CRY1AC, A BACILLUS-THURINGIENSIS TOXIN, TRIGGERS EXTRACELLULAR CA2-FUMIFERANA, LEPIDOPTERA)( INFLUX AND CA2+ RELEASE FROM INTRACELLULAR STORES IN CF1 CELLS (CHORISTONEURA)

Intracellular Ca2+ concentration was measured in single Cf1 cells (Cho ristoneura fumiferana, spruce budworm) loaded with Fura-2, a Ca2+-sens itive fluorescent probe. Cf1 cells displayed Ca2+ surges in response t o Cry1Ac and Cry1C proteins, two Cf1-toxic Bacillus thuringiensis prod ucts, but not to Cry1Aa and Cry3A, which are not toxic to Cf1 cells. I n the presence of extracellular Ca2+, the toxin-induced Ca2+ response was insensitive to methoxyverapamil, a voltage-dependent Ca2+ channel blocker, but was abolished by lanthanum, a general inhibitor of Ca2+ t ransport. In the absence of external Ca2+, Cry1Ac induced a small intr acellular Ca2+ transient which was inhibited by TMB-8, a blocker of Ca 2+ release from inositol-1,4,5-trisphosphate-sensitive pools. Under th ese conditions, thapsigargin, which inhibits intracellular Ca2+ ATPase s, elicited a Ca2+ surge when applied alone. However, subsequent addit ion of Cry1Ac failed to induce a Ca2+ signal, indicating a depletion o f intracellular Ca2+ pools. In Cf1 cells, therefore, bioactive B, thur ingiensis toxins triggered intracellular Ca2+ surges which were mainly due to the influx of extracellular Ca2+ through toxin-made pores, as confirmed by planar lipid bilayer experiments. Furthermore, TMB-8- and thapsigargin-sensitive Ca2+ stores contributed to the Cry1Ac-induced Ca2+ signal.