A novel magnesium-dependent mechanism for the activation of transducin by fluoride.

Activation of transducin-GDP by NaF is mainly mediated by aluminofluorde or beryllofluoride complexes acting as GTP gamma-phosphate analogs. In millimolar magnesium, NaF at concentrations above 3 mM is active even in the absence of aluminium or beryllium. This activation has a Hill coefficient of 3 with respect to F-, and its rate is linear with respect to Mg2+ concentrations above 2 mM. Upon fluoride dilution, inactivation rate is hundreds of times faster than for aluminofluoride-activated T alpha GDP. We propose that at high NaF concentrations, 3 hydrogen-bonded fluorides in the gamma-phosphate site of T alpha GDP entrap a magnesium counterion and this induces the transconformation to the T alpha GTP form.