Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 108, Issue 23, Pages 9390-9395Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1104824108
Keywords
transmembrane receptors; bacterial chemotaxis; transmembrane signaling; protein-protein interaction
Categories
Funding
- National Institutes of General Medical Sciences [GM29963]
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Bacterial chemoreceptors, histidine kinase CheA, and coupling protein CheW form clusters of chemotaxis signaling complexes. In signaling complexes kinase activity is enhanced several hundredfold and placed under receptor control. Activation is necessary to poise enzyme activity such that receptor control has physiologically relevant effects. Thus kinase activation can be considered the underlying core activity of signaling complexes. We defined the minimal physical unit that generates this activity using chemoreceptor Tar from Escherichia coli rendered water soluble by insertion into nanodiscs to (i) measure saturable binding of CheA and CheW to the smallest kinase-activating groups of receptor dimers and (ii) purify and characterize core units of signaling complexes. Purified complexes activated kinase almost as well as signaling complexes formed on arrays of receptors in isolated native membrane. Purified complexes contained two receptor trimers of dimers and two CheW for each CheA dimer, consistent with the approximately 1: 1 CheA: CheW ratio determined by binding measurements. The 2: 2: 1 stoichiometry implied that CheA dimers, the enzymatically active form, connect two chemoreceptor trimers of dimers by interaction of one CheA protomer and a CheW with each trimer, an organization for which specific molecular interactions have previously been identified. The core unit associates six receptor dimers with a CheA dimer, providing sufficient capacity to account for much of the cooperativity and interdimer influence observed experimentally. We conclude that the 2: 2: 1 organization is the core structural and functional unit of chemotaxis signaling complexes and postulate that hexagonal arrays characteristic of signaling complexes are built from this unit.
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