A soluble calcium-binding protein from the terrestrial annelidLumbricus terrestris L.

Abstract

Soluble calcium-binding proteins (SCBP) considerably different from calmodulin were purified from the body wall muscle of the earthwormLumbricus terrestris. Three isoforms were obtained with similar UV absorption spectra and amino acid compositions and an apparent molecular weight close to 20 kDa. They can be distinguished by their histidine and proline content and by their peptide maps. The tissue content, as determined by quantitative ELISA varies individually from 0.1 to 0.3 mmol kg−1. The calcium-binding property can be demonstrated by Ca2+-dependent electrophoretic mobility shift and45Ca2+ autoradiography on nitrocellulose sheets. The apparentKD values for the SCBP-Ca2+ complex is approximately 10−7 mol l−1 as revealed by euquilibrium and flow dialysis experiments. In the presence of 1 mmol l−1 MgCl2 the maximum binding capacity of SCBP was determined to be either 2 mol Ca2+ mol−1 protein (SCBP2) or 3 mol Ca2+ mol−1 protein (SCBP3). Preliminary studies concerning the functional role of SCBP indicate that it facilitates the diffusion of Ca2+ ions by a factor of 2 and is capable of inhibiting the ATPase of isolated body wall muscle actomyosin. The results reveal that earthworm SCBP are similar to vertebrate parvalbumin and to SCBP characterized from aquatic invertebrates.