PHOSDUCIN INDUCES A STRUCTURAL-CHANGE IN TRANSDUCIN BETA-GAMMA

Background: Phosducin binds tightly to the beta gamma subunits (G(t)be ta gamma) of the heterotrimeric G protein transducin, preventing G(t)b eta gamma reassociation with G(t)alpha-GDP and thereby inhibiting the G-protein cycle. Phosducin-like proteins appear to be widely distribut ed and may play important roles in regulating many heterotrimeric G-pr otein signaling pathways. Results: The 2.8 Angstrom crystal structure of a complex of bovine retinal phosducin with G(t)beta gamma shows how the two domains of phosducin cover one side and the top of the seven- bladed beta propeller of G(t)beta gamma. The binding of phosducin indu ces a distinct structural change in the beta propeller of G(t)beta gam ma, such that a small cavity opens up between blades 6 and 7. Electron density in this cavity has been assigned to the farnesyl moiety of th e gamma subunit. Conclusions: beta gamma subunits of heterotrimeric G proteins can exist in two distinct conformations. In the R (relaxed) s tate, corresponding to the structure of the free beta gamma or the str ucture of beta gamma in the alpha beta gamma heterotrimer, the hydroph obic farnesyl moiety of the gamma subunit is exposed, thereby mediatin g membrane association, in the T (tense) state, as observed in the pho sducin-G(t)beta gamma structure, the farnesyl moiety of the gamma subu nit is effectively buried in the cavity formed between blades 6 and 7 of the beta subunit. Binding of phosducin to G(t)beta gamma induces th e formation of this cavity, resulting in a switch from the R to the T conformation. This sequesters py from the membrane to the cytosol and turns off the signal-transduction cascade. Regulation of this membrane association/dissociation switch of G(t)beta gamma by phosducin may be a general mechanism for attenuation of G protein coupled signal trans duction cascades.