Transducin in Rod Photoreceptors: Translocated When Not Terminated

The Journal of Neuroscience, June 13, 2007 • 27(24):6349–6351 • 6349

Journal Club

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Deepak Kalra,1 Rebecca Elsaesser,2 Yi Gu,3 and Kartik Venkatachalam2 1Graduate Program in Biochemistry, Cellular, and Molecular Biology, 2Department of Biological Chemistry, and 3Graduate Program in Biological Chemistry, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205 Review of Lobanova et al. (http://www.jneurosci.org/cgi/content/full/27/5/1151)

Phototransduction in the vertebrate ret- However, it is not clear whether increased R9AP (thereby increasing activity of the ina occurs in the outer segment (OS) of levels of Gt␣-GTP are sufficient to trigger entire GAC) also increased the light photoreceptor cells. Translocation of sig- transducin translocation or whether threshold for transducin translocation naling proteins between the OS and the downstream signaling events also play a [Lobanova et al. (2007), their Fig. 4 inner segment (IS) (Sokolov et al., 2002; role. To distinguish between these possi- (http://www.jneurosci.org/cgi/content/ Lee et al., 2003) allows these cells to adapt bilities, Lobanova et al. (2007) compared full/27/5/1151/F4)]. Because these pertur- to a wide range of light intensities. This light thresholds for transducin transloca- bations increased Gt␣-GTP, a mouse adaptation is evolutionarily conserved in tion in two mice models. RGS9-anchor model with defective signaling down- vertebrates and invertebrates. Lobanova protein (R9AP) knock-out mice lack the stream of transducin but with normal et al. (2007) in their recent study in The GAC and display physiological pheno- RGS9-mediated increase in GTPase activ- Journal of Neuroscience elucidate the types similar to RGS9 knock-outs. The ity, (e.g., mutants with catalytically inac- mechanism for light-induced transloca- W70A mice have a mutation in transdu- tive phosphodiesterase) may enable a bet- tion of the G-protein transducin from the cin that decreases the affinity between ter understanding of the role of OS to the IS, indicating that saturation of phosphodiesterase and Gt␣-GTP, causing downstream signaling events under basal the GTPase-activating complex (GAC) decreased downstream signaling. This Gt␣-GTP levels. during high light intensities triggers mutation also abolishes RGS9-mediated The transducin ␤-␥ dimer (Gt␤␥)- translocation. increase in the GTPase activity of Gt␣- binding protein, phosducin, is phosphor- Transducin translocation in rod pho- GTP. Therefore, after an increase in light ylated in the dark and dephosphorylated toreceptors occurs when light intensity intensity, both R9AP and W70A mice in light. In vitro, dephosphorylated phos- reaches a critical threshold (Sokolov et al., have increased levels of Gt␣-GTP, but ducin binds Gt␤␥ with higher affinity 2002). This threshold is reduced if the reg- only the R9AP mice have increased down- than its phosphorylated counterpart ulator of G-protein signaling 9 (RGS9), a component of the GAC, is knocked out or stream signaling. (Thulin et al., 2001). This interaction pre- Compared with wild-type mice, the vents the association between Gt␤␥ and if a GTPase activity-deficient variant of ␣ the ␣ subunit of transducin (Gt␣)isex- light threshold for transducin transloca- Gt , thereby promoting their light- pressed (Kerov et al., 2005). The resulting tion in R9AP knock-out and W70 mice mediated translocation. The authors ϳ increase in the activated form of the ␣ was reduced by 2.3-fold [Lobanova compared phosphorylation of phosducin subunit of transducin (Gt␣-GTP) in- et al. (2007), their Figs. 1 (http://www. in eye homogenates using phospho- creases downstream signaling events. jneurosci.org/cgi/content/full/27/5/1151/ specific antibodies and light intensities F1), 2 (http://www.jneurosci.org/cgi/ that allowed translocation in R9AP content/full/27/5/1151/F2), 3 (http:// knock-out and W70A mice but not in Received March 28, 2007; revised April 30, 2007; accepted May 1, 2007. We thank Drs. Craig Montell, Richard L. Huganir, and King-Wai Yau and www.jneurosci.org/cgi/content/full/27/5/ wild-type animals. Phosducin dephos- apologizeforanycitationsweleftoutbecauseofspacerestrictions.Noneof 1151/F3)]. This result suggests that the phorylation was complete in R9AP and us have any conflict of interest. threshold for transducin translocation de- wild-type mice but only partial in W70 Correspondence should be addressed to Kartik Venkatachalam at the creases if Gt␣-GTP levels increase, regard- mice [Lobanova et al. (2007), their Table 1 above address. E-mail: [email protected]. DOI:10.1523/JNEUROSCI.1399-07.2007 less of the state of signaling events down- (http://www.jneurosci.org/cgi/content/ Copyright©2007SocietyforNeuroscience 0270-6474/07/276349-03$15.00/0 stream of transducin. Overexpression of full/27/5/1151/T1)]. This strongly sug- 6350 • J. Neurosci., June 13, 2007 • 27(24):6349–6351 Kalra et al. • Journal Club gests that phosducin dephosphorylation is not sufficient for transducin translocation. A conclusive experiment would be to mutate the residues in phosducin that are phosphorylated and determine the effects on light threshold of translocation in vivo. The transducin trimer consists of two posttranslationally added lipid anchors that tether it to membranes. Gt␥ has a far- nesyl group, whereas Gt␣ is modified on its N terminus with one of four different acyl groups. The authors purified Gt␣ from rat retinas and identified all four types of lipid moieties by mass spectrom- etry [Lobanova et al. (2007), Table 2 (http://www.jneurosci.org/cgi/content/ full/27/5/1151/T2)]. Using serial tangen- tial sections of the retina, the authors an- alyzed both the distribution of transducin and the nature of the attached lipid moieties by Western blotting and mass spec- trometry. This approach allowed the authors to directly compare the relationship between the nature of the lipid moiety attached to transducin and its distribution in the OS or IS. In dark-adapted animals, all four acylated isoforms of Gt␣ were distributed evenly in the OS. However, in rats exposed to light, transducin was re- distributed along the entire length of the photoreceptor cell, including the IS [Lo- banova et al. (2007), their Fig. 5 (http:// www.jneurosci.org/cgi/content/full/27/5/ 1151/F5)]. Interestingly, the different acylated isoforms of Gt␣ did not translo- cate to the same extent. The least lipophilic isoform was enriched in the IS by 1.5-fold, whereas the most lipophilic was depleted in the IS by fourfold. The results of this elegant experiment show that the lipophilicity of the acyl group attached to Gt␣ inversely affects its rate of translocation from the OS to the IS, most likely by Figure1. Light-dependenttranslocationoftransducin.Left,Indimlight,RGS9-inducedinactivationoftransducinpreventsits increasing its affinity for photoreceptor rapiddissociationfromthemembrane.Thus,mostofthetransducinremainsintheOSoftherodcells.Right,Inbrightlight,higher membranes. levels of activated transducin overwhelm the RGS9 complex, thereby allowing the accumulation of activated transducin. The The transducin trimer is attached to latterdissociatesfromthemembraneandsubsequentlytranslocatestotherodIS.Rh,Rhodopsin;h␯,light;Pi,phosphategroup. the membrane by two lipid anchors. However, after dissociating from each other in light, Gt␣ and Gt␤␥ are each at- old, GAC immediately inactivates Gt␣- light intensities but to prevent constant tached to the membrane by single lipid GTP, thereby preventing its membrane stimulation of rods when they are not anchors. These individual subunits have dissociation. However, at light intensities contributing to vision. Persistent stimula- lower membrane affinity than the trimer above threshold, the capacity of the GAC tion of all sensory neurons causes cell (Seitz et al., 1999), thereby predicting a to inactivate Gt␣-GTP is exceeded, so that death. Indeed, excessive stimulation of the uniform increase in translocation with in- transducin stays activated and dissociates phototransduction cascade leads to reti- creasing light intensity rather than a criti- from the membrane. nal degeneration in vertebrate and inver- cal light threshold. However, Lobanova et Despite this explanation, the function tebrate models (Yoon et al., 2000; Lee and al. (2007) proposed a model [their Fig. 6 of transducin translocation remains enig- Montell, 2004; Fain, 2006). Therefore, (http://www.jneurosci.org/cgi/content/ matic, because the light threshold of transducin translocation may have a valu- full/27/5/1151/F6)] (Fig. 1) whereby the translocation is in the range at which rods able role in neuroprotection. Elucidation threshold of light activation correlates di- saturate and cones take over vision. One of the mechanisms involved in its translo- rectly with the capacity of the GAC to ac- intriguing explanation for this could be cation may assist in the development of tivate the intrinsic GTPase activity of Gt␣. that the primary function of translocation therapeutic strategies for human retinal Briefly, at light intensities below thresh- is not to enable phototransduction at high degenerations. Kalra et al. • Journal Club J. Neurosci., June 13, 2007 • 27(24):6349–6351 • 6351

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