Sensing calcium in rod cells

NEWS AND VIEWS VISUAL T R A N S D U C T I O N - - - - - - - - - - - - - - - - cal work 11 , however, shows that the handicap principle works even when this assumption is relaxed (A. Grafen, University of Oxford). Receivers may also make use of several signals simultaneously in an attempt to 'average out' the error in their perceptions of each. How pure are 'honest' signalling systems? Costly signalling systems could, in some circumstances, be vulnerable to cheats who form the signal but do not pay its costs (M. Dawkins, University of Oxford). Insects with warning coloration, such as ladybirds, and even some birds 12 , sequester toxins and so may be signalling honestly to potential predators that they are unpalatable (but see ref. 13). However, 'automimic' ladybirds, which produce the warning colour but do not pay the price of sequestering the toxins, could exist at low levels. But cheats do owe their existence to honest systems. The term 'conventional signals' has been applied to those signals for which the costs of the signal are not part of the message 14 . Much debate has focused on whether conventional signals can be used to persuade ( see figure). Birds may use badges of status (D. Harper, University of Sussex), such as a spot on their breasts, to signal their position in the status hierarchy. A smaller, less costly, badge may be used to signal higher status. But even though the meaning of the signal (status) is not related to its cost, the actions required to enforce status will be, and so badges of status may in this sense prove to be 'honest'. Perhaps conventional signals commonly exist in cases where there is no conflict of interest between the two parties? Not necessarily: although a controversial point, genetically identical (save for mutations) somatic cells in the same body may ensure the reliability of signals passed between them by making those signals costly (Zahavi). D Mark Pagel is at the School of Mathematical Sciences, Queen Mary and Westfield College, University of London, London £1 4NS. 1. Ryan, M. J .• Fox, J. H .. Wilczynski, W. & Rand, A. S. Nature 343, 66-67 (1990). 2. Basolo, A. L. Science 250, 808-810 (1990). 3. Searcy, W. A. Am. Zoo/. 32, 71-80 (1992). 4. Kirkpatrick, M. & Ryan, M. J. Nature 350, 33-38 (1991). 5. Maynard Smith, J. Evolution and the Theory of Games (Cambridge University Press, 1982). 6. Zahavi, A. J. theor. Biol. 53, 205--214 (1975). 7. Fitzgibbon, C. D. & Fanshaw, J. H. Behav. Ecol. Sociobio/. 23, 69-74 (1988). 8. Pomiankowski. A. Proc. R. Soc. B127 123 -145 (1987). 9. Grafen, A. J. theor. Biol. 144, 473-516 (1990). 10. Grafen, A. J. theor. Biol. 144, 517-546 (1990). 11. Johnstone, R. A. & Grafen, A. Proc. R. Soc. 8248, 229-233 (1992). 12. Dumbacher, J. P. et al. Science 258, 799-801 (1992). 13. Guilford, T. & Dawkins, M. S. Evolution (in the press). 14. Maynard Smith, J. & Harper, D. Phil. Trans. R. Soc. 8319, 557-570 (1988). Sensing calcium in rod cells James 8. Hurley THE discovery of cyclic GMP-gated proteins in signal transduction are only cation channels in the rod and cone beginninfc to be understood. Initial photoreceptors 1 of vertebrate retinae reports 9 • 0 suggested that one of these unveiled an important and elegantly proteins, bovine recoverin, is a Ca 2 +simple type of sensory transduction mech- sensitive activator of guanylate cyclase anism. Light-stimulated hydrolysis of and that another, frog S-modulin, regintracellular cGMP is linked to inhibi- ulates the light sensitivity of cGMP phostion of Na+ and Ca 2 + influx by direct phodiesterase. But a report to appear in interaction of cGMP with plasma mem- a forthcoming issue of Neuron raises brane cation channels. Recent reports 2- 4 important questions about the relademonstrate that Ca2 + regulates the tionship between recoverin and f'uanyaffinities of these channels for cyclic late cyclase. Gray-Keller et al. 1 have nucleotides both in photoreceptors and found biochemical and electrophysioloin olfactory sensory neurons. Writing on gical evidence that recoverin and related page 76 of this issue 4 , Hsu and Molday proteins do not promote recovery, but report that the affinity of photoreceptor instead prolong the photoresponse. Hsu and Molday 4 show that Ca 2 + in channels for cGMP is regulated by the ubiquitous Ca2 +-binding protein photoreceptors has an additional mode of action that is mediated by calmodulin. calmodulin. Electrophysiological studies of how By measuring the cation permeability of light affects ion currents flowing across vesicles derived from photoreceptor photoreceptor plasma membranes have membranes, they found that Ca2 +defined important characteristics of the calmodulin interacts with the rod photophotoresponse. This has challenged receptor cGMP-gated channel to raise its biochemists to identify molecules and K 112 for cGMP from 19 µM to 33 µM reactions that can explain those charac- cGMP. As the cGMP-dependence of the teristics. Quite a few pieces of the puzzle channel is cooperative, this produced as of phototransduction have been identi- much as a sixfold effect of calmodulin on fied, but few of them have been placed channel activity at low cGMP concentradefinitively into a picture that fully ex- tions. The calmodulin effect occurred plains photoexcitation, recovery and between 50 and 30 nM Ca 2 +, which is adaptation. Hsu and Molday have identi- probably within the range of free Ca2 + fied an important new piece to be fitted concentrations in a photoreceptor 6 . Only calmodulin produced this effect; it was into this puzzle. Here is a popular version of how not mimicked by several other Ca2 +phototransduction works, based on what binding proteins, including recoverin. These findings prompted Hsu and we have found out so far. Two well known second messengers, cGMP and Molday to identify photoreceptor proCa 2 +, have intricately intertwined roles teins that interact with calmodulin. Not in generating the photoresponse 5 . Light surprisingly, they found that the primary stimulates hydrolysis of cGMP through calmodulin target in photoreceptor the actions of rhodopsin, transducin and membranes is the cGMP-gated channel a cGMP phosphodiesterase. In the pre- complex. Calmodulin affinity chromasence of sufficient intracellular cGMP, as tography purified two polypeptides of there would be in darkness, cGMP-~ated relative molecular mass 63K and 240K cation channels allow Na+ and Ca + to which had been previously shown to enter the cell. Light-stimulated cGMP constitute the channel complex. Blots of hydrolysis reduces this channel activity. purified channel preparations probed Consequently, the intracellular concen- with radioactive calmodulin revealed tration of Ca 2 + falls as it is pumped out that the 240K polypeptide is the compoof the cell by a Na+ /Ca2 +-K+ exchanger6 . nent that binds calmodulin. This Ca 2 + is an important regulator of cer- polypeptide may be related to spectrin tain vertebrate phototransduction en- and fodrin, which h (...truncated)