Human HER2 structure
In a recent issue of Nature (421, 756–760, 2003), Cho et al. report a crystal structure of the extracellular portion of human HER2, an epidermal growth factor (EGF)-family receptor linked to breast cancer, in complex with the antigen-binding fragment of Herceptin, a monoclonal antibody used to treat breast cancer. Overexpression of HER2 occurs in 20–30% of breast cancers and causes normal cultured cells to become cancerous in the absence of added ligand. The new structure (together with a second crystal structure solved by Cho et al. of the rat form of the receptor) may explain the apparent ligand-independent activation of HER2. In both structures, the extracellular region adopts an 'open' conformation (green model in inset figure) very different from the 'closed' conformation seen in crystal structures of unliganded HER1 and HER3 (red model in figure). Moreover, the open conformation is similar to that of HER1 bound to EGF or transforming growth factor-α. These comparisons suggest that HER2 may be permanently available to form activated receptor dimers in the absence of ligand, unlike other members of the EGF receptor family. By revealing the Herceptin binding site on HER2's juxtamembrane domain (domain IV in figure), the structure also sheds light on the drug's mechanism of action and hints at strategies for designing new HER2 inhibitors. KA
Will GM mosquitoes fly?
Genetically engineering mosquitoes with an impaired ability to transmit malaria has been proposed as a means to combat this age-old scourge. However, new research by Andrea Crisanti and colleagues (Science 299, 1225–1227, 2003) shows that genetically modified (GM) mosquitoes do not compete well against normal populations. GM mosquitoes carrying genes that block development of the malaria parasite, as well as a mechanism to drive these genes through the wild population during mating, could in theory reduce disease transmission. In the current report, the researchers discovered that transgenes, in this case two different fluorescent proteins, introduced into Anopheles stephensi (a vector species of human malaria) disappeared in just 16 generations after mating the modified mosquitoes with normal laboratory strains. The researchers investigated the cause of the gene loss and determined that both the transgene itself, and inbreeding involved in generating the transgenic lines, were responsible for the reduction in the frequency of the transgenes in the populations. The findings indicate that it may be necessary to release large populations of modified insects that are not inbred to ensure the survival of adequate numbers so that they can replace parasite-transmitting mosquitoes. MS
Antibody cross-reactivity has been postulated to arise from an equilibrium state between discrete structural isomers of an immunoglobulin. These isomers have divergent binding sites and specificities that allow promiscuous binding to unrelated antigens. Now, Tawfik and colleagues (Science 299, 1362–1367, 2003) provide the first structural evidence for the existence of such multiple isomers of distinct antigen specificity in a monoclonal IgE antibody raised against a 2,4-dinitrophenyl hapten. In addition to dinitrophenyl, this anti-body also targets close analogs of dinitrophenyl as well as other unrelated compounds, including protein antigens. The authors obtained X-ray crystallographic data and pre-steady state–binding kinetics measurements, which showed the presence of at least two antigen-independent conformations with distinct antigen-binding functions. Crystal structures of the antibody bound to dinitrophenyl and two other haptens revealed a third conformation, and a complex with a recombinant protein antigen showed yet another conformation. The four antibody conformations were the result of substantial structural rearrangements within the antibody. Proof of the existence of antibody isomers and the resulting insights into antibody interactions should have a great impact on our understanding of processes such as autoimmunity and allergy, where antibody cross-reactivity is known to play a major role. GTO
Recycled vitamin C
Researchers at the University of California, Riverside have increased the vitamin C content of a model and crop plant by overexpressing the gene encoding the ascorbic acid recycling enzyme dehydroascorbate reductase (DHAR) (Proc. Natl. Acad. Sci. USA 100, 3525–3530, 2003). Although increases in vitamin C have been observed in plants over-expressing enzymes in the ascorbic acid biosynthetic pathway (Nat. Biotechnol. 21, 177–181, 2003), this is the first report employing the recycling pathway. DHAR allows plants to recycle oxidized ascorbate, which otherwise is irreversibly hydrolyzed. After isolating the DHAR gene from a wheat cDNA library, the researchers introduced the gene into both tobacco and maize. In leaves from both plants and in maize kernels, DHAR activity was elevated between 11- and 100-fold. In the same tissues, the researchers found levels of ascorbic acid 2.2- to 3.9-fold higher than the levels in controls. In addition, they found a change in the redox state—the ascorbate/DHA ratio was elevated, as was the amount of reduced glutathione. The authors conclude that rescue of oxidized vitamin C is enhanced in their transformed plants. LD
Antibiotics that pack a punch
Molecules that combine antimicrobial qualities with an ability to package DNA potentially could enable dual treatment of genetic diseases that are also associated with bacterial infections (e.g., cystic fibrosis). Bechinger and colleagues (Proc. Natl. Acad. Sci. USA 100, 1564–1568, 2003) set out to determine whether histidine-containing cationic peptides (LAH1–LAH5) with antimicrobial activity were capable of transferring DNA. While all peptides bound DNA on the basis of a gel-mobility shift assay, one peptide (LAH4) that contained four histidine residues was 10-fold more efficient than the others in transfecting human embryonic kidney (HEK)293 and HepG2 hepatoma cells on the basis of GFP reporter assays. LAH4 also achieved gene transfer levels comparable to 25 kDa polyethyleneimine, one of the most efficient transfection reagents currently available. Further assays demonstrated that LAH4 was nontoxic, assumes a helical conformation, and mediates DNA import via pH-dependent membrane insertion. As LAH4 shows significant antibiotic activity, the authors suggest it might enable combined therapy of genetic defects and infectious agents (although optimal dosage of the two therapeutic modalities may present problems). AM
Research News Briefs written by Kathy Aschheim, Laura DeFrancesco, Andrew Marshall, Meeghan Sinclair, and Gaspar Taroncher-Oldenburg.