Transmembrane signaling systems operate in sensory neurons to encode a variety of external signals into fluctuations in membrane potential. These signaling systems display beautiful properties of catalytic signal amplification and extensive feedback regulation that are important for accomplishing their biologic task. How are these properties encoded by the protein networks involved, and how do the biochemical activities of individual proteins contribute to processing external signals? In our laboratory, we use a combination of biophysical, structural, and genetic techniques to study the signaling protein complexes that mediate phototransduction, the process of converting photon flux into a graded electrical output in the photoreceptor cells of the Drosophila compound eye. The model system is chosen for the feasibility of the necessary biophysical and structural techniques, and for the availability of powerful molecular genetic tools to generate useful reagents. We are currently focusing our work in two areas: (1) the study of the cycle of activation and inactivation of the G-protein-coupled light-receptor rhodopsin, and (2) the structural mechanisms of assembling signaling proteins into functional macromolecular complexes.
1) A problem of general importance in the study of G-protein-coupled receptors involves understanding the conformational changes induced by external signals that efficiently cause switching between active and inactive forms of the molecule. Using whole-cell patch clamp techniques, we can measure a transient current generated from the light-induced conformational changes in rhodopsin due to net charge movement within the plasma membrane electric field. The kinetic analysis of these so-called "gating" currents in rhodopsin have begun to yield information about the intermediate states in the photoisomerization of the rhodopsin molecule. Ultimately, these studies may allow us to build a low-resolution dynamic model for state transitions in this membrane protein.
The shutoff of active metarhodopsin involves the direct binding of a cytosolic protein called arrestin; this reaction is rate-limiting for M inactivation, and proceeds with 1:1 stoichiometry. Interestingly, arrestins are rapidly phosphorylated upon photoexcitation, suggesting that this modification may represent a mechanism for controlling the catalytic lifetime of metarhodopsin. We are investigating this issue through both in-vitro biochemical assays for the arrestin-rhodopsin interaction as well as whole-cell patch clamp measurements in transgenic fly photoreceptors carrying mutated forms of arrestin.
2) An emerging principle in intracellular signaling is that many transduction proteins are organized into macromolecular complexes that allow for highly localized and efficient signaling events. In Drosophila photoreceptors, a group of proteins involved in excitation and in feedback regulation are organized at the membrane by an adapter protein known as InaD. InaD consists of five repeated segments that are members of a conserved family of protein interaction modules known as PDZ domains. We are now working on solving the crystal structures of inaD and its PDZ domains to understand how binding specificity is achieved for different signaling molecules and how the tertiary structure contributes to the biological role of this scaffolding protein.
RESEARCH INTERESTS
protein structure, function and dynamics
cellular signaling networks
RECENT PUBLICATIONS
Hatley MA, Lockless SW, Gibson SK, Gilman AG, Ranganathan R, "Allosteric determinants in guanine nucleotide-binding proteins" Proc Natl Acad Sci USA, in press December 2003
Shulman, A.I., C. Larson, D.J. Mangelsdorf, R. Ranganathan, "Structural Determinants of Allosteric Ligand Activation in RXR Heterodimers" Cell, 116:417-429, 2004
Jain, R.K., and R. Ranganathan, "Local Complexity of Amino-acid Interactions in a Protein Core" PNAS, 101:111-116, 2003
Russ, W., D.M. Lowery, P. Mishra, M.B. Yaffe, and R. Ranganathan, "Natural-like Function in Artificial WW Domains" Nature, 437:579-583, September 2005
Socolich, M., S.W. Lockless, H.L. Lee, K. Gardner, and R. Ranganathan, "Evolutionary Information for Specifying a Protein Fold" Nature, 437:512-518, September 2005
SIGNIFICANT PUBLICATIONS
Kiselev A, Ranganathan R, "A molecular pathway for light dependent photoreceptor apoptosis in drosophilia" Neuron, 28:139-152, 2000
Suel GM, Lockless SW, Wall MA, Ranganathan R, "Evolutionarily conserved networks of residues mediate allosteric coupling in proteins" Nature Structural Biology, 10:59-69, January 2003
Lockless SW, Ranganathan R, "Evolutionarily conserved pathways of energetic connectivity in protein families" Science, 286:295-299, 1999
Socolich, M., S.W. Lockless, H.L. Lee, K. Gardner, and R. Ranganathan, "Evolutionary Information for Specifying a Protein Fold" Nature, 437:512-518, September 2005
Russ, W., D.M. Lowery, P. Mishra, M.B. Yaffe, and R. Ranganathan, "Natural-like Function in Artificial WW Domains" Nature, 437:579-583, September 2005
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