Crossing the boundary: how proteins permit entry to cell

The findings, published today [16 October 2008] in Science Express, are important because there are hundreds of transporter proteins in the body and understanding their structure and how they work will help scientists design the next generation of drugs to treat illness and disease.

“Transporter proteins are very difficult to study,” says Professor Peter Henderson of the University of Leeds. “However, in Leeds we developed methods to maximise their expression in bacteria, and purify them very carefully so they retain their biological activity. We could then maintain a ‘pipeline’ to supply them to experts in a technique called protein crystallography, which requires very high intensity x-rays, available, for example, at the Diamond Light Source national synchrotron facility in Oxfordshire and the European Synchrotron Radiation Facility (ESRF) in Grenoble.”

At Diamond, co-author Professor So Iwata and colleagues from Imperial College London’s Department of Life Sciences imaged the transporter protein Microbacterium hydantoin permease (Mhp1) – which lives in the oily membrane that surrounds bacterial cells – transporting molecules of hydantoin across its otherwise impermeable cell membrane. Once inside the cell, these molecules are converted to useful amino acids, which are of commercial importance, as they are used for food and drink supplements and to make pharmaceuticals.

The structure of Mhp1 was analysed both before and after it had taken in a hydantoin molecule from outside the cell. The researchers saw that the Mhp1 protein opens up on its outer-facing side, allowing the hydantoin molecule to move inside. Once the hydantoin is bound, the pathway closes behind it, ensuring that no other substances are let in. The inward-facing side then opens to release the hydantoin into the cell.

Commenting on the significance of the discovery, Professor Iwata says: “Our research has revealed the detailed molecular function of an important membrane protein. We now know how the protein facilitates the movement of hydantoin across the cell membrane without letting any other substances through at the same time. This mechanism is likely to be shared by many cell membrane transport proteins, including those in the human body, so this is an important step forward in our understanding of the fundamental processes which occur in our cells.”

The research was funded in the UK by the BBSRC and the EU and carried out in collaboration with scientists from Japan and Iran.

The function of Mhp1 was initially discovered during a two year visit to Leeds – which began in 2000 – by a Japanese researcher, Dr Suzuki, from Ajinomoto Inc. This work was patented in Japan and the USA.

Professor Peter Henderson adds: “We hope now to gain sufficient detail of the intimate structure of many transporter proteins to help chemists and industrial sponsors to design and develop drugs to manipulate their activities and treat a variety of diseases.”