Pathway for Ca 2+ influx into cells by trichosporin-B-VIa, an α-aminoisobutyric acid-containing peptide, from the fungus Trichoderma polysporum

Abstract

Trichosporin (TS) -B-VIa, a fungal α-aminoisobutyric acid (Aib) -containing peptide consisting of 19 amino acid residues and a phenylalaninol, produced both 45Ca 2+ influx into bovine adrenal chromaffin cells and catecholamine secretion from the cells. The secretion induced by TS-B-VIa at lower concentrations (2–5 μM) was completely dependent on the external Ca 2+, while that induced by TS-B-VIa at higher concentrations (10–30 μM) was partly independent of the Ca 2+. The concentration-response curves (2–5 μM) for the TS-B-VIa-induced Ca 2+ influx and secretion correlated well. The TS-B-VIa (at 5 μM) -induced secretion was not antagonized by diltiazem, a blocker of L-type voltage-sensitive Ca 2+ channels. The treatment of fura-2-loaded C 6 glioma cells with TS-B-VIa (2–5 μM) led to an increase in the intracellular free Ca 2+ concentration ([Ca 2+] i) in a concentration-dependent manner but the stimulatory effects of TS-B-VIa on [Ca 2+]; were only slightly observed in Ca 2+-free medium, indicating that TS-B-VIa causes Ca 21 influx from the external medium into the C 6 cells. The TS-B-VIa-induced increase in [Ca 2+] i in the C 6 cells was not antagonized by diltiazem and by SK&F 96365, a novel blocker of receptor-mediated Ca 2+ entry. High K + increased neither [Ca 2+] i in the C6 cells nor Mn 2+ influx into the cells, while TS-B-VIa increased Mn 2+ influx. Also in other non-excitable cells, bovine platelets, similar results were obtained. These results strongly suggest that the mechanism of Ca 2+ influx by TS-B-VIa at the lower concentrations is distinct from the event of Ca 2+ influx through receptor-operated or L-type voltage-sensitive Ca 2+ channels in both excitable cells (the chromaffin cells) and non-excitable cells (the C 6 cells and the platelets) and that TS-B-VIa per se may form Ca 2+-permeable ion channels in biological membranes. On the other hand, the peptide at the higher concentrations seems to damage cell membranes.