The cell membrane forms the barrier between the cytoplasm inside the cell and the environment. Transport proteins in this membrane allow the controlled intake and output of molecules by the cell. ABC (ATP-binding cassette) transporters are widely distributed. They occur both in simple organisms like bacteria and also in humans. Researchers at the Institute of Molecular Biology & Biophysics have now succeeded in elucidating the structure of the transporter ModBC of the archaebacterium Archaeoglobus fulgidus. The paper appeared in the scientific journal Nature online (1)on Sunday 25 February 2007.

Proteins form a channel

ModBC is responsible for the intake of molybdate, which is essential to life. Its binding protein ModA captures molybdate outside the cell plasma. The loaded ModA binds to the transporter ModBC which functions like a channel. It can open either outwards or towards the cell interior. Assistant professor Kaspar Locher’s researchers crystallised the complex consisting of the transporter and binding protein, and determined its three-dimensional structure by X-ray structural analysis.

Two ModB polypeptide chains are located in the membrane, spanning it several times in a spiral shape and forming a channel that is closed to the exterior. Inside the cell are two copies of the nucleotide binding domain ModC, firmly associated with the ModB sub-units. The ModC molecules form the actual motor of the transport machine. By binding and cleaving the ATP molecule they release the energy needed to open and close the membrane channel. The researchers have now been able to observe a structure that is open to the interior and closed to the exterior without having bound ATP.

However, Kaspar Locher and his colleagues are not interested in the structure alone. To elucidate the transport mechanism they compared the structure of ModBC-A with that of the transporter Sav1866, which belongs to the same family. They had already solved the latter’s structure last year (2).

The ABC transporter ModBC-A with sub-units ModA (red), ModB (blue, yellow) and ModC (green, violet). At the left is a front view in which the channel closed to the exterior is visible. On the right is a side view of the same complex rotated through 90° around its longitudinal axis. (Photo: K. Hollenstein / Institute of Molecular Biology & Biophysics). large

Both function on a similar principle. The only difference: the structure of Sav1866, this time with bound ATP, shows the transporter in the state in which it is open to the exterior. From the combination of the two structures the researchers are able to deduce the general mechanism of ABC transporters. The ATP-free state, illustrated by the molybdate transporter ModBC-A, is closed to the exterior. After binding ATP, the channel opens in an outwards direction and is closed to the interior. This state was captured in the structure of Sav1866. After the ATP is cleaved, the polypeptide chains change their shape again and the channel is open to the cell interior again.

Relevant to medical practice

Thus the two structures permit general conclusions about the mechanism of ABC transporters, which is relevant to clinical medicine. The first author of the study, Kaspar Hollenstein, stresses that “There are ABC transporters in humans as well.” They play a part in various human illnesses such as the resistance of cancer cells to chemotherapy drugs or in cystic fibrosis, a hereditary disease. Elucidating the structure of the transport proteins involved forms an important basis to understanding these processes at the molecular level.