Article Figures & Data
Figures
- FIG. 1.
Schematic diagram of the Tsr homodimer. One of the two subunits is designated as prime (′). The positions of the polar headgroups of the cytoplasmic membrane are indicated in gray. The four helices of the periplasmic domain are labeled α1 through α4, and the two transmembrane helices are shown as TM1 and TM2. HAMP is indicated by H, and the two long helices that participate in the extended four-helix bundle of the cytoplasmic domain are labeled CD1 and CD2. The curved black line represents the unstructured C-terminal region. Serine is depicted as a black oval bound at the periplasmic dimer interface. The two arrows indicate that serine can bind at either of two rotationally symmetrical sites.
- FIG. 2.
HAMP structure. (A) The top drawing shows a linear representation of HAMP. The precise boundary between TM2 and AS1 is not known; the boundary shown is based on the Af1503 HAMP structure (11). Beneath that, the Tsr and Tar amino acid sequences are shown, using the same color scheme as in the upper diagram. Identical residues are shown in boldface type. Residues in the Tsr connector that are susceptible to replacements that destroy or impair receptor function are underlined. (B) Ribbon diagram of the four-helix bundle of the Tsr HAMP domain modeled on the nuclear magnetic resonance structure from Af1503. The color scheme is the same as in panel A. (C) Space-filling model of one Tsr HAMP subunit. In the connector, only the residues at which substitutions conferred strong chemotaxis phenotypes are labeled. The color scheme is the same as in panel A. (Images in B and C are courtesy of J. S. Parkinson.)
- FIG. 3.
Two models for Tsr HAMP function. The diagram at the upper left is based on data reported by Williams and Stewart (20). In the “on” (CW) conformation of the receptor, the AS1 and AS1′ helices are oriented nearly parallel to, and embedded in, the inner face of the cell membrane, with the hydrophobic face of the helix in contact with the hydrophobic interior of the membrane and the hydrophilic face in contact with the cytoplasm or the phospholipid head groups (indicated in gray). The diagram at the upper right is based on the data described previously by Hulko et al. (11). In the “on” conformation, the four-helix bundle is in one of its structurally stable arrangements, with either knob-on-knob or knob-in-hole packing. The bottom diagram shows the receptor in an “off” (CCW) configuration. One of the TM2 helices is pushed down slightly, and HAMP is in the “off” conformation. The four-helix bundle has rotated 26° from the hypothetical “on” state shown at the upper right.
