An eye-specific Gβ subunit essential for termination of the phototransduction cascade

Abstract

Heterotrimeric G proteins couple various receptors to intracellular effector molecules. Although the role of the G alpha subunit in effector activation, guanine nucleotide exchange and GTP hydrolysis has been well studied, the cellular functions of the G beta subunits are less well understood. G beta gamma dimers bind G alpha subunits and anchor them to the membrane for presentation to the receptor. In specific systems, the G beta subunits have also been implicated in direct coupling to ion channels and to effector molecules. We have isolated Drosophila melanogaster mutants defective in an eye-specific G-protein beta-subunit (G beta e), and show here that the beta-subunit is essential for G-protein-receptor coupling in vivo. Remarkably, G beta mutants are also severely defective in the deactivation of the light response, demonstrating an essential role for the G beta subunit in terminating the active state of this signalling cascade.

Phototransduction in transgenic mice after targeted deletion of the rod transducin α-subunit

“…1E) or by overexpression of a mutant Tc␣ (44). Flies deficient in G␤e maintained normal retinal levels of the ␣-subunit (45). It follows that free T␤␥ should have been greatly increased in Tr␣ Ϫ͞Ϫ rods.…”

Phototransduction and retinal degeneration in Drosophila

“…The effector for the light-activated rhodopsin is the heterotrimeric G-protein, G q and it is encoded by Gα q [108][109][110], Gβ e [111,112], and Gγ e [113,114], all of which are eyeenriched. After exchange of the guanosine diphosphate (GDP) bound to the Gα q for GTP, the Gα q and Gβγ subunits dissociate and the activated Gα q -GTP binds to the PLC encoded by norpA (no receptor potential A) [5,115].…”

“…A strong allele of Gβ e mutant, Gβ 1 e , displays a ∼100-fold reduction in light sensitivity [111], and a mutation in the farnesylation site of Gγ e also results in a reduction in light sensitivity [114]. Moreover, light-simulated PLC activity is deficient in the Gβ 1 e mutant [116].…”

“…The Gβγ subunit is also required for termination of phototransduction, as the rate of deactivation of photoresponse is decreased in Gb 1 e flies [111]. Other Gβγ subunits have been shown to be necessary for phosphorylation of GPCRs by recruiting GPCR kinases to the plasma membrane [119].…”

Light-regulated translocation of signaling proteins in Drosophila photoreceptors

“…In these studies the Drosophila eye-specific G q α (DG q α) translocates to the cytosol during illumination and subsequently returns to the membrane as an inactive G q α-GDP form, via binding to the βγ dimmer. The role of the β-subunit in DG q α movement was studied in Drosophila photoreceptors by using the G βe 1 mutant (Dolph et al, 1994), which expresses highly reduced amounts of the eye-specific DG q β. Analysis of the heterozygot G βe 1 revealed that the kinetics of the light-dependent translocation of DG q α, and in particular the subsequent recovery were markedly slowed down.…”