Breakthrough Reveals Important Human Receptor Structure That Will Aid Drug Development, UK

Main Category: Biology / Biochemistry
Article Date: 23 Oct 2007 - 4:00 PDT

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Scientists at the MRC Laboratory of Molecular Biology (LMB) in Cambridge have obtained the first clear images of the proteins that relay vital messages to cells. G protein-coupled receptors (GPCRs) bridge the cell membrane, relaying crucial information from the outside of the cell to the inside. They constitute a major target for drug development. Now scientists have been able to produce and crystallise GPCRs and obtain the first structure of a recombinant GPCR using a method known as high brilliance microcrystallography. Their findings are published in Nature.

The 700 or so GPCRs in man belong to the largest family of cell membrane proteins in the human genome. Their function is to sense molecules outside the cell, hence 'receptors', and trigger cellular reactions. GPCRs are essential for the body to complete a wide range of physiological responses. They allow us to process light and smells, regulate our behaviour, mood and immune response. They are essential in autonomic nervous system transmission. They also control blood pressure, heart rate and digestive processes. This means GPCRs play a crucial role in many diseases and are targets of around half of all modern drugs and a major focus for pharmaceutical companies.

There has been significant progress in understanding GPCRs over the last two decades, including important work on their structure by Dr Gebhard Schertler and colleagues at the LMB, yet little was known about how its structure influences the way the receptor transmits signals through the cell membrane.

Until now, understanding of GPCR structure has been based largely on the crystal structures of the inactive state of rhodopsin, a type of GPCR. Rhodopsin is more easily obtained and also a remarkably stable GPCR, retaining its function under laboratory conditions that denature other GPCRs.

Trying to determine the structure of GPCRs in order to see how they react to hormones and neurotransmitters has been hindered by their low natural abundance and the instability of these proteins in the laboratory. But now Dr Schertler, in collaboration with Dr Daniel Oprian from Brandeise University in Boston, Massachusetts, has been able to stabilise and crystallise recombinant rhodopsin and finally obtain the first structure of a recombinant GPCR using high brilliance microcrystallography.

Using the latest techniques in protein expression crystallisation and micro-crystallography, Dr Schertler in collaboration with Dr Brian Kobilkas' team from Stanford has now obtained a high-definition crystal structure of the unmodified human beta 2 adrenoceptor, a receptor for adrenaline that plays important roles in cardiovascular and pulmonary physiology. This is the first non-rhodopsin, pharmacologically-relevant GPCR to have its structure determined at an atomic level. The breakthrough came when the team of scientists used a monoclonal antibody fragment to create a more stable 'receptor antibody complex', in which the G protein binding site of the receptor could be clearly defined. This success is the end of a very long quest for the structure of an adrenergic receptor and the beginning of a truly molecular pharmacology of GPCRs.

Crystal structure of human β2 adrenergic receptor in complex with an antibody fragment solved using microcrystallography

Dr Schertler said, "This is a case of basic research, which after many years has finally paid off. We have been studying the structure of GPCRs since 1993 when we published the first image of a GPCR by cryo-electron microscopy. Our development of high brilliance micro-crystallography techniques in collaboration with the European Synchrotron Facility has played an essential role in this effort."

"The methodology, involving the expression and stabilisation of the receptor followed by high-brilliance micro-crystallography provides a good strategy for solving the structure of many GPCRs and other clinically important membrane proteins."

Learning more about the structure of GPCRs and how they transmit signals over the cell membrane will help drug development. It will not only enable medical scientists to identify the molecular targets responsible for therapeutic effects more rapidly, but will also help identify the potential side-effects of approved and candidate medications. This newly identified structure will be widely used as a model for other GPCRs and will play a role in a wide range of drug discovery programmes.

Crystal structure of the human ß2 adrenergic G-protein-coupled receptor Søren G. F. Rasmussen, Hee-Jung Choi1, Daniel M. Rosenbaum, Tong Sun Kobilka, Foon Sun Thian, Patricia C. Edwards, Manfred Burghammer, Venkata R. P. Ratnala, Ruslan Sanishvili, Robert F. Fischetti, Gebhard F. X. Schertler, William I. Weis & Brian K. Kobilka. Nature. doi:10.1038/nature06325

Previous papers leading up to this work and alluded to in the text:

Protein crystallography microdiffraction Christian Riekel, Manfred Burghammer and Gebhard Schertler. Current Opinion in Structural Biology 15, 556-62. doi: 10.1016/j.sbi.2005.08.013

Crystal structure of a thermally stable rhodopsin mutant Jorg Standfuss, Guifu Xie, Patricia C. Edwards, Manfred Burghammer, Daniel D. Oprian, Gebhard F.X. Schertler. Journal of Molecular Biology 372, 1179-88. doi: 10.1016/j.jmb.2007.03.007

http://www.mrc.ac.uk