Crystal structure of rhodopsin: A G protein-coupled receptor.

Heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) respond to a variety of different external stimuli and activate G proteins. GPCRs share many structural features, including a bundle of seven transmembrane alpha helices connected by six loops of varying lengths. We determined the structure of rhodopsin from diffraction data extending to 2.8 angstroms resolution. The highly organized structure in the extracellular region, including a conserved disulfide bridge, forms a basis for the arrangement of the seven-helix transmembrane motif. The ground-state chromophore, 11-cis-retinal, holds the transmembrane region of the protein in the inactive conformation. Interactions of the chromophore with a cluster of key residues determine the wavelength of the maximum absorption. Changes in these interactions among rhodopsins facilitate color discrimination. Identification of a set of residues that mediate interactions between the transmembrane helices and the cytoplasmic surface, where G-protein activation occurs, also suggests a possible structural change upon photoactivation.

X-Ray diffraction analysis of three-dimensional crystals of bovine rhodopsin obtained from mixed micelles.

Rhodopsin, a prototypic G protein-coupled receptor responsible for absorption of photons in retinal rod photoreceptor cells, was selectively extracted from bovine rod outer segment membranes, employing mixed micelles of nonyl beta-d-glucoside and heptanetriol. Highly purified rhodopsin was crystallized from solutions containing varying amounts of detergent and amphiphile. The crystals contained ground state rhodopsin molecules as judged by their red color and the linear dichroism originating from the 11-cis-retinal chromophore. However, when exposed to visible light, even at 4 degrees C, rhodopsin was bleached and the crystals decomposed. Reflections in the diffraction pattern were observed out to 3.5-A resolution at 100 K for the most ordered crystals. Diffraction data have been processed to 3.85-A resolution. The symmetry of the diffraction pattern and the systematic absences indicate that the crystals have tetragonal symmetry, space group P4(1)22 or P4(3)22, a = b = 96.51 A, c = 148.55 A. A value of 4.12 A(3)/Da for V(M) was obtained for one monomer in the asymmetric unit (eight molecules per unit cell). Our study is the first characterization of a three-dimensional crystal of a G protein-coupled receptor and may be valuable for future structural studies on related receptors of this important superfamily.

Structural determinants of the bifunctional corn Hageman factor inhibitor: x-ray crystal structure at 1.95 A resolution.

Corn Hageman factor inhibitor (CHFI) is a bifunctional 127 residue, 13.6 kDa protein isolated from corn seeds. It inhibits mammalian trypsin and Factor XIIa (Hageman Factor) of the contact pathway of coagulation as well as alpha-amylases from several insect species. Among the plasma proteinases, CHFI specifically inhibits Factor XIIa without affecting the activity of other coagulation proteinases. We have isolated CHFI from corn and determined the crystallographic structure at 1.95 A resolution. Additionally, we have solved the structure of the recombinant protein produced in Escherichia coli at 2.2 A resolution. The two proteins are essentially identical. The proteinase binding loop is in the canonical conformation for proteinase inhibitors. In an effort to understand alpha-amylase inhibition by members of the family of 25 cereal trypsin/alpha-amylase inhibitors, we have made three-dimensional models of several proteins in the family based on the CHFI coordinates and the coordinates determined for wheat alpha-amylase inhibitor 0.19 [Oda, Y., Matsunaga, T., Fukuyama, K., Miyazaki, T., and Morimoto, T. (1997) Biochemistry 36, 13503-13511]. From an analysis of the models and a structure-based sequence analysis, we propose a testable hypothesis for the regions of these proteins which bind alpha-amylase. In the course of the investigations, we have found that the cereal trypsin/alpha-amylase inhibitor family is evolutionarily related to the family of nonspecific lipid-transfer proteins of plants. This is a new addition to the group which now consists of the trypsin/alpha-amylase inhibitors, 2S seed storage albumins, and the lipid-transfer family. Apparently, the four-helix conformation has been a successful vehicle in plant evolution for providing protection from predators, food for the embryo, and lipid transfer.

Activation of programmed cell death (apoptosis) by cisplatin, other anticancer drugs, toxins and hyperthermia.

Cell death induced by cisplatin was studied in Chinese hamster ovary cell lines, one proficient and the other deficient (100-fold sensitive) in DNA excision repair. Previous experiments demonstrated that cells progressed to and arrested in the G2 phase of the cell cycle before dying. DNA double-strand breaks were detected following G2 arrest and prior to loss of membrane integrity. These DNA breaks have been studied in more detail. DNA fragments were observed consisting of multimers of approximately 180 base pairs. These fragments are consistent with internucleosomal cleavage of chromatin by an endonuclease. At LC90 concentrations, DNA digestion began 48 hr cisplatin treatment followed by loss of membrane integrity and cell shrinkage 24 hr later. High concentrations of cisplatin (170 logs of kill) induced DNA digestion 12 hr after drug treatment but loss of membrane integrity occurred 12 hr later. Both cell death and DNA fragmentation were inhibited by cycloheximide, suggesting the requirement for new protein synthesis. Cells incubated with many other agents demonstrated the same characteristic pattern of DNA degradation. At 90% lethal conditions, DNA digestion was induced within 30 min by hyperthermia, 18 hr by methotrexate, and 48-72 hr by all other agents tested. DNA digestion always preceded loss of membrane integrity and cell shrinkage. These observations are consistent with cell death occurring by the process of apoptosis, or prorammed cell death, and demonstrate the importance of DNA digestion as an early and presumably essential step in cell death. The results suggest that, irrespective of the primary site of action of a drug, cell death by most pharmacologic agents is mediated by activation of the signal transduction pathway for apoptosis. The results also suggest two signal pathways for apoptosis, one directly associated with drug action and a second that requires cell cycle-related events.