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cabps are 50% homologous to cam , and have the following characteristics consistent with roles as cav1 channel modulators in neurons .
first , cabps ( cabp1 , 2 , 4 , and 5 ) inhibit cdi when coexpressed with cav1.2 or cav1.3 channels in transfected hek293 t cells and in xenopus oocytes ( fig .  1 ) .
this effect results from cabps competitively displacing cam from the cav1 1 subunit , as well as non - competitive actions that may be due to cabps binding to other site(s ) on the channel .
like cam , cabps have an n - terminal and c - terminal lobe separated by an inter - lobe -helical linker domain .
each lobe contains 2 ef - hand ca binding domains , at least one of which has amino acid substitutions that would inhibit ca binding . for cabp1 ,
the key determinants for suppression of cav1.2 cdi are the n - terminal lobe and a glutamate residue in the interlobe linker .
this glutamate residue ( e94 ) , conserved among cabp family members , abolishes cdi suppression by cabp1 when mutated to alanine . unlike cam , which is expressed in most cells , cabps are expressed primarily in neuronal cell - types in the brain , retina , and inner ear . in each of these tissues , cabps are localized in similar cell - types as cav1 channels ( table  1 ) , although alterative splice variants of cabps may be expressed at varying levels .
for example , there are 3 cabp1 splice variants ( cabp1-s , cabp1-l , and caldendrin ) of which caldendrin is the most abundant in the brain .
our understanding of the physiological relevance of cabps as cav1 channel modulators has emerged largely from genetically modified mice lacking expression of particular cabps , as well as human genetic studies . 

table 1.tissue distribution of cabpscabpregionreferencescabp1/caldendrinbraincabp1 , cabp2 , cabp4 , and cabp5cochleacabp4 and cabp5retina 
  tissue distribution of cabps
the first evidence suggesting that cabp4 is an essential regulator for cav1.4 channels in photoreceptor nerve terminals was the similar visual phenotypes of mice lacking cabp4 or cav1.4 ( cabp4 ko and cav1.4 ko , respectively ) . in both strains of mice , there is a loss of synaptic transmission from rod photoreceptors to second - order rod bipolar neurons , which is evident as a diminished  b - wave  in the electroretinogram .
cav1.4 channels containing a ctm exhibit little cdi , even in the absence of cabps .
however , coexpression of cabp4 leads to enhanced voltage - dependent activation of the cav1.4 in transfected hek293 t cells .
thus , cav1.4 channels would be expected to activate at more positive voltages in cabp4 ko mice , which may explain the loss - of function of photoreceptor transmission in these animals .
in addition , mutations in the genes encoding cabp4 and cav1.4 cause similar visual phenotypes in humans . unlike cabp4
, cabp5 is expressed primarily in bipolar cells in the retina , where it colocalizes with cav1.2 . in transfected hek293 t cells
, cabp5 causes a modest suppression of cdi , an effect that could explain the reduced rod - mediated ganglion cell responses to light in mice lacking cabp5 .
cochlear ihcs express cabp1 , cabp2 , cabp4 , and cabp5 , which were proposed to serve as suppressors of cdi of the native cav1.3 channel . in transfected hek293 t cells , each of these cabps except cabp2 inhibited cdi of cav1.3 .
however , subsequent work showed that expression of higher levels of cabp2 induced strong cdi suppression .
moreover , a mutation that leads to premature truncation of cabp2 causes moderate to severe hearing loss in humans .
when tested in hek293 t cells , the mutant cabp2 was less effective than the wild - type cabp2 in suppressing cav1.3 cdi . in individuals affected by
the mutation , stronger cdi of cav1.3 might impair sound - evoked transmission at the ihc synapse .
it is possible that a more severe phenotype is not observed due to potential compensation by the other cabps in ihcs .
while cabp2 , cabp4 , and cabp5 are largely restricted in expression to the retina and inner ear , cabp1 splice variants including caldendrin ( cabp1/caldendrin ) are also expressed in the brain .
cabp1/caldendrin is localized to subgroups of neurons known to express cav1.2 and cav1.3 channels , such as in the cerebral cortex and hippocampus . within these neurons ,
cabp1/caldendrin strongly suppresses cdi of cav1.2 and cav1.3 in transfected hek293 t cells and xenopus oocytes . in the brain ,
these channels have roles in shaping incoming synaptic inputs , sustaining regenerative dendritic spikes , and activating ca - dependent k currents that curtail cell excitability .
some forms of hippocampal synaptic plasticity and learning and memory depend on cav1 channels , particularly cav1.2 .
thus , cabp1/caldendrin could have important roles in regulating diverse cav1 functions in the brain .
since data are not yet available regarding the neurophysiological phenotypes of mice lacking cabp1/caldendrin , one can only speculate on the potential role of cabp1 in modulating cav1 channels in neurons . during a train of depolarizations at 100  hz
, a physiologically relevant frequency often used for inducing synaptic plasticity in brain slices , cam - dependent cdi causes a robust depression of ca influx through cav1.2 channels at the end of the train in transfected hek293 t cells .
this effect is completely blocked by coexpression of cabp1 , such that channel availability remains as strong at the end of the train as it was at the beginning ( fig .  1 ) .
thus , during high frequency bursts in vivo , cabp1 may help support postsynaptic cav1.2 ca signals that are involved in synaptic plasticity , activity - dependent gene transcription , and learning and memory .
the coexistence of cam and cabp1/caldendrin in neurons may allow for a push - pull modulation to fine - tune plasticity involving cav1 channels .
cav channels are essential and versatile regulators of ca signals in excitable cells . compared to the molecular diversity within the family of voltage - gated k channels , there are relatively few genes encoding the pore - forming subunit of cav channels .
the interaction of cav channels with proteins that can modulate their function represents another route by which the activity of cav channels can be adjusted according to cell - type . the opposing regulation of cav channels by cam and cabps represent 2 extremes on the modulatory spectrum . as in the heart , cav channels in some neuronal cell - types
however , in other cells , such as ihcs in the cochlea , sustained ca currents due to cabp1/caldendrin - modulated cav1.3 channels may be required for faithful transmission of sensory input .
studies of how cabps oppose cdi of cav1 channels in heterologous expression systems have revealed major insights into the molecular and biophysical mechanisms controlling cdi ( reviewed in 7 and 9 ) .
however , direct evidence that cabps do indeed suppress cdi of cav channels is currently lacking .
while phenotypes in the cabp4 and cabp5 ko mice are consistent with roles for these cabps in regulating cav1 channels in vivo , voltage clamp recordings of cav1 currents in retinal photoreceptors and bipolar cells have not been done to confirm that there is indeed a loss of cabp modulation in these cells . a definitive role for cabp1/caldendrin in suppressing cdi of neurons in the brain
it also is important to note that cabps can interact with partners other than cav channels .
therefore , phenotypes in cabp ko mice might not necessarily arise from altered cav1 channel regulation .
detailed studies of cabp knockouts , or knock - in cav mutants with disrupted cabp but not cam binding , will provide further clues as to the physiological role of cabps as modulators of neuronal cav1 channels .
supported by the nih ( dc009433 , ns084190 ) and a carver research program of excellence award .
ca2 + -dependent inactivation ( cdi ) is a negative feedback regulation of voltage - gated cav1 and cav2 channels that is mediated by the ca2 + sensing protein , calmodulin ( cam ) , binding to the pore - forming cav 1 subunit . 
 david yue and his colleagues made seminal contributions to our understanding of this process , as well as factors that regulate cdi . 
 important in this regard are members of a family of ca2 + binding proteins ( cabps ) that are related to calmodulin . 
 cabps are expressed mainly in neural tissues and can antagonize cam - dependent cdi for cav1 l - type channels . 
 this review will focus on the roles of cabps as cav1-interacting proteins , and the significance of these interactions for vision , hearing , and neuronal ca2 + signaling events .