Essential Role of Isoprenylation of Transducin in Regulation of Phototransduction in Rod Cells

Abstract

Purpose: : Isoprenylation is a posttranslational protein modification, and one of the two types of isoprenoids, the 15–carbon farnesyl and the 20–carbon geranylgeranyl, is linked to the conserved cysteine residue of a variety of signaling proteins such as G proteins. Transducin (Tα and Tß γ), a member of heterotrimeric G protein family, plays a pivotal role in phototransduction. Tγ is selectively modified with the farnesyl, which is indispensable for the signal–transducing function of transducin. When the farnesyl of Tγ is replaced by the geranylgeranyl, the biochemical properties of Tßγ are markedly altered in in vitro expreiments. This study was performed in order to understand the role of farnesylation of Tγ in photoreceptor function of the rods in vivo.

Methods: : We generated knock–in mice, in which the farnesylation signal in Tγ gene was switched either to a geranylgeranylation signal (S74L mutation) or to a form producing unmodified Tγ.

Results: : Knock–in mice expressing geranylgeranylated Tγ showed normal rod responses to dim flashes under dark–adapted condition, but light adaptation to stronger light was severely inhibited in the mutant mice. The impaired light adaptation was associated with slowdown of the light–dependent translocation of Tßγ from the rod outer segment to the inner part. These results indicate the physiological importance of selective farnesylation over geranylgeranylation in regulation of the degree of the light–signal amplification. On the other hand, when the isoprenylation signal (for farnesylation or geranylgeranylation) was deleted in Tγ gene, the intracellular distribution of transducin subunits became extremely abnormal in the retina of the knock–in mice. Although the retina was normal in morphology even in the adulthood, GTPγS–binding activity of transducin in the mutant mice retinas was markedly reduced, and the photoresponses from the rods of the mutant mice were largely impaired when compared with wild–type mice.

Conclusions: : Our results demonstrate a very important role of the selective farnesylation of transducin in physiological function of rod photoreceptor cells.

Keywords: signal transduction • protein modifications-post translational • transgenics/knock-outs