LETTER REPLY TO HECK ET AL.: Signal amplification at the rhodopsin-to- LETTER transducin·phosphodiesterase step in rod phototransduction K.-W. Yaua,1, W. W. S. Yueb, and D. Silvermana In PNAS (1), we estimate ∼12–14 active transducin· In point iia, Heck et al. question our analysis of the phosphodiesterase complexes (GT*·PDE*s) produced data in figure 3A, right (1), pointing out the disparity in per active rhodopsin (Rho*) in mouse rods. This is the waveform between the ensemble variance and the en- effective gain—more informative by not including the semble mean square of the flash responses. We do empty gain from GT*s failing to activate PDE. Nearly not completely disagree about possibly some small dis- all previous estimates were on the total number of GT* parity between the two waveforms, but these measure- produced per Rho* merely because no one before ments were difficult, given the small variance (10 times us could measure the individual GT*·PDE*-triggered smaller than that in WT experiments). Moreover, instead electrical event for direct comparison with the single- of stemming necessarily from variability in the underly- Rho* response. ing elementary events as Heck et al. suggest, the dis- We do not state in our paper that previous crepancy may come from variability in the timing of measurements of the rate of GT* production per Rho* activation of individual GT molecules by REY-Rho* due were all incorrect, but rather they spanned a wide to the much reduced affinity between REY-Rho* and GT. range of values with no consensus regarding which Point iib questions a constancy of the GT*·PDE*- value is valid, owing to the different experimental triggered events produced by apo-opsin (Opn*), pro- preparations, methodologies, temperature, interpre- posing instead a stochastic variability in event size/wave- tations, etc. [see SI appendix in our paper (1)]. Most form, but admits that it is not straightforward to take this importantly, unlike previous work, we obtained our into account in the analysis. Given no a priori evidence data exclusively from intact, live rods, and thus our for stochastic variations, we extracted the average event results are more straightforward to interpret. Heck size and shape by adopting the simplest model of fairly et al.’s Letter (2) selects without justification particular constant events. This possibility is not unfounded but values from a wide range to claim agreement with guided by our previous findings on olfactory trans- ours and fails to recognize the significance of our find- duction (6), where the response to Golf*·AC*(adenylyl ings in the broader history of the problem. cyclase)—functionally equivalent to GT*·PDE* in vision— Regarding specific point i in Heck et al.’s Letter, appears quite constant in amplitude and waveform. the authors again selectively discuss just one model Furthermore, the time-integrated single-GT*·PDE*– [namely, their own (3, 4)] out of several about func- triggered responses from our two independent meth- tional symmetry/asymmetry of PDE. This model is ods match each other well (figure 7B in ref. 1). neither recent (see ref. 5) nor widely accepted in In sum, relative to the “historical,” textbook-dogma the field currently because of limited experimental value of 500 (7), we do consider our value very low and evidence. Our paper provides an unbiased discus- would still consider it low even if our estimate were, say, sion of this and other models. higher by another factor of 2. aSolomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, MD 21205; and bDepartment of Physiology, University of California, San Francisco, CA 94158 Author contributions: K.-W.Y., W.Y., and D.S. designed research; W.Y. and D.S. performed research; W.Y. and D.S. analyzed data; and K.-W.Y., W.Y., and D.S. wrote the paper. The authors declare no conflict of interest. Published under the PNAS license. 1To whom correspondence should be addressed. Email: [email protected]. Published online April 30, 2019. www.pnas.org/cgi/doi/10.1073/pnas.1904339116 PNAS | April 30, 2019 | vol. 116 | no. 18 | 8655–8656 Downloaded by guest on September 26, 2021 1 Yue WWS, et al. (2019) Elementary response triggered by transducin in retinal rods. Proc Natl Acad Sci USA 116:5144–5153. 2 Heck M, Hofmann KP, Kraft TW, Lamb TD (2019) Phototransduction gain at the G-protein, transducin, and effector protein, phosphodiesterase-6, stages in retinal rods. Proc Natl Acad Sci USA 116:8653–8654. 3 Lamb TD, Heck M, Kraft TW (2018) Implications of dimeric activation of PDE6 for rod phototransduction. Open Biol 8:180076. 4 Qureshi BM, et al. (2018) It takes two transducins to activate the cGMP-phosphodiesterase 6 in retinal rods. Open Biol 8:180075. 5 Bennett N, Clerc A (1989) Activation of cGMP phosphodiesterase in retinal rods: mechanism of interaction with the GTP-binding protein (transducin). Biochemistry 28:7418–7424. 6 Bhandawat V, Reisert J, Yau K-W (2005) Elementary response of olfactory receptor neurons to odorants. Science 308:1931–1934. 7 Vuong TM, Chabre M, Stryer L (1984) Millisecond activation of transducin in the cyclic nucleotide cascade of vision. Nature 311:659–661. 8656 | www.pnas.org/cgi/doi/10.1073/pnas.1904339116 Yau et al. Downloaded by guest on September 26, 2021.