Featured Article - December 2009
Short description: The structure of a sensor histidine kinase in complex with its effector response regulator reveals the details of the phosphoryl transfer reaction.
Bacteria sense their environment through signaling systems composed of two proteins, a sensor and an effector. A histidine kinase (HK) acts as the sensor and phosphorylates the response regulator (RR). These highly conserved proteins carry out a four-step reaction, starting with autophosphorylation of the HK at a histidine residue, using ATP as the source of the phosphoryl group.
HKs are generally homodimeric transmembrane membrane receptors. They have an extracellular amino-terminal sensor domain that is connected via a transmembrane helix to the carboxy-terminal cytoplasmic region that contains the catalytic activity. The catalytic portion of class I HKs, which are the biggest group of HKs, comprises a dimerization and histidine-containing phosphotransfer (DHp) domain.
Previous biochemical studies of the HKs EnvZ and NtrB indicated that autophosphorylation takes place in trans, with the catalytic domain of one HK subunit binding ATP and catalyzing phosphorylation of the phosphoacceptor His on the other subunit. And so it was widely thought that all HKs autophosphorylate in trans.
When Casino et al. solved the X-ray crystallographic structure of the Thermotoga maritima class I HK853 and its response regulator RR468 at 2.8 Å resolution, however, they were surprised to find that in the complex, the β-phosphorus of bound ADPβN (the product of hydrolysis of an ATP analog) is only 11 Å from the His260 εN on the same subunit, compared with 24 Å from the equivalent atom on the other subunit.
These distances suggested that HK853 autophosphorylates in cis rather than in trans. To confirm this, point mutations of the key residues were generated, and these agreed with the structural findings, pointing to an in cis reaction.
In addition, the new structures suggest that extracellular signals are transmitted through the N terminus of HK, which is unstructured in the complex and part of a coiled-coil motif when free. This change seems to derive from a 20° rotation of the coiled-coil in the complex that causes the initial three turns to unfold, revealing hydrophobic residues that presumably promote new protein–protein interactions.
P. Casino, V. Rubio and A. Marina. Structural insight into partner specificity and phosphoryl transfer in two-component signal transduction.
Cell 139, 1-12 (2009). doi:10.1016/j.cell.2009.08.032