How are signalling molecules organized into different pathways within the same cell? In Drosophila, the inaD gene encodes a protein consisting of five PDZ domains which serves as a scaffold to assemble different components of the phototransduction cascade, including the principal light-activated ion channels, the effector phospholipase C-beta and protein kinase C. Null inaD mutants have a dramatically reorganized subcellular distribution of signalling molecules, and a total loss of transduction complexes. Also, mutants defective in a single PDZ domain produce signalling complexes that lack the target protein and display corresponding defects in their physiology. A picture emerges of a highly organized unit of signalling, a 'transduclisome', with PDZ domains functioning as key elements in the organization of transduction complexes in vivo.

Heterotrimeric G proteins mediate a variety of signaling processes by coupling seven-transmembrane receptors to intracellular effector molecules. The Drosophila phototransduction cascade is a G protein-coupled signaling cascade that utilizes a phospholipase C (PLC beta) effector. PLC beta has been shown to be activated by Gq alpha in reconstituted systems. To determine whether a Gq-like protein couples rhodopsin to PLC, and to study its function, we isolated a mutant defective in a photoreceptor-specific Gq protein, DGq. We now demonstrate that Gq is essential for the activation of the phototransduction cascade in vivo. We also generated transgenic flies expressing DGq under an inducible promoter and show that it is possible to manipulate the sensitivity of a photoreceptor cell by controlled expression of DGq. Characterization of quantum bumps in mutants expressing less that 1% of the levels of DGq revealed that the rhodopsin-G protein interaction does not determine the gain of the single photon responses. Together, these results provide significant insight into the role of Gq in regulating the output of a photoreceptor cell.

Calmodulin (CAM) participates in a variety of intracellular transduction processes by modulating signaling molecules in response to calcium changes. We report the characterization of Drosophila Cam mutants and the role of CAM in photoreceptor cell function. Contrary to current models of excitation and TRP channel function, we demonstrate that the transient phenotype of trp mutants can be explained by CAM regulation of the TRPL channel rather than by the loss of a store-operated conductance leading to depletion of the internal stores. We also analyzed light responses in a variety of mutant and transgenic backgrounds and demonstrate the importance of calmodulin in mediating calcium-dependent negative regulation of phototransduction. Our results show that CAM coordinates termination of the light response by modulating receptor and ion channel activity.

No abstract

No abstract

In Drosophila photoreceptors the multivalent PDZ protein INAD organizes the phototransduction cascade into a macromolecular signaling complex containing the effector PLC, the light-activated TRP channels, and a regulatory PKC. Previously, we showed that the subcellular localization of INAD signaling complexes is critical for signaling. Now we have examined how INAD complexes are anchored and assembled in photoreceptor cells. We find that trp mutants, or transgenic flies expressing inaD alleles that disrupt the interaction between INAD and TRP, cause the mislocalization of the entire transduction complex. The INAD-TRP interaction is not required for targeting but rather for anchoring of complexes, because INAD and TRP can be targeted independently of each other. We also show that, in addition to its scaffold role, INAD functions to preassemble transduction complexes. Preassembly of signaling complexes helps to ensure that transduction complexes with the appropriate composition end up in the proper location. This may be a general mechanism used by cells to target different signaling machinery to the pertinent subcellular location.

We present a worm-type Monte Carlo study of several typical models in the three-dimensional (3D) U(1) universality class, which include the classical 3D XY model in the directed flow representation and its Villain version, as well as the 2D quantum Bose-Hubbard (BH) model with unitary filling in the imaginary-time world-line representation. From the topology of the configurations on a torus, we sample the superfluid stiffness ρs and the dimensionless wrapping probability R. From the finite-size scaling analyses of ρs and of R, we determine the critical points as Tc(XY) = 2.201 844 1(5) and Tc(Villain) = 0.333 067 04 (7) and (t/U )c(BH) = 0.059 729 1(8), where T is the temperature for the classical models, and t and U are respectively the hopping and on-site interaction strength for the BH model. The precision of our estimates improves significantly over that of the existing results. Moreover, it is observed that at criticality, the derivative of a wrapping probability with respect to T suffers from negligible leading corrections and enables a precise determination of the correlation length critical exponent as ν = 0.671 83(18). In addition, the critical exponent η is estimated as η = 0.038 53(48) by analyzing a susceptibility-like quantity. We believe that these numerical results would provide a solid reference in the study of classical and quantum phase transitions in the 3D U(1) universality, including the recent development of the conformal bootstrap method.

No abstract