The PDZ scaffold protein INAD abolishes apparent store-dependent regulation of the light-activated cation channel TRP.
In fly photoreceptor cells, light initiates a G protein-coupled phospholipase Cb-dependent signaling cascade that results in the depolarization of the cell membrane, which is mediated by the cation channels TRP and TRPL. Together with phospholipase Cb and an eye-specific protein kinase C, TRP is tethered to the scaffolding protein INAD, which forms a multimolecular signaling complex. Divergent data from expressed TRP and studies from photoreceptor cells have brought up a controversy whether or not a capacitative calcium entry (CCE) mechanism is involved in the Drosophila phototransduction pathway. Our initial characterization of TRP from photoreceptors of Calliphora vicina supported the hypothesis of a CCE mechanism, as heterologously expressed TRP was stimulated after application of thapsigargin. The situation changed when the PDZ domain protein INAD was coexpressed with TRP. In cells coexpressing TRP and INAD, no calcium entry was detectable on application of store depletion protocols. Suppression of CCE by INAD was not observed when the described interaction was disrupted by mutations in TRP and INAD. Our data show that apparent activation of TRP by CCE is abolished by INAD. Within the complex, the proteins necessary for phototransduction mutually influence their activities. The results support the hypothesis of a store-independent activation of TRP.
Institut für Pharmakologie, Universitätsklinikum Benjamin Franklin, Freie Universität Berlin, 14195 Berlin, Germany. hartenek@zedat.fu-berlin.de
Christian
C
Harteneck
Sabina Naranjo
SN
Kuchta
Armin
A
Huber
Reinhard
R
Paulsen
Günter
G
Schultz
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000235:genetics / Q000502:physiology* / Q000528:radiation effects; D002122:Calcium Chloride / Q000494:pharmacology; D002460:Cell Line; D029721:Drosophila Proteins; D004533:Egtazic Acid / Q000494:pharmacology; D005136:Eye Proteins / Q000235:genetics / Q000502:physiology*; D005822:Genetic Vectors / Q000235:genetics; D008027:Light; D009154:Mutation; D011485:Protein Binding / Q000187:drug effects / Q000235:genetics; D050052:TRPC Cation Channels; D019284:Thapsigargin / Q000494:pharmacology; D014162:Transfection
D015220:Calcium Channels; D029721:Drosophila Proteins; D005136:Eye Proteins; D050052:TRPC Cation Channels; C091568:inaD protein, Drosophila; D004533:Egtazic Acid; D019284:Thapsigargin; D002122:Calcium Chloride; D002118:Calcium