The aspartate receptor from E. coli is a dimeric transmembrane-signaling protein that mediates chemotaxis behavior and is the most studied system among the chemotaxis receptors to understand the molecular mechanism for transmembrane signaling. However, there is an unresolved issue for the structural event which initiates the transmembrane signal upon binding to the ligand. Biochemical and genetic evidence implies an intrasubunit mechanism (monomeric model) whereas crystallographic evidence implies an intersubunit mechanism (dimeric model). Crystallographic evidence has been ambiguous because all the apo protein structures contained a pseudoligand sulfate, and a completely ligand-free structure has not been available thus far. Here we present the crystal structure of the ligand binding domain of the aspartate receptor free of the ligand aspartate or pseudoligand sulfate. The structural comparison of this structure with those of ligand-bound and pseudoligand-bound forms revealed that, on ligand or pseudoligand binding, the conformational change in the ligand-binding domain is relatively small, but there is a considerable rotation between two subunits, supporting the dimeric model.
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Acknowledgements. We would like to thank Chan-Kyu Park of Korea Advanced Institute of Science and Technology for a gilt of the E. coli aspartate receptor (Tar) clone and Joanne Yeh for construction of the MBP clone and contribution to the purification of MBP. We also would like to thank Rosalind Kim for valuable discussions and Edward Berry lbr assistance with data collection. This work was supported by the U.S. Department of Energy (DE-AC03-76SF00098), Korea Research Institute of Science and Biotechnology, and Asahi Chemical Industry, Inc.
- Aspartate receptor
- Crystal structure
- Transmembrane signaling
- X-ray crystallogrpahy