Calcium/Calmodulin Modulation of Olfactory and Rod Cyclic Nucleotide-gated Ion Channels*

Matthew C. Trudeau and William N. Zagotta {ddagger}

From the Department of Physiology and Biophysics, Howard Hughes Medical Institute, University of Washington Medical School, Seattle, Washington 98195

Received for publication, March 7, 2003
    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
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.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
Cyclic nucleotide-gated (CNG)1 ion channels mediate the final step of the enzymatic cascade in sensory cells of the olfactory and visual systems (1, 2, 3). 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) (4). Upon binding, cyclic nucleotides cause a rearrangement in the CNBD and promote the opening of a channel pore via an allosteric mechanism (5, 6). 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) (2, 7, 8). In the olfactory system Ca2+ ions bind to calmodulin (CaM), and the Ca2+/CaM complex directly inhibits olfactory CNG channels (Fig. 1) (9), constituting a major mechanism underlying olfactory adaptation (7, 10, 11). In the visual system, Ca2+ ions interact with several members of the phototransduction cascade to cause negative feedback and visual adaptation (8, 12). Ca2+/CaM inhibits native rod CNG channels (13, 14); however, the role for Ca2+/CaM in visual adaptation is apparently not as large as in olfactory adaptation (15). Ca2+/CaM also inhibits CNG channels in retinal cones (16, 17); 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.



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FIG. 1.
Signal transduction in sensory cells. Top, the enzymatic cascade in olfactory sensory neurons that underlies olfactory transduction (black arrows) operates through cAMP. Ca2+ ions mediate negative feedback (red arrows) through CaM. CNG channels are found primarily in the cilia and knob. R and R*, resting and active olfactory receptor; G, G-protein; AC, adenylyl cyclase; PDE, phosphodiesterase. Bottom, the enzymatic cascade in rod photoreceptor cells (black arrows) operates through cGMP. Negative feedback is through Ca2+ ions via a Ca2+/CaM-dependent component (red arrow) and other pathways (dotted arrows). The plasma membrane of the outer segment contains a high density of rod CNG channels. GT, transducin; GC, guanylate cyclase. This figure is derived from A. L. Zimmerman ((1995) Cyclic nucleotide-gated ion channels. Cell Physiology (Sperelakis, N., ed) pp. 495–505, Academic Press, San Diego).

 

Native olfactory and rod CNG channels are inhibited by nanomolar levels of CaM in a Ca2+- and time-dependent manner (9, 13, 14). 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 (9). Quantitatively, however, inhibition is different between the channel types. Ca2+/CaM decreases the apparent affinity of olfactory channels for cyclic nucleotide about 10-fold (9), whereas that for rod channels decreases about 2-fold (13, 14). A mechanistic explanation for this difference will be discussed below.


    Physiological Role for Ca2+/CaM Modulation of CNG Channels
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
Olfactory 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 (10). 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 (19, 20).

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 (13). 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 (15, 21, 22). 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 (15, 21). 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 (8, 12, 15, 23). 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 (15, 24).


    Cloned CNG Channel Subunits
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
Advances 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 (25). 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 (26, 27).



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FIG. 2.
Membrane 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.

 


    Subunit Composition of Olfactory and Rod CNG Channels
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
Functional 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 (28, 29). 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) (11, 28, 29, 30). 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 (30, 31). 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 (28, 29, 32).


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TABLE I
Properties of olfactory and rod CNG channels

 

The role of individual olfactory CNG channel subunits in adaptation has recently become clearer by studies with a mouse containing an engineered deletion of the CNGA4 subunit (11). Olfactory cells from these mutant mice do not adapt to repeated application of odorants, unlike cells from wild-type mice. In addition, the cells from CNGA4-deficient mice have 200-fold slower kinetics of modulation by Ca2+/CaM than do wild-type mice. In a complementary study, CNGA2/CNGA4/CNGB1b channels exhibit native-like kinetics of Ca2+/CaM inhibition, whereas CNGA2/CNGB1b channels have much slower kinetics, similar to those of the CNGA4 knock-out mouse (30). Although it does not bind Ca2+/CaM directly, the CNGA4 subunit allows for state-independent association of Ca2+/CaM with the CNGA2/CNGA4/CNGB1b channel complex. The on-rate for Ca2+/CaM does not decrease as the channel open probability (Po) changes from 0 to 1 in CNGA2/CNGA4/CNGB1b channels whereas the on-rate decreases >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 (11, 30). Consistent with these findings, the behaving CNGA4-deficient mouse has impaired odor detection in the presence of an adapting odor (33).

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 (34, 35, 36). 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) (34, 35, 36, 37). Heteromeric rod channels contain three CNGA1 subunits and one CNGB1 subunit (Fig. 2) (38, 39, 40, 41). CNGA1 subunits form functional homomeric CNG channels in heterologous systems but lack several features of native rod channels (Table I) (37, 42). 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 (43, 44).


    Molecular Mechanisms of Ca2+/CaM Inhibition
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
The 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 Subunits
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
Olfactory CNGA2 subunits contain a site in their N-terminal region (68FQRIVRLVGVIRDW81) that is necessary and sufficient to bind to Ca2+/CaM (9). Deletion of this site results in CNGA2 channels that are insensitive to Ca2+/CaM, suggesting a critical role for this site in functional inhibition (9, 45). 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) (46).

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 (47, 48, 49). This site is critical for functional inhibition as its deletion in CNGB1 subunits results in heteromeric channels (after co-expression with CNGA1) that are insensitive to Ca2+/CaM (47, 48, 49). Several key residues (underlined) are similar to those in the IQ type of CaM binding motifs (IQXXXRGXXXRXX(F/W)); however, the CNGB1 region is "unconventional" as it lacks the central glycine, and the final hydrophobic residue is not amphipathic and requires Ca2+ for CaM binding, unlike IQ motifs (46). CNGB1 also contains a C-terminal region that binds to Ca2+/CaM in biochemical assays, but the functional significance of this site is unclear, because when deleted, channels retain wild-type Ca2+/CaM dependence (47, 48).


    Differential Role in Gating of the N-terminal Region in Olfactory and Rod Channels
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
In addition to binding to Ca2+/CaM, the 1-8-14 site from the N-terminal region of CNGA2 has an autoexcitatory effect on channel gating. When this site is deleted, the apparent affinity for cyclic nucleotides and the fractional activation by cAMP in CNGA2 homomers decreases about 10-fold (9, 45). In addition, transplanting this region to CNGA1 subunits increases the apparent affinity, which can be completely explained by an autoexcitatory effect of the N-terminal region on the final opening transition (50, 51). This effect is closely linked with the Ca2+/CaM binding ability of this region; mutations that disrupt binding also disrupt autoexcitatory properties (52).

In CNGA1/CNGB1 channels, however, the Ca2+/CaM site from CNGB1 does not appear to have an autoexcitatory role in channel gating. Deletion of this site from CNGB1 subunits yields channels (after co-expression with CNGA1) that have the same apparent affinity for cGMP and fractional activation by cAMP as found in wild-type channels (49). This suggests a fundamentally different mechanism for Ca2+/CaM-dependent inhibition in olfactory and rod channels.


    N- and C-terminal Interactions in CNG Channels
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
In CNGA2 channels the N-terminal region forms an interaction with the C-terminal region (45). Specifically, the 1-8-14 site is necessary to form an interaction with the C-linker region (which connects the S6 to the CNBD) and the CNBD (Fig. 3). This interdomain interaction may promote channel opening by helping to stabilize conformations of the C-linker and CNBD that are permissive of the open conformation of the channel.



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FIG. 3.
Mechanism of Ca2+/CaM inhibition in CNG channels. Top,an interdomain interaction in CNGA2 olfactory channels (gray) involving an N-terminal 1-8-14 motif and the C-terminal C-linker and CNBD. Inhibition is due to disruption of the interaction (light gray) in the presence of Ca2+/CaM (yellow). Bottom, intersubunit interaction in rod channels involving the Ca2+/CaM-binding site in the N-terminal region from CNGB1 (red) and the distal C-terminal region of CNGA1 (blue). The interaction is disrupted (light red) in the presence of Ca2+/CaM (yellow).

 

The cytoplasmic N- and C-terminal regions also form an interaction in rod CNGA1/CNGB1 channels (49, 53). The Ca2+/CaM-binding site in the N-terminal region of CNGB1 is necessary to form an interaction with a short C-terminal region of CNGA1 that is distal to the CNBD. The C-linker and CNBD of CNGA1 are not involved in the interaction, unlike the case in olfactory channels (Fig. 3). Thus, the N- and C-terminal regions that interact in olfactory versus rod channels are fundamentally different on the molecular level (Fig. 3).


    Ca2+/CaM Disrupts Interdomain Interactions in CNG Channels
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
Ca2+/CaM disrupts the N- and C-terminal interactions in both olfactory CNGA2 and rod CNGA1/CNGB1 channels (45, 49). This result suggests a mechanism for inhibition in both channels; Ca2+/CaM binds to the CaM-binding site in the N-terminal region and disrupts the interaction with the C-terminal region. For CNGA2 channels, Ca2+/CaM disruption removes an autoexcitatory interaction between the N- and C-terminal regions, which accounts for inhibition. In CNGA1/CNGB1 channels, the N-terminal region of CNGB1 does not have an autoexcitatory effect, suggesting a different mechanism of inhibition. Ca2+/CaM may directly inhibit CNGA1/CNGB1 channels.


    Ca2+/CaM Modulation of Cone CNG Channels and Other Ion Channels
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
Like native olfactory and rod CNG channels, native cone CNG channels are inhibited by internal Ca2+ (17, 54). Although exogenous Ca2+/CaM inhibits native cone channels, Ca2+ appears to operate through an unidentified factor that diffuses from channels even at elevated Ca2+ levels, arguing against a physiological role for CaM (17, 54). Ca2+/CaM does bind to cloned cone (CNGA3) channels from bovine or human (43) but does not inhibit them (52, 55). However, Ca2+/CaM does inhibit chick CNGA3 channels (56). Identification of a cone modulatory subunit (CNGB3) (44) may help sort out a role for Ca2+/CaM in heteromeric CNGA3/CNGB3 channels.

Ca2+/CaM also regulates several other ion channels via mechanisms distinct from those in olfactory and rod CNG channels (57). The SK class of Ca2+-activated K+ channels is activated by Ca2+ binding to a pre-existing CaM channel complex (58, 59). L-type Ca2+ channels bind Ca2+-free CaM and inactivate upon binding Ca2+ ion (60). N-Methyl-D-aspartate receptors inactivate when Ca2+/CaM binds and displaces a C-terminal region that interacts with the cytoskeleton (61).


    Conclusions
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 
Several recent studies in olfactory and rod CNG channels highlight intriguing similarities and differences in the mechanisms underlying Ca2+/CaM inhibition. Many interesting questions and implications continue to arise from these studies. In olfactory CNG channels much of the original investigations into mechanism focused on CNGA2 homomeric channels. As native channels have now been shown to contain three different subunit types (CNGA2/CNGA4/CNGB1b), two of which have Ca2+/CaM-binding sites (CNGA2 and CNGB1b), it will be of interest to see whether the CNGB1b site also plays a role in the fast Ca2+/CaM-dependent modulation of heteromeric olfactory channels.

The unusual subunit stoichiometry of rod CNG channels (three CNGA1 and one CNGB1 subunit) suggests that CNGB1 subunits have a special impact on Ca2+/CaM-dependent modulation. Because CNGB1 is the only subunit in the tetramer that binds Ca2+/CaM, rod channels may be inhibited by a single Ca2+/CaM molecule.


    FOOTNOTES
 
* This minireview will be reprinted in the 2003 Minireview Compendium, which will be available in January, 2004. This work was supported by National Institutes of Health Grant T32 EY07031 (to W. N. Z.) and by the Howard Hughes Medical Institute. Back

{ddagger} To whom correspondence should be addressed: Dept. of Physiology and Biophysics, Howard Hughes Medical Inst., University of Washington Medical School, Box 357370, Seattle, WA 98195. Tel.: 206-685-3878; Fax: 206-543-0934.

1 The abbreviations used are: CNG, cyclic nucleotide-gated; CNBD, cyclic nucleotide-binding domain; CaM, calmodulin. Back


    ACKNOWLEDGMENTS
 
We thank Dr. M. Varnum for helpful discussions.



    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 Physiological Role for Ca2+/CaM...
 Cloned CNG Channel Subunits
 Subunit Composition of Olfactory...
 Molecular Mechanisms of Ca2+/CaM...
 Ca2+/CaM-binding Sites in the...
 Differential Role in Gating...
 N- and C-terminal Interactions...
 Ca2+/CaM Disrupts Interdomain...
 Ca2+/CaM Modulation of Cone...
 Conclusions
 REFERENCES
 

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