The Calliphora rpa mutant lacks the PDZ domain-assembled INAD signalling complex

The visual transduction cascade of fly photoreceptors is a G protein-couple d phospholipase C-signalling pathway which is assembled into a supramolecul ar signalling complex by the PDZ (postsynaptic density protein-95, discs la rge, Z0-1) domain protein INAD (inactivation no afterpotential D). The norp A-encoded phospholipase C beta, the light-activated transient receptor pote ntial (TRP) Ca2+ channel and an eye-specific protein kinase C are bound to INAD and together form the core of the signalling complex. In the present s tudy we show that the Calliphora rpa mutant, which has previously been hypo thesized to represent an equivalent of Drosophila norpA mutants, has normal amounts of norpA mRNA but fails to express inaD mRNA. Electrophysiological recordings from the eyes of the rpa mutant reveal that the electroretinogr am is reduced (about 12% of wild type) but not completely absent, and that it exhibits markedly prolonged deactivation kinetics. Furthermore, rpa muta nts display a slow, light-dependent degeneration of the photoreceptor cells . With respect to the INAD signalling complex, the rpa mutant is similar to the Drosophila inaD null mutant: not only INAD itself, but also the other core components of the INAD signalling complex, are reduced or absent in ph otoreceptor membranes of rpa flies. Residual TRP is localized throughout th e plasma membrane of the photoreceptor cell, rather than being restricted t o the microvillar photoreceptor membrane. [S-35]methionine-labelling of new ly synthesized retinal proteins reveals that TRP is synthesized in the rpa mutant at wild-type level, but is transported to or incorporated into the m icrovillar photoreceptor membrane at a much lower rate. We thus suggest, th at the formation of the INAD signalling complex is required for specificall y targeting its components to the photoreceptor membrane.