Subunit Dissociation is Sufficient for Light‐Induced Translocation of Rod Transducin

Abstract

Photoreceptor illumination causes a massive redistribution of G protein subunits, arrestins and recoverin between inner and outer compartments. In light, rod G protein transducin (Gt) moves from the outer segments (OS) to the inner compartments, whereas arrestin moves from the inner compartments to the OS. This phenomenon is thought to be important for light and dark adaptation. Many groups presented evidence that movement of Gt and arrestin requires molecular motors. However, we show that ATP is not required for redistribution, ruling out active transport or gating mechanisms. We demonstrate that GTP binding causes rod Gt release from membranes and movement to the inner compartments. Moreover, we show that rod Gt translocates if the GαGβ γ subunits are induced to dissociate by a synthetic peptide (mSIRK) in the dark. Thus, our results support a simple model of Gt translocation: upon activation, subunits dissociate, leave the OS membranes and disperse throughout the cell. We also investigated the striking difference between rod Gt, which is capable of re-localization and cone Gt, which does not re-localize upon activation. Our experiments suggest that cone Gt subunits do not dissociate upon activation (GTPγS binding) and remain localized to the OS. We infer from our studies that in vivo, most G proteins do not dissociate upon activation. Rod transducin has this unique ability in order to re-localize within the cells. This research is supported by NIH grants GM 060019 (V.Z.S) and AHA 0515233B (D.H.R)