The University of Tokyo : : Research Center for Advanced Science and Technology (RCAST) : : Quantitative Biology and Medicine
G-protein-coupled receptors (GPCRs) are receptors for hormones and neurotransmitters and are one of the most important targets of drug discovery programs. Elucidation of the detailed three-dimensional structures of GPCRs will facilitate the development of desirable drugs.
However, elucidating the structures of these target proteins via crystallization and X-ray diffraction is very difficult, because many of the proteins are embedded in cell membranes, i.e., they are membrane proteins. Few studies have successfully elucidated the three-dimensional structures of membrane proteins, particularly those of mammals including humans.
Dr. Iwata of Kyoto University has developed a method involving the use of an antibody as a crystallizing probe for these membrane proteins. Among membrane proteins, GPCRs are known to be the most difficult to crystallize. It is also difficult to produce antibodies against GPCRs. Thus far, successful crystallization of GPCRs has been achieved by attaching a protein, e.g., T4 lysozyme, to assist crystallization or by using a single-chain antibody obtained by immunizing a camelid (llama).
Presently, in collaboration with the groups of Dr. Murata of Chiba University and Dr. Kobayashi and Dr. Iwata of Kyoto University, we successfully obtained a mouse antibody against the A2a adenosine receptor, a drug target for Parkinson’s disease. This antibody was screened from more than 500 positive wells and was found to possess the activity of an inverse agonist, which locks the receptor in an inactive conformation and prevents the binding of adenosine (agonist) but not the inhibitor (antagonist). (The yellow region in the upper right part of the figure is the A2a receptor, and the red part is the Fab fragment of the antibody).
Crystal structure analysis shows that a part of the heavy chain of the antibody, called CDR3, extends like a finger and sticks into the pocket made by 7 -helices of the GPCR, and locks the receptor in an inactive conformation.
This kind of structural information provides us important tools for drug discovery based on computer simulation and accelerates the development of new technologies.