Pivots or pistons?

Transmembrane receptors monitor the extracellular environment and transmit information about changes in specific ligand concentration to the cytoplasm. It is not clear how the message gets across the cell membrane, but one proposal is that ligand binding induces a transmembrane conformational change in the receptor. Writing in Science late last year, members of Kim and Koshland's laboratories reported the determination of the three-dimensional structure of the extracellular domain of Tar, the bacterial aspartate receptor, with and without bound ligand, the hope being that the structures would provide a clear image of the conformational changes involved in signalling. Unfortunately, however, the picture remains blurred.