Department of Biochemistry

Daniel Oprian

Louis and Bessie Rosenfield Professor of Biochemistry

Research Description

Structure-function studies of visual pigments and other cell surface receptors

Our laboratory is interested in the structure and function of G protein-coupled receptors with particular focus on the subgroup of receptors known as visual pigments. The pigments are major components of rod and cone photoreceptor cells and form the basis of phototransduction in the vertebrate retina. As is typical of G protein-coupled receptors, the visual pigments are integral membrane proteins composed of seven transmembrane helical segments. However, what is atypical is that each pigment is bound covalently to a small molecule ligand, 11-cis-retinal, which is a chromophore for the absorption of light.

Spectral Tuning

There are four visual pigments in the human retina: rhodopsin, the pigment of rod photoreceptor cells, and the blue, green and red color vision pigments of cone photoreceptor cells. These four pigments have absorption spectra which span (actually define) the visible region of the electromagnetic spectrum. And yet, the small molecule chromophore responsible for absorption of light is the same in each pigment. One of the goals of our research is to understand the underlying mechanism by which the spectrum of each pigment is “tuned” by interactions of the retinal chromophore with amino acid side chains in the active site of the proteins.

Mechanism of Retinal Disease

A large number of mutations in rhodopsin are known to cause inherited diseases of the retina. We are interested in a small group of these which result in constitutive activation of rhodopsin - that is, these mutations cause the protein to activate the G protein transducin in the absence of light. The activating mutations are known to cause two different diseases: a devastating degenerative disease of the retina known as retinitis pigmentosa and, in comparison, a relatively benign disease known as congenital stationary night blindness. Our goal is to elucidate the underlying molecular mechanisms responsible for pathophysiology in these diseases. To do this, we employ a broad array of techniques ranging from classical biochemistry and mutagenesis for in vitro characterization of the mutant proteins to the development of transgenic animals combined with single-cell electrophysiology to test models of disease under in vivo conditions.

Structure of the Active State

Rhodopsin is the only G protein-coupled receptor for which a structure has been determined by x-ray crystallography. While the structure of rhodopsin has had an enormous impact on our understanding of the protein it is clear that we are at the very beginning of these structural studies and major questions remain. In particular, we still do not have a structure for the active conformation of the protein – the published rhodopsin structure is for the inactive- or dark-state. Our laboratory is currently undertaking a major effort to determine the x-ray crystal structure of the active state of rhodopsin. In addition, we are undertaking efforts to determine the crystal structure of other G protein-coupled receptors.