ADP-RIBOSE GATES THE FERTILIZATION CHANNEL IN ASCIDIAN OOCYTES

We report an ion channel in the plasma membrane of unfertilized oocyte s of the ascidian Ciona intestinalis that is directly gated by the sec ond messenger ADP-ribose. The ion channel is permeable to Ca2+ and Na and is characterized by a reversal potential between 0 and +20 mV and a unitary conductance of 140 pS. Preinjection of the Ca2+ chelator 2- bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) or antag onists of intracellular Ca2+ release channels into oocytes did not inh ibit the ADP-ribose current, demonstrating that the channel is activat ed in a Ca2+-independent manner. Both the fertilization current and th e current induced by the injection of nicotinamide nucleotides are blo cked by nicotinamide, suggesting that the ADP-ribose channel is activa ted at fertilization in a nicotinamide-sensitive manner. These data su ggest that ascidian sperm trigger the hydrolysis of nicotinamide nucle otides in the oocyte to ADP-ribose and that this mechanism is responsi ble for the production of the fertilization current.