Calcium/Calmodulin Modulation of Olfactory and Rod Cyclic Nucleotide-gated Ion Channels
2003; Elsevier BV; Volume: 278; Issue: 21 Linguagem: Inglês
10.1074/jbc.r300001200
ISSN1083-351X
AutoresMatthew C. Trudeau, William N. Zagotta,
Tópico(s)Insect Pheromone Research and Control
ResumoCyclic nucleotide-gated (CNG) ion channels mediate sensory transduction in olfactory sensory neurons and retinal photoreceptor cells. In these systems, internal calcium/calmodulin (Ca2+/CaM) inhibits CNG channels, thereby having a putative role in sensory adaptation. Functional differences in Ca2+/CaM-dependent inhibition depend on the different subunit composition of olfactory and rod CNG channels. Recent evidence shows that three subunit types (CNGA2, CNGA4, and CNGB1b) make up native olfactory CNG channels and account for the fast inhibition of native channels by Ca2+/CaM. In contrast, two subunit types (CNGA1 and CNGB1) appear sufficient to mirror the native properties of rod CNG channels, including the inhibition by Ca2+/CaM. Within CNG channel tetramers, specific subunit interactions also mediate Ca2+/CaM-dependent inhibition. In olfactory CNGA2 channels, Ca2+/CaM binds to an N-terminal region and disrupts an interaction between the N- and C-terminal regions, causing inhibition. Ca2+/CaM also binds the N-terminal region of CNGB1 subunits and disrupts an intersubunit, N- and C-terminal interaction between CNGB1 and CNGA1 subunits in rod channels. However, the precise N- and C-terminal regions that form these interactions in olfactory channels are different from those in rod channels. Here, we will review recent advances in understanding the subunit composition and the mechanisms and roles for Ca2+/CaM-dependent inhibition in olfactory and rod CNG channels. Cyclic nucleotide-gated (CNG) ion channels mediate sensory transduction in olfactory sensory neurons and retinal photoreceptor cells. In these systems, internal calcium/calmodulin (Ca2+/CaM) inhibits CNG channels, thereby having a putative role in sensory adaptation. Functional differences in Ca2+/CaM-dependent inhibition depend on the different subunit composition of olfactory and rod CNG channels. Recent evidence shows that three subunit types (CNGA2, CNGA4, and CNGB1b) make up native olfactory CNG channels and account for the fast inhibition of native channels by Ca2+/CaM. In contrast, two subunit types (CNGA1 and CNGB1) appear sufficient to mirror the native properties of rod CNG channels, including the inhibition by Ca2+/CaM. Within CNG channel tetramers, specific subunit interactions also mediate Ca2+/CaM-dependent inhibition. In olfactory CNGA2 channels, Ca2+/CaM binds to an N-terminal region and disrupts an interaction between the N- and C-terminal regions, causing inhibition. Ca2+/CaM also binds the N-terminal region of CNGB1 subunits and disrupts an intersubunit, N- and C-terminal interaction between CNGB1 and CNGA1 subunits in rod channels. However, the precise N- and C-terminal regions that form these interactions in olfactory channels are different from those in rod channels. Here, we will review recent advances in understanding the subunit composition and the mechanisms and roles for Ca2+/CaM-dependent inhibition in olfactory and rod CNG channels. Cyclic nucleotide-gated (CNG) 1The abbreviations used are: CNG, cyclic nucleotide-gated; CNBD, cyclic nucleotide-binding domain; CaM, calmodulin.1The abbreviations used are: CNG, cyclic nucleotide-gated; CNBD, cyclic nucleotide-binding domain; CaM, calmodulin. ion channels mediate the final step of the enzymatic cascade in sensory cells of the olfactory and visual systems (1Yau K.W. Baylor D.A. Annu. Rev. Neurosci. 1989; 12: 289-327Crossref PubMed Scopus (434) Google Scholar, 2Zufall F. Munger S.D. Trends Neurosci. 2001; 24: 191-193Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar, 3Kaupp U.B. Seifert R. Physiol. Rev. 2002; 82: 769-824Crossref PubMed Scopus (917) Google Scholar). CNG channels gate (open and close) in response to the direct binding of cyclic nucleotides to a cytoplasmic C-terminal region of the channel known as the cyclic nucleotide-binding domain (CNBD) (4Zagotta W.N. Siegelbaum S.A. Annu. Rev. Neurosci. 1996; 19: 235-263Crossref PubMed Scopus (417) Google Scholar). Upon binding, cyclic nucleotides cause a rearrangement in the CNBD and promote the opening of a channel pore via an allosteric mechanism (5Richards M.J. Gordon S.E. Biochemistry. 2000; 39: 14003-14011Crossref PubMed Scopus (41) Google Scholar, 6Flynn G.E. Johnson Jr., J.P. Zagotta W.N. Nat. Rev. Neurosci. 2001; 2: 643-651Crossref PubMed Scopus (65) Google Scholar). In both olfactory sensory neurons and retinal photoreceptor cells, activated CNG channels conduct an inward cation current that is carried primarily by sodium (Na+) and calcium (Ca2+) ions, resulting in membrane depolarization and elevated Ca2+ levels. In olfactory sensory neurons, odorants activate G-protein-coupled receptors starting a signaling cascade that increases the cytosolic concentration of cAMP, thus opening previously closed CNG channels and causing membrane depolarization (Fig. 1). In rod photoreceptor cells, light activates rhodopsin, which begins a cascade that decreases the cytosolic concentration of cGMP, resulting in closure of previously open CNG channels and membrane hyperpolarization (Fig. 1). Thus, odorants in the olfactory system and light in the visual system produce opposite effects on membrane voltage. For both systems, Ca2+ ions feed back to down-regulate the enzymatic cascade (Fig. 1) (2Zufall F. Munger S.D. Trends Neurosci. 2001; 24: 191-193Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar, 7Menini A. Curr. Opin. Neurobiol. 1999; 9: 419-426Crossref PubMed Scopus (105) Google Scholar, 8Fain G.L. Matthews H.R. Cornwall M.C. Koutalos Y. Physiol. Rev. 2001; 81: 117-151Crossref PubMed Scopus (435) Google Scholar). In the olfactory system Ca2+ ions bind to calmodulin (CaM), and the Ca2+/CaM complex directly inhibits olfactory CNG channels (Fig. 1) (9Chen T.Y. Yau K.W. Nature. 1994; 368: 545-548Crossref PubMed Scopus (251) Google Scholar), constituting a major mechanism underlying olfactory adaptation (7Menini A. Curr. Opin. Neurobiol. 1999; 9: 419-426Crossref PubMed Scopus (105) Google Scholar, 10Kurahashi T. Menini A. Nature. 1997; 385: 725-729Crossref PubMed Scopus (273) Google Scholar, 11Munger S.D. Lane A.P. Zhong H. Leinders-Zufall T. Yau K.W. Zufall F. Reed R.R. Science. 2001; 294: 2172-2175Crossref PubMed Scopus (108) Google Scholar). In the visual system, Ca2+ ions interact with several members of the phototransduction cascade to cause negative feedback and visual adaptation (8Fain G.L. Matthews H.R. Cornwall M.C. Koutalos Y. Physiol. Rev. 2001; 81: 117-151Crossref PubMed Scopus (435) Google Scholar, 12Pugh Jr., E.N. Nikonov S. Lamb T.D. Curr. Opin. Neurobiol. 1999; 9: 410-418Crossref PubMed Scopus (268) Google Scholar). Ca2+/CaM inhibits native rod CNG channels (13Hsu Y.T. Molday R.S. Nature. 1993; 361: 76-79Crossref PubMed Scopus (307) Google Scholar, 14Gordon S.E. Downing-Park J. Zimmerman A.L. J. Physiol. (Lond.). 1995; 486: 533-546Crossref Scopus (96) Google Scholar); however, the role for Ca2+/CaM in visual adaptation is apparently not as large as in olfactory adaptation (15Koutalos Y. Yau K.W. Trends Neurosci. 1996; 19: 73-81Abstract Full Text PDF PubMed Scopus (211) Google Scholar). Ca2+/CaM also inhibits CNG channels in retinal cones (16Haynes L.W. Yau K.W. J. Physiol. (Lond.). 1990; 429: 451-481Crossref Scopus (59) Google Scholar, 17Hackos D.H. Korenbrot J.I. J. Gen. Physiol. 1997; 110: 515-528Crossref PubMed Scopus (49) Google Scholar); however, this topic is outside of our present scope. We will focus on recent, intriguing similarities and differences in the molecular mechanisms underlying Ca2+/CaM-dependent inhibition in olfactory and rod CNG channels.Native olfactory and rod CNG channels are inhibited by nanomolar levels of CaM in a Ca2+- and time-dependent manner (9Chen T.Y. Yau K.W. Nature. 1994; 368: 545-548Crossref PubMed Scopus (251) Google Scholar, 13Hsu Y.T. Molday R.S. Nature. 1993; 361: 76-79Crossref PubMed Scopus (307) Google Scholar, 14Gordon S.E. Downing-Park J. Zimmerman A.L. J. Physiol. (Lond.). 1995; 486: 533-546Crossref Scopus (96) Google Scholar). For both channels Ca2+/CaM decreases the apparent affinity for cyclic nucleotide, i.e. more cyclic nucleotide is required to open the same number of Ca2+/CaM-bound channels than for unbound channels. One way for this to occur is if Ca2+/CaM destabilizes the opening allosteric transition, as proposed for olfactory channels (9Chen T.Y. Yau K.W. Nature. 1994; 368: 545-548Crossref PubMed Scopus (251) Google Scholar). Quantitatively, however, inhibition is different between the channel types. Ca2+/CaM decreases the apparent affinity of olfactory channels for cyclic nucleotide about 10-fold (9Chen T.Y. Yau K.W. Nature. 1994; 368: 545-548Crossref PubMed Scopus (251) Google Scholar), whereas that for rod channels decreases about 2-fold (13Hsu Y.T. Molday R.S. Nature. 1993; 361: 76-79Crossref PubMed Scopus (307) Google Scholar, 14Gordon S.E. Downing-Park J. Zimmerman A.L. J. Physiol. (Lond.). 1995; 486: 533-546Crossref Scopus (96) Google Scholar). A mechanistic explanation for this difference will be discussed below.Physiological Role for Ca2+/CaM Modulation of CNG ChannelsOlfactory adaptation, a decrease in the electrical response of the cell to repeated application of odorants, depends on the concentration of internal Ca2+ ions. The time courses of adaptation to either pulses of odorant or to photolysis of caged cAMP are the same, suggesting that adaptation works though olfactory CNG channels (10Kurahashi T. Menini A. Nature. 1997; 385: 725-729Crossref PubMed Scopus (273) Google Scholar). Moreover, adapted channels and Ca2+/CaM-inhibited channels have a similar apparent affinity for cAMP, suggesting that odorant adaptation is due to Ca2+/CaM-dependent inhibition of CNG channels (Fig. 1). Long term olfactory adaptation may work though a different pathway (19Zufall F. Leinders-Zufall T. Chem. Senses. 2000; 25: 473-481Crossref PubMed Scopus (217) Google Scholar, 20Takeuchi H. Kurahashi T. J. Physiol. (Lond.). 2002; 541: 825-833Crossref Scopus (43) Google Scholar).Unlike the case for olfactory neurons, the physiological role for Ca2+/CaM modulation of CNG channels in rod photoreceptors is not well established. In theory, the interaction of Ca2+/CaM with CNG channels is sufficient to form a negative feedback loop in native rods (13Hsu Y.T. Molday R.S. Nature. 1993; 361: 76-79Crossref PubMed Scopus (307) Google Scholar). In the dark, in high levels of Ca2+ and cGMP, Ca2+/CaM-bound channels would be inhibited, thereby having a lower apparent affinity for cGMP. In the light, Ca2+ and cGMP levels drop, Ca2+/CaM would not be bound, and channels would exhibit a higher apparent affinity for cGMP. Through Ca2+/CaM, rod channels would be perfectly tuned to respond to changes in the cGMP concentration in different levels of light, thereby extending the range of the photoresponse and aiding in visual adaptation. However, the significance of such a mechanism in native cells has been questioned because Ca2+/CaM alters the apparent affinity of rod channels by about 2-fold, which is a relatively small change compared with the 10,000-fold range in intensity over which visual adaptation occurs (15Koutalos Y. Yau K.W. Trends Neurosci. 1996; 19: 73-81Abstract Full Text PDF PubMed Scopus (211) Google Scholar, 21Koutalos Y. Nakatani K. Yau K.W. J. Gen. Physiol. 1995; 106: 891-921Crossref PubMed Scopus (86) Google Scholar, 22Nakatani K. Koutalos Y. Yau K.W. J. Physiol. (Lond.). 1995; 484: 69-76Crossref Scopus (81) Google Scholar). Also, in a computed model of the response-intensity relation in rods the contribution of direct Ca2+/CaM inhibition of CNG channels in light adaptation was minimal (15Koutalos Y. Yau K.W. Trends Neurosci. 1996; 19: 73-81Abstract Full Text PDF PubMed Scopus (211) Google Scholar, 21Koutalos Y. Nakatani K. Yau K.W. J. Gen. Physiol. 1995; 106: 891-921Crossref PubMed Scopus (86) Google Scholar). One major target for Ca2+-dependent adaptation in rods is guanylate cyclase-activating protein. Its down-regulation by Ca2+ reduces the activity of guanylate cyclase, which reduces the rate of formation of cGMP and in turn closes CNG channels (8Fain G.L. Matthews H.R. Cornwall M.C. Koutalos Y. Physiol. Rev. 2001; 81: 117-151Crossref PubMed Scopus (435) Google Scholar, 12Pugh Jr., E.N. Nikonov S. Lamb T.D. Curr. Opin. Neurobiol. 1999; 9: 410-418Crossref PubMed Scopus (268) Google Scholar, 15Koutalos Y. Yau K.W. Trends Neurosci. 1996; 19: 73-81Abstract Full Text PDF PubMed Scopus (211) Google Scholar, 23Burns M.E. Mendez A. Chen J. Baylor D.A. Neuron. 2002; 36: 81-91Abstract Full Text Full Text PDF PubMed Scopus (183) Google Scholar). A second major target for Ca2+ ion is the Ca2+-binding protein recoverin; its activation inhibits the rhodopsin kinase, which keeps light-activated rhodopsin and ultimately phosphodiesterase active, and that decreases the cGMP concentration and leads to CNG channel closure (15Koutalos Y. Yau K.W. Trends Neurosci. 1996; 19: 73-81Abstract Full Text PDF PubMed Scopus (211) Google Scholar, 24Baylor D.A. Burns M.E. Eye. 1998; 12: 521-525Crossref PubMed Scopus (40) Google Scholar).Cloned CNG Channel SubunitsAdvances in understanding the molecular mechanism(s) of Ca2+/CaM inhibition have been made by studies of cloned CNG channels. Currently, six types of mammalian CNG channel subunits are divided into two classes; the CNGA class contains CNGA1, CNGA2, CNGA3, and CNGA4 subunits, and the CNGB class contains the CNGB1 and CNGB3 subunits. CNGB also contains CNGB1b, an olfactory-specific splice variant of CNGB1. There is no clone designated CNGB2 (25Bradley J. Frings S. Yau K.-W. Reed R.R. Science. 2001; 294: 2095Crossref PubMed Scopus (60) Google Scholar). Channel subunits are 35–75% similar, and all have the same proposed transmembrane arrangement with intracellular N- and C-terminal regions and six transmembrane domains (Fig. 2). Four subunits coassemble to form a tetrameric channel with a central pore region (26Varnum M.D. Zagotta W.N. Biophys. J. 1996; 70: 2667-2679Abstract Full Text PDF PubMed Scopus (69) Google Scholar, 27Liu D.T. Tibbs G.R. Siegelbaum S.A. Neuron. 1996; 16: 983-990Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar).Fig. 2Membrane topology, subunit stoichiometry, and arrangement of CNG channels.Top, left, cartoon version of the proposed membrane topology of the olfactory subunits CNGA2 (black), CNGA4 (green), and CNGB1b (red). Top, right, top-down view of a putative subunit arrangement in a tetrameric olfactory CNG channel with the nearest subunit removed. Bottom, left, transmembrane topology of rod CNGA1 (blue) and CNGB1 (red) subunits. Bottom, right, top-down view of subunit stoichiometry and arrangement in a tetrameric rod CNG channel. Several regions of CNG channel subunits are listed, including the glutamic acid-rich protein region (GARP), the CaM-binding site, the pore domain, C-linker, and the CNBD.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Subunit Composition of Olfactory and Rod CNG ChannelsFunctional inhibition of CNG channels by Ca2+/CaM depends on the subunit composition of CNG channels. CNG channels in olfactory sensory neurons are formed by three different channel subunits, CNGA2, CNGA4, and CNGB1b (Fig. 2). These three subunits are all present in olfactory neurons as determined by molecular and biochemical studies (28Sautter A. Zong X. Hofmann F. Biel M. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 4696-4701Crossref PubMed Scopus (106) Google Scholar, 29Bonigk W. Bradley J. Muller F. Sesti F. Boekhoff I. Ronnett G.V. Kaupp U.B. Frings S. J. Neurosci. 1999; 19: 5332-5347Crossref PubMed Google Scholar). All three subunits are necessary to form channels that reproduce key functional properties of native olfactory channels, including the apparent affinities for cGMP and cAMP, the single channel kinetics, the presence of substrates at the single-channel level, and the fast kinetics of Ca2+/CaM inhibition (Table I) (11Munger S.D. Lane A.P. Zhong H. Leinders-Zufall T. Yau K.W. Zufall F. Reed R.R. Science. 2001; 294: 2172-2175Crossref PubMed Scopus (108) Google Scholar, 28Sautter A. Zong X. Hofmann F. Biel M. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 4696-4701Crossref PubMed Scopus (106) Google Scholar, 29Bonigk W. Bradley J. Muller F. Sesti F. Boekhoff I. Ronnett G.V. Kaupp U.B. Frings S. J. Neurosci. 1999; 19: 5332-5347Crossref PubMed Google Scholar, 30Bradley J. Reuter D. Frings S. Science. 2001; 294: 2176-2178Crossref PubMed Scopus (92) Google Scholar). Expressed alone, CNGA2 subunits form functional homomeric CNG channels but lack some properties of native channels (Table I). CNGA2 homomers are inhibited by Ca2+/CaM but with slow, non-native kinetics (30Bradley J. Reuter D. Frings S. Science. 2001; 294: 2176-2178Crossref PubMed Scopus (92) Google Scholar, 31Dhallan R.S. Yau K.W. Schrader K.A. Reed R.R. Nature. 1990; 347: 184-187Crossref PubMed Scopus (513) Google Scholar). The CNGA4 and CNGB1b subunits do not form functional homomeric channels when expressed alone but rather form heteromeric channels (with CNGA2) and are thus considered modulatory subunits (28Sautter A. Zong X. Hofmann F. Biel M. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 4696-4701Crossref PubMed Scopus (106) Google Scholar, 29Bonigk W. Bradley J. Muller F. Sesti F. Boekhoff I. Ronnett G.V. Kaupp U.B. Frings S. J. Neurosci. 1999; 19: 5332-5347Crossref PubMed Google Scholar, 32Liman E.R. Buck L.B. Neuron. 1994; 13: 611-621Abstract Full Text PDF PubMed Scopus (213) Google Scholar).Table IProperties of olfactory and rod CNG channelsOlfactory CNG channelsK½ cGMPK½ cAMPSingle channel conductanceCa2+/CaM inhibitionCa2+/CaM kineticsRefs.μMμMpSsCNGA2255-8435Yes>4028Sautter A. Zong X. Hofmann F. Biel M. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 4696-4701Crossref PubMed Scopus (106) Google Scholar, 30Bradley J. Reuter D. Frings S. Science. 2001; 294: 2176-2178Crossref PubMed Scopus (92) Google Scholar, 31Dhallan R.S. Yau K.W. Schrader K.A. Reed R.R. Nature. 1990; 347: 184-187Crossref PubMed Scopus (513) Google ScholarCNGA2/CNGA42-410-1219Yes>4029Bonigk W. Bradley J. Muller F. Sesti F. Boekhoff I. Ronnett G.V. Kaupp U.B. Frings S. J. Neurosci. 1999; 19: 5332-5347Crossref PubMed Google Scholar, 30Bradley J. Reuter D. Frings S. Science. 2001; 294: 2176-2178Crossref PubMed Scopus (92) Google Scholar, 32Liman E.R. Buck L.B. Neuron. 1994; 13: 611-621Abstract Full Text PDF PubMed Scopus (213) Google ScholarCNGA2/CNGB1b2.218-30NDaNot determined.Yes>4028Sautter A. Zong X. Hofmann F. Biel M. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 4696-4701Crossref PubMed Scopus (106) Google Scholar, 29Bonigk W. Bradley J. Muller F. Sesti F. Boekhoff I. Ronnett G.V. Kaupp U.B. Frings S. J. Neurosci. 1999; 19: 5332-5347Crossref PubMed Google ScholarCNGA2/CNGA4/CNGB1b1.44.8-5.512Yes<128Sautter A. Zong X. Hofmann F. Biel M. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 4696-4701Crossref PubMed Scopus (106) Google Scholar, 29Bonigk W. Bradley J. Muller F. Sesti F. Boekhoff I. Ronnett G.V. Kaupp U.B. Frings S. J. Neurosci. 1999; 19: 5332-5347Crossref PubMed Google Scholar, 30Bradley J. Reuter D. Frings S. Science. 2001; 294: 2176-2178Crossref PubMed Scopus (92) Google ScholarNative1.8414.5Yes 10-fold in homomeric CNGA2 channels over the same change in Po. This allows channels with a high open probability to be inhibited by Ca2+/CaM, a property that enables negative feedback by Ca2+/CaM in a system where the opening of channels controls membrane depolarization (11Munger S.D. Lane A.P. Zhong H. Leinders-Zufall T. Yau K.W. Zufall F. Reed R.R. Science. 2001; 294: 2172-2175Crossref PubMed Scopus (108) Google Scholar, 30Bradley J. Reuter D. Frings S. Science. 2001; 294: 2176-2178Crossref PubMed Scopus (92) Google Scholar). Consistent with these findings, the behaving CNGA4-deficient mouse has impaired odor detection in the presence of an adapting odor (33Kelliher K.R. Ziesmann J. Munger S.D. Reed R.R. Zufall F. Proc. Natl. Acad. Sci. U. S. A. 2003; 103: 4299-4304Crossref Scopus (59) Google Scholar).The native retinal rod CNG channel is formed exclusively by co-assembly of CNGA1 and CNGB1 subunits into heteromeric channels (Fig. 2). Molecular and biochemical evidence shows the presence and interaction of these two proteins in rod cells (34Chen T.Y. Illing M. Molday L.L. Hsu Y.T. Yau K.W. Molday R.S. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 11757-11761Crossref PubMed Scopus (123) Google Scholar, 35Korschen H.G. Illing M. Seifert R. Sesti F. Williams A. Gotzes S. Colville C. Muller F. Dose A. Godde M. et al.Neuron. 1995; 15: 627-636Abstract Full Text PDF PubMed Scopus (207) Google Scholar, 36Bauer P.J. J. Physiol. (Lond.). 1996; 494: 675-685Crossref Scopus (52) Google Scholar). In functional expression studies, CNGA1/CNGB1 heteromers have properties similar to those of native channels, including similar apparent affinity for cGMP and cAMP, sensitivity to L-cis-diltiazem, fast single channel gating, and inhibition by Ca2+/CaM (Table I) (34Chen T.Y. Illing M. Molday L.L. Hsu Y.T. Yau K.W. Molday R.S. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 11757-11761Crossref PubMed Scopus (123) Google Scholar, 35Korschen H.G. Illing M. Seifert R. Sesti F. Williams A. Gotzes S. Colville C. Muller F. Dose A. Godde M. et al.Neuron. 1995; 15: 627-636Abstract Full Text PDF PubMed Scopus (207) Google Scholar, 36Bauer P.J. J. Physiol. (Lond.). 1996; 494: 675-685Crossref Scopus (52) Google Scholar, 37Chen T.Y. Peng Y.W. Dhallan R.S. Ahamed B. Reed R.R. Yau K.W. Nature. 1993; 362: 764-767Crossref PubMed Scopus (271) Google Scholar). Heteromeric rod channels contain three CNGA1 subunits and one CNGB1 subunit (Fig. 2) (38Weitz D. Ficek N. Kremmer E. Bauer P.J. Kaupp U.B. Neuron. 2002; 36: 881-889Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar, 39Zheng J. Trudeau M.C. Zagotta W.N. Neuron. 2002; 36: 891-896Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar, 40Zhong H. Molday L.L. Molday R.S. Yau K.W. Nature. 2002; 420: 193-198Crossref PubMed Scopus (189) Google Scholar, 41Zimmerman A.L. Neuron. 2002; 36: 997-999Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar). CNGA1 subunits form functional homomeric CNG channels in heterologous systems but lack several features of native rod channels (Table I) (37Chen T.Y. Peng Y.W. Dhallan R.S. Ahamed B. Reed R.R. Yau K.W. Nature. 1993; 362: 764-767Crossref PubMed Scopus (271) Google Scholar, 42Kaupp U.B. Niidome T. Tanabe T. Terada S. Bonigk W. Stuhmer W. Cook N.J. Kangawa K. Matsuo H. Hirose T. et al.Nature. 1989; 342: 762-766Crossref PubMed Scopus (505) Google Scholar). CNGB1 subunits are considered modulatory subunits as they do not form functional channels in heterologous expression systems but co-assemble with CNGA1 to form channels with native-like properties, including inhibition by Ca2+/CaM. Thus, although both are tetrameric, olfactory and rod channels differ in the type (CNGA2, CNGA4, and CNGB1b versus CNGA1 and CNGB1) and number (3 versus 2, respectively) of component subunits. Further, the olfactory CNGA2 subunit contains sufficient machinery for an elementary form of Ca2+/CaM modulation, whereas the rod CNGA1 subunit requires the CNGB1 subunit for Ca2+/CaM modulation (Table I). Although not reviewed here, cone CNG channels are likely formed by CNGA3 and CNGB3 subunits (43Weyand I. Godde M. Frings S. Weiner J. Muller F. Altenhofen W. Hatt H. Kaupp U.B. Nature. 1994; 368: 859-863Crossref PubMed Scopus (231) Google Scholar, 44Gerstner A. Zong X. Hofmann F. Biel M. J. Neurosci. 2000; 20: 1324-1332Crossref PubMed Google Scholar).Molecular Mechanisms of Ca2+/CaM InhibitionThe mechanisms that underlie Ca2+/CaM inhibition of olfactory and rod channels are broadly similar. For both channels, Ca2+/CaM binds to an N-terminal region of the channel and disrupts an interaction between this region and a C-terminal region, causing inhibition. Upon comparison, however, the precise molecular mechanisms of inhibition are quite different and we review those here.Ca2+/CaM-binding Sites in the N-terminal Regions of CNG Channel SubunitsOlfactory CNGA2 subunits contain a site in their N-terminal region (68FQRIVRLVGVIRDW81) that is necessary and sufficient to bind to Ca2+/CaM (9Chen T.Y. Yau K.W. Nature. 1994; 368: 545-548Crossref PubMed Scopus (251) Google Scholar). Deletion of this site results in CNGA2 channels that are insensitive to Ca2+/CaM, suggesting a critical role for this site in functional inhibition (9Chen T.Y. Yau K.W. Nature. 1994; 368: 545-548Crossref PubMed Scopus (251) Google Scholar, 45Varnum M.D. Zagotta W.N. Science. 1997; 278: 110-113Crossref PubMed Scopus (133) Google Scholar). The CaM-binding region in CNGA2 is an archetypal "1-8-14" site, characterized by hydrophobic residues at positions 1 and 14 and long chain aliphatic residues at position 8 (as underlined) (46Rhoads A.R. Friedberg F. FASEB J. 1997; 11: 331-340Crossref PubMed Scopus (734) Google Scholar).Rod CNGA1 subunits do not contain a Ca2+/CaM-binding site; however, CNGB1 subunits have an N-terminal site (682LQELVKLFKERTEKVKEKLI701) that is necessary for Ca2+/CaM binding (47Grunwald M.E. Yu W.P. Yu H.H. Yau K.W. J. Biol. Chem. 1998; 273: 9148-9157Abstract Full Text Full Text PDF PubMed Scopus (94) Google Scholar, 48Weitz D. Zoche M. Muller F. Beyermann M. Korschen H.G. Kaupp U.B. Koch K.W. EMBO J. 1998; 17: 2273-2284Crossref PubMed Scopus (93) Google Scholar, 49Trudeau M.C. Zagotta W.N. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 8424-8429Crossref PubMed Scopus (51) Google Scholar). This site is critical for functional inh
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