The separation and characterization of two forms of Torpedo electric organ calelectrin

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.

Inhibition of intrinsic protein tyrosine kinase activity of EGF-receptor kinase complex from human breast cancer cells by the marine sponge metabolite (+)-aeroplysinin-1

1. (+)-Aeroplysinin-1, a naturally occurring tyrosine metabolite from the marine sponge Verongia aerophoba, was found to inhibit the phosphorylation of lipocortin-like proteins by a highly purified preparation of the epidermal growth factor (EGF) receptor-tyrosine protein kinase complex from MCF-7 breast carcinoma cells. 2. (+)-Aeroplysinin-1 blocked the EGF-dependent proliferation of both MCF-7 and ZR-75-1 human breast cancer cells and inhibited the ligand-induced endocytosis of the EGF receptor in vitro. 3. Treatment with aeroplysinin-1 in the concentration range at 0.25-0.5 microM resulted in a time- and dose-dependent total tumor cell death in vitro. 4. At a 10-fold higher concentration the compound did not reveal any cytostatic activity in normal human fibroblasts. 5. From these data we conclude that (+)-aeroplysinin-1 represents a compound which displays a strong anti-tumor effect on EGF-dependent tumor cell lines.

Role of phospholipase A2 in the stimulation of sponge cell proliferation by homologous lectin

Using the Geodia cydonium system, we showed that after incubation of competent sponge cells in the presence of lectin, phospholipase A2 was released from the cells. The substrates for this enzyme, phosphatidylethanolamine and phosphatidylcholine, were identified in the extracellular material of sponge tissue. In addition, the phospholipase A2 inhibitor calelectrin was identified by immunobiochemical techniques; this molecule was associated with the aggregation factor. Reconstitution experiments strongly suggested that phospholipase A2 catalyzed the release of arachidonic acid, which is then taken up by the cells. Intracellularly, arachidonic acid was metabolized primarily to prostaglandin E2. Inhibition studies revealed that prostaglandin E2 is involved in the ultimate increase of DNA synthesis. These findings suggest that the phospholipase A2-arachidonic acid system is involved in the matrix-initiated signal transduction pathway in sponges.

Ca2+- and phospholipid-binding properties of Torpedo electric organ calelectrin

The Ca2+-regulated lipid-binding properties of the H- and L-forms of calelectrin present in the electric organ of Torpedo marmorata have been compared. Binding of H-calelectrin required Ca2+ in millimolar concentrations, whereas that of L-calelectrin occurred in the micromolar range. Dissociation of H-calelectrin previously bound to lipids in the presence of 2 mM Ca2+ took place only when the Ca2+ concentration was reduced to micromolar concentrations. Binding was most effective to acidic phospholipids such as phosphatidylserine. Both forms of calelectrin promoted the aggregation of membrane vesicles in the presence of Ca2+.Mg2+, Na+ and K+ inhibited the Ca2+-induced binding to phospholipid, decreasing in effectiveness in that order. Binding was also inhibited by high pH. The surface activity and hydrophobicity index showed that H-calelectrin is a hydrophilic molecule. It may represent a less active, more highly phosphorylated "down-regulated" form of L-calelectrin. The role of calcium in H-calelectrin binding to lipid appeared to be consistent with the formation of a ternary complex of the protein, an acidic lipid and Ca2+, rather than with a direct interaction of lipid with hydrophobic sequences in H-calelectrin whose accessibility is Ca2+-regulated.