cDNA structure and expression of calpactin, a peptide involved in Ca2(+)-dependent cell aggregation in sponges

Aggregation of cells of the marine sponge Geodia cydonium is mediated by an aggregation factor (AF) particle of Mr 1.3 X 10(8). It is now reported that the AF particle is associated with calpactin, which was ascribed a role in the cell-adhesion process. In order to identify the sequence similarity to other members of the lipocortin family, the cDNA of sponge calpactin was cloned and found to display an 80% sequence similarity to vertebrate calpactin II but only a 47% similarity to calpactin I. The calpactin gene, which contains the consensus sequence coding for the amino acids G-T-D-E, was expressed in Escherichia coli and subsequently purified to a 37000-Mr polypeptide. Both the p32 and the p37 are provided with approximately two Ca2+ ions/molecule and the property to bind to phospholipids. The dissociation constant (calpactin-Ca2+) was in the absence of phospholipids in the range 500-700 microM-Ca2+ but in their presence about 20-30 microM-Ca2+. On the basis of (i) inhibition studies with antibodies to calelectrin and (ii) competition experiments with soluble phospholipids (both chemically defined as well as total homologous membrane lipids) we conclude that the AF-associated calpactin and plasma-membrane-bound phospholipid(s) are involved in cell-cell aggregation in sponges.