Structure of a conserved receptor domain that regulates kinase activity: the cytoplasmic domain of bacterial taxis receptors

Abstract

Many bacteria are motile and use a conserved class of transmembrane sensory receptor to regulate cellular taxis toward an optimal living environment. These conserved receptors are typically stimulated by extracellular signals, but also undergo adaptation via covalent modification at specific sites on their cytoplasmic domains. The function of the cytoplasmic domain is to integrate the extracellular and adaptive signals, and to use this integrated information to regulate an associated histidine kinase. The kinase, in turn, triggers a cytoplasmic phosphorylation pathway of the two-component class. The high-resolution structure of a receptor cytoplasmic domain has recently been determined by crystallographic methods and is largely consistent with a structural model independently generated by chemical studies of the domain in the full-length, membrane-bound receptor. These results represent an important step toward a mechanistic understanding of receptor-to-kinase information transfer.

Figure 1

Schematic model of the chemotaxis receptor structure, including the cytoplasmic domain architecture established by recent chemical and crystallographic studies [1••,2••]. The different structural and functional regions are highlighted, including the adaptive methylation sites (filled circles). The aspartate receptor possesses four such sites per subunit, whereas the serine receptor includes a fifth site (not shown) per subunit. The two subunits of the dimer are shown in red and gold. HP, helical hairpin; TM, transmembrane helix.

Figure 2

Schematic structure of the dimeric aspartate receptor cytoplasmic domain determined by chemical methods (adapted from [2••]), illustrating the 14 functional, symmetric, intersubunit disulfide bonds (dashed lines). The architecture is a four-helix bundle, with one subunit shown in red and the other in gold; the spacing between helices is exaggerated for clarity. Lock-on disulfide bonds that constitutively activate the kinase are indicated by blue spheres and italicized labels; signal-retaining disulfides that allow normal ligand-induced kinase regulation are denoted by white spheres and roman labels. Faded spheres indicate positions completely hidden from view. Adaptive methylation sites are indicated by black spheres and positions implicated in kinase docking are denoted by black squares. Breaks in helices indicate regions where helix continuity has not been confirmed by chemical analysis, in most cases because the region has not yet been cysteine scanned.

Figure 3

Ribbon diagram illustrating the dimer structure of the serine receptor cytoplasmic domain (adapted from [1••]). Major adaptive methylation sites are shown as yellow spheres in one subunit and blue spheres in the other. One subunit is in purple and the other is cyan.

Figure 4

Model of an intact serine receptor dimer spanning the membrane (adapted from [1••]). (a) Ribbon diagram of the model, scaled to match the dimensions in (b). One subunit is in purple and the other is in cyan. Methylation sites are marked by yellow spheres and the ligand serine is drawn as a red sphere (partially hidden at upper left corner). (b) Schematic diagram of the model denoting one subunit in cyan and the other in purple. The presumed location of the membrane bilayer is represented by a gray band. Landmark residues are indicated by arrows and the number of residues in each helical section is specified.

Figure 5

Trimer formation by three dimeric cytoplasmic domains of the serine receptor (adapted from [1••]). Stereo diagram of the trimer of dimers, in which each monomer is colored differently. The methylation sites are shown as small spheres.