TIBS 17 - FEBRUARY 1 9 9 2
An ion channela e u a u e n y lyl
The desire to generalize and to categorize is as immanent to the sciences as it is to mankind, but one that is often defeated by the reality of nature. Take the concept of receptors. Originally j i defined as a specific binding site on the surface of a signal-receiving cell 1, at least three types of receptor systems are known: (1) catalytical receptors which stimulation of cAMP synthesis correhave an intrinsic catalytic activity, often lates with the hyperpolarization of the tyrosine kinase activity [e.g. the recep- resting potential and therefore can also tots for insulin, epidermal growth be induced by other ions, e.g. Na + and factor (EGF) and platelet-derived Ca2+, that hyperpolarize the membrane growth factor 0°DGF)]; (2) ligand-gated potential and affect the K+ conductance, ion channels which carry an ion chan- Further evidence in favour of K+channel within their molecular moiety [e.g. nel control of cAMP production is prothe nicotinic acetylcholine receptor and vided by the dose-dependent blockade the receptors for glutamate, glycine and of the enzyme's activity by typical K+ ~,-aminobutyric acid (GABA)]; and (3) channel blockers such as tetraethylG protein-coupled receptors which ammomium (TEA), Cs + and quinine 2. have no inbuilt signalling capabilities The Paramecium mutant restless is but which are associated with a G oversensitive to low extracellular K+ protein and a (adenylyl or guanynyi) concentrations in that it cannot curb its cyclase (e.g. the muscarinic acetyl- membrane hyperpolarization in recholine receptor and the ~adrenergic sponse to K+ dilution 5. The increase in receptor). And what about rhodopsin cAMP production in response to K+ and voltage-gated ion channels? In line dilution is several fold higher in restless with the receptor hypothesis, they are than in wild-type paramecia 2. In the receiving units within the plasma mere- extragenic suppressor mutant manxA, brane for extracellular signals (light and the oversensitivity to low extracellular electric field, respectively) that are K+ is corrected without affecting other coupled by established principles (G ion currents 6. The double mutant restprotein and ion channel, respectively) less-manxA does not show over-producto the cell's intracellular response. To tion of cAMP in response to dilution of further complicate this matter, we now extracellular K+ (Ref. 2), reinforcing the have a K+ channel-controlled adenylyl link between K+ conductance and adencyclase which may be considered a new ylyl cyclase activity in Paramecium. subtype of catalytic receptor 2. Co-localization within the s a m e Already several years ago, the group polypeptide of the cyclase activity and of Joachim E. Schultz from the the controlling K÷ channel was shown University of Tfibingen made the inter- by reconstitution of the purified cyclase esting discovery that there exists in in planar lipid bilayers 2. Starting from the freshwater ciliate P a r a m e c i u m , an cilia of P a r a m e c i u m (which contain adenylyl cyclase which does not appear about haft of the cellular cyclase activity), to be coupled to a classical hormone the enzyme was solubilized and purlreceptor but rather is controlled by the fled to apparent homogeneity. Analysis cell's membrane potentiaP ,4. The en- by SDS-polyacrylamide gel electrophorzyme is unresponsive to cholera toxin, esis showed one major band of 96 kDa; pertussis toxin, fluoride, GTP and non- the enzyme activity was similar to that hydrolysable GTP derivatives, which of adenylyl cyclase from bovine brain 7. are all known to interfere with G protein When reconstituted into artificial function, but its activity is triggered by planar lipid bilayers, the purified prochanges in the concentration of extra- tein showed pore-forming activity cellular ions that cause depolarization, characterized by a channel conducThus, when the K ÷ concentration of the tance of 320 pS and a preferred ion surrounding buffer is rapidly reduced selectivity for K ÷ and Cs ÷ (Ref. 2). Poreby dilution, paramecia previously forming activity and enzyme activity adapted to an environment of high K ÷ were strictly interdependent. concentration respond by a rapid The primary structure of an adenylyl increase in cAMP production 2. The cyclase from bovine brain has recently
c Tclase
© 1992, Elsevier Science Publishers, (UIC) 0376-5067/92/$05.00
~oo° det ermined 8. Secondary structure calculations suggest 12 membranespanning mhelicaf domains and further topographical similarities with ion channels. The protozoan enzyme may be an evolutionary ancestor of the metazoan enzyme. What may be the biological function of the K+ conductance-controlled adenylyl cyclase of Paramecium, and is it more than just a caprice of nature? As a fresh water ciliate, Paramecium must actively regulate its K+ conductance to maintain the membrane potential within narrow limits within an ever-changing ionic environment 9,t°. The ion channel of the cyclase thus appears to function as the cell's 'ampere-meter', constantly monitoring the K+ resting current across the membrane. Another K+ conductance-coupled cAMP producing systern has recently been described in Drosophila u, but its molecular structure has not yet been resolved. Membrane current-regulated intracellular enzyme activity seems a very practical signal pathway not only for single cell organisms but also for cells participating in organized structures with changing ionic environments, such as in nervous tissue. We therefore will probably have to get used to this new facet of signal transduction systems.
I~elerences 1 Dale,H. H. (1914)J. Pharmacol.Exp. Ther.6, 147-161
2 Schultz, J. E. et al. Science (in press) 3 Schultz, J. E. et al. (1984) FEBS Lett. 167,
113-116 4 Klumpp,S., Gierlich,D. and Schultz,J. E. (1984) FEBS Lett. 171, 95-99 5 Richard,E. A., Hinrichsen,R. D. and Kung,C. (1985) J. Neurogenet. 2,239-252 6 Yellen,G. (1987) Annu. Rev. Biophys. Biochem. 16, 227-246 7 Smigel, M. D. (1986) J. Biol. Chem. 261, 1976-1982 8 Krupinski,J. et al. (1989) Science 244, 1558-1564 9 Naitoh,Y. (1982) in ElectricConduction and Behaviour in Simple Invertebrates (G. A. B. Shelton,ed.), pp. 1-48, Clarendon 10 Oka,T., Nakaoka,Y. and Oosawa,F. (1986) J. Exp. Biol. 126, 111-117 11 Delgada, R. et al. (1991) Proc. Natl Acad. Sci. USA 88, 557-560
ALFRED M,~EMCI(IE
Institute of Physiological Chemistry, University of Mainz, Duesbergweg6, D6500 Mainz, FRG. 51
An ion channela e u a u e n y lyl
The desire to generalize and to categorize is as immanent to the sciences as it is to mankind, but one that is often defeated by the reality of nature. Take the concept of receptors. Originally j i defined as a specific binding site on the surface of a signal-receiving cell 1, at least three types of receptor systems are known: (1) catalytical receptors which stimulation of cAMP synthesis correhave an intrinsic catalytic activity, often lates with the hyperpolarization of the tyrosine kinase activity [e.g. the recep- resting potential and therefore can also tots for insulin, epidermal growth be induced by other ions, e.g. Na + and factor (EGF) and platelet-derived Ca2+, that hyperpolarize the membrane growth factor 0°DGF)]; (2) ligand-gated potential and affect the K+ conductance, ion channels which carry an ion chan- Further evidence in favour of K+channel within their molecular moiety [e.g. nel control of cAMP production is prothe nicotinic acetylcholine receptor and vided by the dose-dependent blockade the receptors for glutamate, glycine and of the enzyme's activity by typical K+ ~,-aminobutyric acid (GABA)]; and (3) channel blockers such as tetraethylG protein-coupled receptors which ammomium (TEA), Cs + and quinine 2. have no inbuilt signalling capabilities The Paramecium mutant restless is but which are associated with a G oversensitive to low extracellular K+ protein and a (adenylyl or guanynyi) concentrations in that it cannot curb its cyclase (e.g. the muscarinic acetyl- membrane hyperpolarization in recholine receptor and the ~adrenergic sponse to K+ dilution 5. The increase in receptor). And what about rhodopsin cAMP production in response to K+ and voltage-gated ion channels? In line dilution is several fold higher in restless with the receptor hypothesis, they are than in wild-type paramecia 2. In the receiving units within the plasma mere- extragenic suppressor mutant manxA, brane for extracellular signals (light and the oversensitivity to low extracellular electric field, respectively) that are K+ is corrected without affecting other coupled by established principles (G ion currents 6. The double mutant restprotein and ion channel, respectively) less-manxA does not show over-producto the cell's intracellular response. To tion of cAMP in response to dilution of further complicate this matter, we now extracellular K+ (Ref. 2), reinforcing the have a K+ channel-controlled adenylyl link between K+ conductance and adencyclase which may be considered a new ylyl cyclase activity in Paramecium. subtype of catalytic receptor 2. Co-localization within the s a m e Already several years ago, the group polypeptide of the cyclase activity and of Joachim E. Schultz from the the controlling K÷ channel was shown University of Tfibingen made the inter- by reconstitution of the purified cyclase esting discovery that there exists in in planar lipid bilayers 2. Starting from the freshwater ciliate P a r a m e c i u m , an cilia of P a r a m e c i u m (which contain adenylyl cyclase which does not appear about haft of the cellular cyclase activity), to be coupled to a classical hormone the enzyme was solubilized and purlreceptor but rather is controlled by the fled to apparent homogeneity. Analysis cell's membrane potentiaP ,4. The en- by SDS-polyacrylamide gel electrophorzyme is unresponsive to cholera toxin, esis showed one major band of 96 kDa; pertussis toxin, fluoride, GTP and non- the enzyme activity was similar to that hydrolysable GTP derivatives, which of adenylyl cyclase from bovine brain 7. are all known to interfere with G protein When reconstituted into artificial function, but its activity is triggered by planar lipid bilayers, the purified prochanges in the concentration of extra- tein showed pore-forming activity cellular ions that cause depolarization, characterized by a channel conducThus, when the K ÷ concentration of the tance of 320 pS and a preferred ion surrounding buffer is rapidly reduced selectivity for K ÷ and Cs ÷ (Ref. 2). Poreby dilution, paramecia previously forming activity and enzyme activity adapted to an environment of high K ÷ were strictly interdependent. concentration respond by a rapid The primary structure of an adenylyl increase in cAMP production 2. The cyclase from bovine brain has recently
c Tclase
© 1992, Elsevier Science Publishers, (UIC) 0376-5067/92/$05.00
~oo° det ermined 8. Secondary structure calculations suggest 12 membranespanning mhelicaf domains and further topographical similarities with ion channels. The protozoan enzyme may be an evolutionary ancestor of the metazoan enzyme. What may be the biological function of the K+ conductance-controlled adenylyl cyclase of Paramecium, and is it more than just a caprice of nature? As a fresh water ciliate, Paramecium must actively regulate its K+ conductance to maintain the membrane potential within narrow limits within an ever-changing ionic environment 9,t°. The ion channel of the cyclase thus appears to function as the cell's 'ampere-meter', constantly monitoring the K+ resting current across the membrane. Another K+ conductance-coupled cAMP producing systern has recently been described in Drosophila u, but its molecular structure has not yet been resolved. Membrane current-regulated intracellular enzyme activity seems a very practical signal pathway not only for single cell organisms but also for cells participating in organized structures with changing ionic environments, such as in nervous tissue. We therefore will probably have to get used to this new facet of signal transduction systems.
I~elerences 1 Dale,H. H. (1914)J. Pharmacol.Exp. Ther.6, 147-161
2 Schultz, J. E. et al. Science (in press) 3 Schultz, J. E. et al. (1984) FEBS Lett. 167,
113-116 4 Klumpp,S., Gierlich,D. and Schultz,J. E. (1984) FEBS Lett. 171, 95-99 5 Richard,E. A., Hinrichsen,R. D. and Kung,C. (1985) J. Neurogenet. 2,239-252 6 Yellen,G. (1987) Annu. Rev. Biophys. Biochem. 16, 227-246 7 Smigel, M. D. (1986) J. Biol. Chem. 261, 1976-1982 8 Krupinski,J. et al. (1989) Science 244, 1558-1564 9 Naitoh,Y. (1982) in ElectricConduction and Behaviour in Simple Invertebrates (G. A. B. Shelton,ed.), pp. 1-48, Clarendon 10 Oka,T., Nakaoka,Y. and Oosawa,F. (1986) J. Exp. Biol. 126, 111-117 11 Delgada, R. et al. (1991) Proc. Natl Acad. Sci. USA 88, 557-560
ALFRED M,~EMCI(IE
Institute of Physiological Chemistry, University of Mainz, Duesbergweg6, D6500 Mainz, FRG. 51
