POSTERS - BLAST X - University of Utah

BLAST X Poster #17
TESTING THE YIN-YANG MODEL OF SIGNAL TRANSDUCTION IN A BACTERIAL
CHEMORECEPTOR CYTOPLASMIC DOMAIN
Ka Lin E. Swain and Joseph J. Falke
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Campus Box 215,
Boulder, CO, 80309
The bacterial transmembrane aspartate receptor (Tar) of E. coli and S. typhimurium
chemotaxis is a homodimer that assembles to form larger oligomers, most likely a trimer-of
dimmers. The homodimer can be divided into three modules: (i) the transmembrane signaling
module comprised of the periplasmic ligand binding domain and the transmembrane helices, (ii)
the cytoplasmic HAMP domain which serves as a signal conversion module, and (iii) the
cytoplasmic kinase control module possessing the adaptation sites and a protein interaction
region that binds the CheA kinase. The kinase control module is a 4-helix bundle essential for
transmitting the integrated signal output of the HAMP domain and the adaptation sites to the
bound histidine kinase CheA. Considerable evidence indicates that structural rearrangments of
the subunit-subunit interface within the homodimeric 4-helix bundle are important in signaling.
This study further probes the mechanism of signal transduction in the kinase control module of
the S. Typhimurium aspartate receptor. The approach mutates the conserved “knob” residues
of “sockets” that stabilize helix-helix contacts to alanines, in order to destabilize the packing of
adjacent helices in the 4- helix bundle. Knob mutations that lock the receptor in “on” and “off”
signaling states are identified by their opposite effects on CheA kinase and receptor methylation
rates. The results suggest a novel “Yin-Yang” mechanism in which the helix packing states of
the adaptation region (I) and the protein interaction region (II) have opposing effects on receptor
on-off switching: stable helix packing in region I and unstable packing in region II drive the
receptor into the kinase-activating on-state, while unstable packing in region I and stable
packing in region II favor the kinase-inactivating off-state.
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