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 (SCBP1-3) 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 SCBP2a/2b. Both of them together with SCBP3 are present in the body wall muscle. In the gizzard solely, SCBP2b and no SCBP2a 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 SCBP2a, SCBP2b, 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.