This paper describes the interaction of
apamin, a
bee venom neurotoxin, with the mouse
neuroblastoma cell membrane. Voltage-clamp analyses have shown that
apamin at low concentrations specifically blocks the Ca2+-dependent K+ channel in differentiated
neuroblastoma cells. Binding experiments with highly radiolabeled toxin indicate that the dissociation constant of the
apamin-receptor complex in differentiated
neuroblastoma cells is 15-22 pM and the maximal binding capacity is 12 fmol/mg of
protein. The receptor is destroyed by
proteases, suggesting that it is a
protein. The binding capacity of
neuroblastoma cells for radiolabeled
apamin dramatically increases during the transition from the nondifferentiated to the differentiated state. The number of Ca2+-dependent K+ channels appears to be at most 1/5th the number of fast Na+ channels in differentiated
neuroblastoma. The binding of radiolabeled
apamin to its receptor is antagonized by monovalent and
divalent cations. Na+ inhibition of the binding of 125I-labeled
apamin is of the competitive type (Kd(Na+) = 44 mM).
Guanidinium and guanidinated compounds such as
amiloride or
neurotensin prevent binding of 125I-labeled
apamin, the best antagonist being
neurotensin.