Phototactic and chemotactic signal transduction by transmembrane receptors and transducers in microorganisms

Abstract

Microorganisms show attractant and repellent responses to survive in the various environments in which they live. Those phototaxic (to light) and chemotaxic (to chemicals) responses are regulated by membrane-embedded receptors and transducers. This article reviews the following: (1) the signal relay mechanisms by two photoreceptors, Sensory Rhodopsin I (SRI) and Sensory Rhodopsin II (SRII) and their transducers (HtrI and HtrII) responsible for phototaxis in microorganisms; and (2) the signal relay mechanism of a chemoreceptor/transducer protein, Tar, responsible for chemotaxis in E. coli. Based on results mainly obtained by our group together with other findings, the possible molecular mechanisms for phototaxis and chemotaxis are discussed.

Keywords: chemotaxis; phototaxis; rhodopsin; signal transduction; transducer.

Figure 1.

Light and chemical signal transfer cascades in microorganisms. Light stimulation activates sensory rhodopsins (SRs) and triggers trans-cis isomerization of the retinal chromophore. Relaxation of the retinal leads to functional processes during the photocycle. SRs transmit light signals to their cognate transducer proteins (Htrs) in the membrane. Htrs form a complex with CheA and CheW, and the complex activates phosphorylation cascades that modulate the autokinase protein, CheY and controls the direction of rotation of the flagellar motor. On the other hand, the cognate chemicals (attractant and repellent) bind to the extracellular domain of the chemoreceptors (MCP) and the binding induces the structural changes of MCP. The signal transfer pathway of MCP is thought to be similar to that of the phototaxis.

Figure 2.

Photochemical reaction cycles of SRII (A) and SRI (B). SRII absorbs blue light and forms K (K₅₄₀), L (L₄₈₈), M (M₃₉₀), and O (O₅₆₀) intermediates [53]. The M and O intermediates are thought to be active states. SRI absorbs orange light and forms K (K₆₂₀), L (L₅₄₀) and the long-lived M intermediates (M₃₇₃), which forms the P intermediate (P₅₂₀) upon the second photon absorption in the near-UV region [54].

Figure 3.

X-ray crystal structure of the SRII/HtrII complex (PDB code 1H2S) [17]. The structure of the ground state of SRII in the complex is very similar to that in the crystal structure of SRII alone. This structure reveals the formation of two specific hydrogen bonds between Tyr199^SRII and Asn74^HtrII and between Thr189^SRII and Glu43^HtrII/Ser62^HtrII. The membrane normally is roughly in the vertical plane of this image, and the top and bottom regions correspond to the extracellular and cytoplasmic sides, respectively.

Figure 4.

(A) Detail of the SRII-HtrII X-ray structure, which focuses on the midmembrane SRII-HtrII interface containing the core signal relay structure. SRII and HtrII are colored orange and blue, respectively. The numbers in the smaller font are the length between the respective amino acid residues. Using FTIR spectroscopy and photochemical techniques, we and another group reported that Thr199^SRII forms a hydrogen bond with Asn74^HtrII [56,79]. A functionally important residue, Thr204^SRII, forms a hydrogen bond with Tyr174^SRII. Tyr174^SRII is also essential for the phototaxis function [81]. (B) The structural change of the retinal chromophore upon formation of the K intermediate of SRII. All seven monodeuterated all-trans retinal analogues were synthesized, and the FTIR spectra were measured at 77 K. The enhanced C₁₄-D stretch in the K intermediate was assigned as the band originating from the local steric constraint between C₁₄-D and Thr204^SRII [75,80]. We reported that the band intensity correlated well with the phototaxis signaling efficiency, indicating its functional importance [80].

Figure 5.

Model for signal transduction mechanism of SRII-HtrII complex.

Figure 6.

(A) Spectral comparison of the HOOP vibrations of the retinal chromophore upon formation of the K intermediate. SrSRI_K minus SrSRI (a) and HsSRI_K minus HsSRI (b), difference infrared spectra measured at pH 7.0 and 8.5, respectively. The spectrum of HsSRI was deleted at < 872 cm⁻¹. The HsSRI_K minus HsSRI spectra are multiplied by 4.2 for the sake of comparison. The samples were hydrated with H₂O. The SRII_K minus SRII (c) and BR-T_K minus BR-T (d) difference FTIR spectra are reproduced from reference [70] and [83], respectively, for comparison. (B) Spectral comparison of the amide-I vibration upon formation of the M intermediate. SrSRI_M minus SrSRI (a) difference infrared spectra measured at 260 K at pH 7.0 in the 1,780–1,600 cm⁻¹ region. The samples were hydrated with H₂O (red) or D₂O (blue). The HsSRI_M minus HsSRI (b) and SRII_M minus SRII (c) difference FTIR spectra are reproduced from reference [54] and [72], respectively, for comparison. Adopted from reference [113].

Figure 7.

Putative chloride binding site of SrSRI (left, top view; right, side view). The structure was generated using a theoretical model of HsSRI (PDB ID: 1SR1) [121]. It was assumed that a positive charge located on the Schiff base nitrogen is likely to move to the b-ionone ring by chloride ion binding to His131.

Figure 8.

Structure of a dimeric bacterial chemoreceptor. (Left) Schematic illustration of the chemoreceptor. Each receptor monomer (∼60 kDa) consists of an N-terminal periplasmic ligand-binding domain, two transmembrane regions (TM1, TM2), a linker region and a C-terminal signaling/adaptation domain. Note that the HAMP domain structure is not integrated. (Center) Atomic structural model of the chemoreceptor generated by combining crystal structures of the fragments. The two symmetric subunits of the homodimer are shown as different colors. (Right) Deduced structure of the HAMP domain of the aspartate chemoreceptor Tar. Adopted from reference [140] and [145].

Figure 9.

Visualization and identification of chemoreceptor arrays. (a) Low-dose cryo-projection image of the polar region in a wild-type E. coli cell, with the chemoreceptor array shown in greater detail in the Inset. (b) Schematic representation of the polar region of a wild-type E. coli cell illustrating the assembly and orientation of the chemotaxis receptor array, based on a and b. (c) The trimer of dimers of the cytoplasmic fragment of Tsr as a crystal unit. Adopted from reference [162] and [164].

Figure 10.

Model of gain control by covalent modification of receptors. At a cell pole, the chemoreceptors form clusters that are made of trimer of dimer units. The cytoplasmic interdimer interaction within trimers of dimers is thought to be important for the signaling. Our results newly demonstrate that receptor dimers interact at the periplasmic tips and these units are organized into a well-defined array. Two-state models of the chemoreceptor function assume two extreme states: one activating and the other inactivating CheA (kinase ON and OFF states, respectively). The efficiency of cross-linking at a given position is highest when the amidation state of the protein is QQQQ (red), EEEE (blue), or QEQE (magenta). Cross-linking at position 36 (yellow) is not detectably changed by amidation. The observed cross-linking is consistent with the notion that methylation (amidation) counteracts the attractant binding; attractant binding favors the OFF state, whereas methylation favors the ON state. The results also suggest that receptor methylation restricts the rearrangement (rotation) of the dimers by attractant binding (denoted by the blue arrows for the demethylated state and the red arrows for the methylated state), leading to a smaller gain for the same input signal. Adopted from reference [168].