RETINAL TARGETS FOR CALMODULIN INCLUDE PROTEINS IMPLICATED IN SYNAPTIC TRANSMISSION

Ca2+ influxes regulate multiple events in photoreceptor cells includin g phototransduction and synaptic transmission. An important Ca2+ senso r in Drosophila vision appears to be calmodulin since a reduction in l evels of retinal calmodulin causes defects in adaptation and terminati on of the photoresponse. These functions of calmodulin appear to be me diated, at least in part, by four previously identified calmodulin-bin ding proteins: the TRP and TRPL ion channels, NINAC and INAD, To ident ify additional calmodulin-binding proteins that may function in photot ransduction and/or synaptic transmission, we conducted a screen for re tinal calmodulin-binding proteins. We found eight additional calmoduli n-binding proteins that were expressed in the Drosophila retina. These included six targets that were related to proteins implicated in syna ptic transmission. Among these six were a homolog of the diacylglycero l-binding protein, UNC13, and a protein, CRAG, related to Rab3 GTPase exchange proteins. Two other calmodulin-binding proteins included Poll ux, a protein with similarity to a portion of a yeast Rab GTPase activ ating protein, and Calossin, an enormous protein of unknown function c onserved throughout animal phylogeny, Thus, it appears that calmodulin functions as a Ca2+ sensor for a broad diversity of retinal proteins, some of which are implicated in synaptic transmission.