Functional properties of Drosophila inositol trisphosphate receptors

The functional properties of the only inositol trisphosphate (IP₃) receptor subtype expressed in Drosophila were examined in permeabilized S2 cells. The IP₃ receptors of S2 cells bound (1,4,5)IP₃ with high affinity (K_d = 8.5±1.1nM), mediated positively co-operative Ca²⁺ release from a thapsigargin-sensitive Ca²⁺ store (EC₅₀ = 75±4nM, Hill coefficient = 2.1±0.2), and they were recognized by an antiserum to a peptide conserved in all IP₃ receptor subtypes in the same way as mammalian IP₃ receptors. As with mammalian IP₃ receptors, (2,4,5)IP₃ (EC₅₀ = 2.3±0.3μM) and (4,5)IP₂ (EC₅₀ approx. 10μM) were approx. 20- and 100-fold less potent than (1,4,5)IP₃. Adenophostin A, which is typically approx. 10-fold more potent than IP₃ at mammalian IP₃ receptors, was 46-fold more potent than IP₃ in S2 cells (EC₅₀ = 1.67±0.07nM). Responses to submaximal concentrations of IP₃ were quantal and IP₃-evoked Ca²⁺ release was biphasically regulated by cytosolic Ca²⁺. Using rapid superfusion to examine the kinetics of IP₃-evoked Ca²⁺ release from S2 cells, we established that IP₃ (10μM) maximally activated Drosophila IP₃ receptors within 400ms. The activity of the receptors then slowly decayed (t_1/2 = 2.03±0.07s) to a stable state which had 47±1% of the activity of the maximally active state. We conclude that the single subtype of IP₃ receptor expressed in Drosophila has similar functional properties to mammalian IP₃ receptors and that analyses of IP₃ receptor function in this genetically tractable organism are therefore likely to contribute to understanding the roles of mammalian IP₃ receptors.