Differential effects of ninaC proteins (p132 and p174) on light-activated currents and pupil mechanism in Drosophila photoreceptors

The Drosophila ninaC locus 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 studied via whole-cell recording and through measurements of the pupil mechanism, i.e. the pigment migration in the photoreceptor cells, in the ninaC mutants, P[ninaC(Delta 132)] (p132 absent), P[ninaC(Delta 174)] (p174 absent), and ninaC(P235) (null mutant). Voltage-clamped flash responses in P[ninaC(Delta 174)] and ninaC(P235) showed delayed response termination. In response to steady light, plateau responses in both P[ninaC(Delta 174)] and ninaC(P235) were also large. In both cases the defect was significantly more severe in ninaC(P235). Responses in P[ninaC(Delta 132)] were apparently normal. P[ninaC(Delta 174)] and ninaC(P235) were 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[ninaC(Delta 174)] and ninaC(P235) was also defective, although in this case the rate of return to baseline in both mutants was more or less the same. In all ninaC mutants, 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[ninaC(Delta 174)]. The light sensitivity of the pupil mechanism of P[ninaC(Delta 174)] compared to that of wild type was reduced by 1.3 log units. Remarkably, the light sensitivity of P[ninaC(Delta 132)] and ninaC(P235) was ca. 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 in Drosophila photoreceptors.