A soluble calcium-binding protein (SCBP) present in Drosophila melanogaster and Calliphora erythrocephala muscle cells

Soluble calcium-binding proteins (SCBPs) of the earthworm Lumbricus terrestris: possible role as relaxation factors in muscle

The soluble Ca²⁺-binding protein (SCBP) from the earthworm Lumbricus terrestris was analyzed with regard to its role as a soluble muscle relaxation factor. The actomyosin ATPase activity was inhibited by the addition of decalcified SCBP as it binds Ca²⁺ stronger than the regulatory proteins associated with the actomyosin. Competitive ⁴⁵Ca²⁺-binding assays with decalcified actomyosin and SCBP showed that ⁴⁵Ca²⁺ is first bound to actomyosin and is subsequently taken over by SCBP with increasing incubation time. Ca²⁺ competition experiments carried out with ⁴⁵Ca²⁺ loaded SCBP and fragmented sarcoplasmic reticulum vesicles revealed that ⁴⁵Ca²⁺ bound to SCBP can be deprived by the ATP-dependent Ca²⁺ uptake of the sarcoplasmic reticulum. Furthermore, experiments in a diffusion chamber showed that the addition of SCBP significantly enhances the ⁴⁵Ca²⁺ flux in a concentration dependent manner. The amount of the Ca²⁺ flux increase tends to reach a maximum value of about 70%. With all protein components isolated from the obliquely striated muscle, our in vitro experiments consistently show that SCBP may accelerate muscle relaxation similar as assumed for vertebrate parvalbumin.

Soluble calcium-binding proteins (SCBPs) of the earthworm Lumbricus terrestris: molecular characterization and localization by FISH in muscle and neuronal tissue

Soluble calcium-binding proteins (SCBPs) of invertebrates probably serve like their vertebrate counterpart-the parvalbumins-as soluble relaxing factors in muscles. Three SCBP isoforms (SCBP_1-3) have been isolated and biochemically characterized in the earthworm Lumbricus terrestris (Huch et al. in J Comp Physiol B 158:325-334, 1988). For SCBP₂, we found two isoforms named SCBP_2a/2b. Both of them together with SCBP₃ are present in the body wall muscle. In the gizzard solely, SCBP_2b and no SCBP_2a or SCBP₃ could be detected. The coding sequences of all three isoforms consist of 534 bp for 178 amino acids and contain four EF-hand motifs, of which the second EF-hands are truncated. Recombinant proteins show heat stability and a Ca²⁺-dependent mobility shift similar to the native proteins, indicating comparable calcium-binding properties. All three isoforms are encoded by three distinct and differentially expressed genes. The genes for SCBP_2a, SCBP_2b, and SCBP₃ are interrupted by only one intron, inserting at nearly the same positions. Northern blot analysis revealed two mRNA transcripts for SCBP₂ of approximately 1250 and 1500 kb and one transcript for SCBP₃ of approximately 1250 kb. SCBP mRNA was localized by fluorescent in situ hybridization in the body wall and the gizzard. The distribution of the staining intensities resembles that for the myosin ATPase activity and indicates a correlation between the amount of SCBP and speed of muscle contraction. In addition, SCBP mRNA was localized within the nervous tissue, the cerebral and subesophageal ganglia and the ventral nerve cord.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.