powder. Purification of natural actin was carried out by the polymerization-depolymerization cycle. The content of inactivated actin remaining in the supernatant is much less than at a similar purification of Straub-type actin. A comparative investigation was performed between the natural mussel actin and the Straub-type rabbit skeletal actin in terms of the key properties of actin: polymerization, activation of Mg-ATPase activity of myosin, and the electron-microscopic structure of actin polymers.

Hybrid and non-hybrid actomyosins reconstituted with actin, myosin and tropomyosin from skeletal and catch muscles.

In this study, we investigated hybrid and non-hybrid actomyosin models including key contractile proteins: actin, myosin, and tropomyosin. These proteins were isolated from the rabbit skeletal muscle and the catch muscle of the mussel Crenomytilus grayanus. Our results confirmed literature data on an unusual ability of bivalve's tropomyosin to inhibit Mg-ATPase activity of skeletal muscle actomyosin. We have shown that the degree of inhibition depends on the environmental conditions and may vary within a wide range. The inhibitory effect of mussel tropomyosin was not detected in non-hybrid model (mussel myosin + mussel actin + mussel tropomyosin). This effect was revealed only in hybrid models containing mussel tropomyosin + rabbit (or mussel) actin + rabbit myosin. We assume that mussel and rabbit myosins have mismatched binding sites for actin. In addition, mussel tropomyosin interacting with actin is able to close the binding sites of rabbit myosin with actin, which leads to inhibition of Mg-ATPase activity.

Molluscan smooth catch muscle contains calponin but not caldesmon.

We isolated Ca(2+)-regulated thin filaments from the smooth muscle of the mussel Crenomytilus grayanus and studied the protein composition of different preparations from this muscle: whole muscle, heat-stable extract, fractions from heat-stable extract, thin filaments and intermediate stages of thin filaments purification. Among the protein components of the above-listed preparations, we did not find caldesmon (CaD), although two isoforms of a calponin-like (CaP-like) protein, which along with CaD is characteristic of vertebrate smooth muscle, were present in thin filaments. Thus, CaD is not Ca(2+)-regulator of thin filaments of this muscle. On the other hand, the mussel CaP-like protein is also not such Ca(2+)-regulator since we have shown that this protein can be selectively removed from isolated mussel thin filaments without loss of their Ca(2+)-sensitivity. We suggest that thin filaments in the smooth catch muscle possess other type of Ca(2+)-regulation, different from that in vertebrate smooth muscles.