Proteins of rod outer segments of toad retina: binding with calmodulin and with GTP

Retinal targets for calmodulin include proteins implicated in synaptic transmission

Ca2+ influxes regulate multiple events in photoreceptor cells including phototransduction and synaptic transmission. An important Ca2+ sensor in Drosophila vision appears to be calmodulin since a reduction in levels of retinal calmodulin causes defects in adaptation and termination of the photoresponse. These functions of calmodulin appear to be mediated, at least in part, by four previously identified calmodulin-binding proteins: the TRP and TRPL ion channels, NINAC and INAD. To identify additional calmodulin-binding proteins that may function in phototransduction and/or synaptic transmission, we conducted a screen for retinal calmodulin-binding proteins. We found eight additional calmodulin-binding proteins that were expressed in the Drosophila retina. These included six targets that were related to proteins implicated in synaptic transmission. Among these six were a homolog of the diacylglycerol-binding protein, UNC13, and a protein, CRAG, related to Rab3 GTPase exchange proteins. Two other calmodulin-binding proteins included Pollux, a protein with similarity to a portion of a yeast Rab GTPase activating protein, and Calossin, an enormous protein of unknown function conserved 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.

Modulation of the cGMP-gated ion channel in frog rods by calmodulin and an endogenous inhibitory factor

1. Outer segment patches excised in the light were used to investigate the effects of exogenous calmodulin and an endogenous inhibitory factor on the cGMP-gated channel of frog rods. 2. Calmodulin shifted to the right the dose-response relation for activation of the channels by 8-Br-cGMP, but did not change the maximum current or the form of the relation. Reversal of this effect by removal of calmodulin was accelerated by brief exposure to saturating [8-Br-cGMP]. Inhibition by calmodulin required calcium and gave as much as a 5-fold decrease in current for an [8-Br-cGMP] functionally comparable to the presumed physiological [cGMP]. 3. Exposure to low [Ca2+]i (tens of nanomolar) appeared to irreversibly remove or inactivate an endogenous channel inhibitory factor from the patches, increasing the current at low [8-Br-cGMP]. Like calmodulin, this factor slowed the voltage-dependent channel-gating kinetics and did not change the maximum current. However, unlike calmodulin, the endogenous factor remained stably associated with the patches at high [Ca2+]i (1 microM), even with exposure to saturating [8-Br-cGMP]. 4. After the low-Ca2+ treatment increased the current, calmodulin reduced the current to about the same level as it had before the low-Ca2+ treatment, giving a larger fractional suppression. Furthermore, patches with high initial sensitivity to 8-Br-cGMP had small low-Ca2+ effects and large calmodulin effects, while the reverse was true for patches with low initial agonist sensitivity. 5. Application of trypsin to the intracellular surface of the patch prevented the responses to calmodulin and to low [Ca2+]i, suggesting involvement of a cytoplasmic portion of the channel. However, trypsin also reduced the total agonist-induced patch current. 6. Our results are consistent with a model in which calmodulin and an endogenous calcium-binding protein compete for the same site, inhibiting channel opening or cGMP binding. The tight association of the endogenous factor with the channel even at relatively low [Ca2+]i suggests that in the transducing rod it may inhibit the channels most of the time in darkness and in dim light, preventing any potential inhibitory effects of calmodulin. The endogenous factor would be expected to leave the channel only in bright or prolonged light, when the [Ca2+]i is thought to be very low.

Detection of novel guanine nucleotide binding proteins in bovine retinal rod outer segments

In the rod outer segment membranes of the bovine retina at least two members of the small molecular weight guanine nucleotide binding proteins were identified by means of the technique of binding radiolabeled GTP to nitrocellulose Western blots of proteins separated by sodium dodecyl sulphate gel electrophoresis. Such proteins, of 23 and 25 kDa, are able to specifically bind guanine nucleotides after denaturing treatments, and are not labeled by pertussis or cholera toxin-catalyzed ADP-ribosylation. The binding site is specific for GTP.