Differential effects ofninaCproteins (p132 and p174) on light-activated currents and pupil mechanism inDrosophilaphotoreceptors

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Abstract
TheDrosophila ninaClocus encodes two retinal specific proteins (p132 and p174) both consisting of a protein kinase joined to a myosin head domain and a C terminal with a calmodulin-binding domain. The role of p132 and p174 was studiedviawhole-cell recording and through measurements of the pupil mechanism, i.e. the pigment migration in the photoreceptor cells, in theninaCmutants, P[ninaCΔ132] (p132 absent), P[ninaCΔ174] (p174 absent), andninaCp235(null mutant). Voltage-clamped flash responses in P[ninaCΔ174] andninaCp235showed delayed response termination. In response to steady light, plateau responses in both P[ninaCΔ174] andninaCp235were also large. In both cases the defect was significantly more severe inninaCp235. Responses in P[ninaCΔ132] were apparently normal. P[ninaCΔ174] andninaCP235were also characterized by spontaneous quantum bump-like activity in the dark and by larger and longer light-induced quantum bumps. The turn-off of the pupil mechanism in P\ninaCΔ174] andninaCp235was also defective, although in this case the rate of return to baseline in both mutants was more or less the same. In allninaCmutants, the amplitudes of the pupillary pigment migration were distinctly smaller than that in the wild type. The reduction of the amplitude was largest in P[ninaCΔ174]. The light sensitivity of the pupil mechanism of P[ninaCΔ174] compared to that of wild type was reduced by 1.3 log units. Remarkably, the light sensitivity of P[ninaCΔ132] andninaCP235wasca. 0.5 log units higher than that of the wild type. The results suggest that the p174 protein is required for normal termination of the transduction cascade. The diverse phenotypes observed may suggest multiple roles for calmodulin distribution for controlling response termination and regulating pigment migration inDrosophilaphotoreceptors.