Phosducin Regulates the Expression of Transducin βγ Subunits in Rod Photoreceptors and Does Not Contribute to Phototransduction Adaptation

Abstract

For over a decade, phosducin's interaction with the βγ subunits of the G protein, transducin, has been thought to contribute to light adaptation by dynamically controlling the amount of transducin heterotrimer available for activation by photoexcited rhodopsin. In this study we directly tested this hypothesis by characterizing the dark- and light-adapted response properties of phosducin knockout (Pd−/−) rods. Pd−/− rods were notably less sensitive to light than wild-type (WT) rods. The gain of transduction, as measured by the amplification constant using the Lamb-Pugh model of activation, was 32% lower in Pd−/− rods than in WT rods. This reduced amplification correlated with a 36% reduction in the level of transducin βγ-subunit expression, and thus available heterotrimer in Pd−/− rods. However, commonly studied forms of light adaptation were normal in the absence of phosducin. Thus, phosducin does not appear to contribute to adaptation mechanisms of the outer segment by dynamically controlling heterotrimer availability, but rather is necessary for maintaining normal transducin expression and therefore normal flash sensitivity in rods.

Transducin γ-Subunit Sets Expression Levels of α- and β-Subunits and Is Crucial for Rod Viability

“…G␥ 1 contains specific ubiquitination sites tagging G␤ 1 ␥ 1 for proteolysis by the 26S proteasome (Obin et al, 2002). This ubiquitination has been shown to be regulated by phosducin in vitro (Obin et al, 2002), and phosducin knock-out results in ϳ36% loss of G␤ 1 ␥ 1 in rods (Sokolov et al, 2004;Krispel et al, 2007).…”

The physiological roles of phosducin: from retinal function to stress-dependent hypertension

“…By interacting with transducin, phosducin can curb rod visual signalling. [56,83,84] However, single cell experiments based on suction electrode recordings have revealed that the above-mentioned effect on rod light adaptation is not influenced by the lack of phosducin under moderate light conditions [69]. The facilitation of transducin translocation seems rather to be a matter of light intensity playing a role in brightly illuminated environments.…”

“…The facilitation of transducin translocation seems rather to be a matter of light intensity playing a role in brightly illuminated environments. Yet, the gain of transductionmeasured by the amplification constant-was diminished in Pdc -/-rods compared to wild-type cells in single cell recordings [69]. It has been suggested that, rather than adjusting the availability of transducin heterotrimer and thereby controlling light sensitivity, phosducin exerts longer-term regulatory functions allocating transducin expression levels.…”

“…Only recently, Herrmann et al [37,69] described phosducin to be a regulator of light sensitivity at the photoreceptor-to-ON-bipolar cell synapse rather than a mediator of phototransduction. By evaluating ERG b-wave responses in mice deficient in phosducin and wild-type animals, phosducin was shown to be important both for dark-adapted sensitivity of flash responses as well as for maintenance of b-wave sensitivity under sustained background illumination.…”

Splice Isoforms of Phosducin-like Protein Control the Expression of Heterotrimeric G Proteins

“…PhLP1 (often called PhLP) is ubiquitously expressed (15) and is shown to act as a CCT co-factor during the folding of G protein ␤ (16 -20). In contrast, phosducin is expressed predominantly in rod and cone photoreceptors, where it helps maintain high lev-els of and assists in the trafficking of the visual G protein transducin without any known connection to CCT (21)(22)(23). Subgroup II consists of PhLP2A and PhLP2B, which were both discovered in humans (13).…”