Scaffolding proteins that bind to and organize components of signaling pathways into macromolecular complexes are crucial to the efficiency, specificity, and speed of various signaling pathways. For instance, INAD (inactivation, no after-potential D), a PDZ domain-containing scaffolding protein that, together with three of its binding partners, constitutes the core complex of the Drosophila phototransduction cascade, is critical to the electrical response to light (see Montell). Using x-ray crystallography, Mishra et al. found that, in the oxidized state, the fifth INAD PDZ protein interaction domain (PDZ5) assumed a distorted conformation inconsistent with canonical ligand binding. This conformation depended on an intramolecular disulfide bond and, in the reduced state, PDZ5 structurally resembled other PDZ domains. In transgenic flies expressing wild-type INAD, light exposure led to formation of the disulfide bond (and thus the oxidized conformation), whereas subsequent darkness was associated with return to the reduced form; development of the oxidized form was attenuated in flies lacking the major isoform of rhodopsin. Electrophysiological analysis of isolated photoreceptor cells expressing a mutant form of INAD in which PDZ5 only existed in the reduced conformation indicated that the sensitivity of the mutant form to light was normal but that loss of a refractory period led to defective termination of the response to bright light. Moreover, flies with the mutant INAD showed a defective escape response to a brief exposure to darkness (mimicking the shadow of a predator). Thus, rather than performing a purely passive function in organizing signaling proteins, INAD appears to play a dynamic role in the Drosophila phototransduction pathway.
P. Mishra, M. Socolich, M. A. Wall, J. Graves, Z. Wang, R. Ranganathan, Dynamic scaffolding in a G protein-coupled signaling system. Cell 131, 80-92 (2007). [PubMed]
C. Montell, Dynamic regulation of the INAD signaling scaffold becomes crystal clear. Cell 131, 19-21 (2007). [PubMed]