Fluoride activation of G proteins requires the presence of aluminium or beryllium and it has been suggested that AIF4‐ acts as an analogue of the gamma‐phosphate of GTP in the nucleotide site. We have investigated the action of AIF4‐ or of BeF3‐ on transducin (T), the G protein of the retinal rods, either indirectly through the activation of cGMP phosphodiesterase, or more directly through their effects on the conformation of transducin itself. In the presence of AIF4‐ or BeF3‐, purified T alpha subunit of transducin activates purified cyclic GMP phosphodiesterase (PDE) in the absence of photoactivated rhodopsin. Activation is totally reversed by elution of fluoride or partially reversed by addition of excess T beta gamma. Activation requires that GDP or a suitable analogue be bound to T alpha: T alpha‐GDP and T alpha‐GDP alpha S are activable by fluorides, but not T alpha‐GDP beta S, nor T alpha that has released its nucleotide upon binding to photoexcited rhodopsin. Analysis of previous works on other G proteins and with other nucleotide analogues confirm that in all cases fluoride activation requires that a GDP unsubstituted at its beta phosphate be bound in T alpha. By contrast with alumino‐fluoride complexes, which can adopt various coordination geometries, all beryllium fluoride complexes are tetracoordinated, with a Be‐F bond length of 1.55 A, and strictly isomorphous to a phosphate group. Our study confirms that fluoride activation of transducin results from a reversible binding of the metal‐fluoride complex in the nucleotide site of T alpha, next to the beta phosphate of GDP, as an analogue of the gamma phosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluoride activation of the cGMP cascade of vision requires the presence of aluminum, and is shown to be mediated by the binding of one AlF‐4 to the GDP/GTP‐binding subunit of transducin. The presence of GDP in the site is required: AlF4 − is ineffective when the site is empty or when GDPßS is substituted for GDP. This sensitivity to the sulfur of GDPßS suggests that AlF4 − is in contact with the GDP. Striking structural similarities between AlF4 − and PO4 −1 lead us to propose that AlF4 − mimics the role of the γ‐phosphate of GTP.

Bacteriorhodopsin (BR) is a transmembrane protein in the purple membrane (PM) of Halobacterium salinarum. Its function as a light-driven proton pump is associated with a cycle of photointermediates which is strongly hydration-dependent. Using energy-resolved neutron scattering, we analyzed the thermal motions (in the nanosecond-to-picosecond time range) in PM at different hydration levels. Two main populations of motions were found that responded differently to water binding. Striking correlations appeared between these "fast" motions and the "slower" kinetic constants (in the millisecond time range) of relaxations and conformational changes occurring during the photocycle.