Atypical chemoreceptor arrays accommodate high membrane curvature

Abstract

The prokaryotic chemotaxis system is arguably the best-understood signaling pathway in biology. In all previously described species, chemoreceptors organize into a hexagonal (P6 symmetry) extended array. Here, we report an alternative symmetry (P2) of the chemotaxis apparatus that emerges from a strict linear organization of the histidine kinase CheA in Treponema denticola cells, which possesses arrays with the highest native curvature investigated thus far. Using cryo-ET, we reveal that Td chemoreceptor arrays assume an unusual arrangement of the supra-molecular protein assembly that has likely evolved to accommodate the high membrane curvature. The arrays have several atypical features, such as an extended dimerization domain of CheA and a variant CheW-CheR-like fusion protein that is critical for maintaining an ordered chemosensory apparatus. Furthermore, the previously characterized Td oxygen sensor ODP influences CheA ordering. These results suggest a greater diversity of the chemotaxis signaling system than previously thought.

Alternative Architecture of the E. coli Chemosensory Array

“…The question thus emerges: what are the molecular origins of the p2 symmetric architecture seen in this study? Interestingly, a recent publication by Muok et al describes a p2 chemosensory array organisation in the spirochete Treponema denticola [45], which exhibits a linear CheA arrangement, including rings involving interactions between a classical CheW and a spirochete-specific CheW variant that are analogous to the (A.P5/W/W) 2 rings seen in our model. Due to the orientation of the linear CheA strands, which always appear to run parallel to the cell axis, and because of the very high curvature of these cells perpendicular to the cell axis, the authors propose that the array organisation seen in T. denticola evolved specifically to accommodate the spirochetes' high curvature.…”

Studying bacterial chemosensory array with CryoEM

“…V. cholera (PM7, pPM045, PM15, PM16, PM049, PM22) EMD-3398 [49] H. pylori (7.13) EMD-8460 [23] In vitro construction of Tar in nanodisc [15] S. enterica minicell (TH16943) [50] Lysed V. cholerae (C6706) [22] A. brasilense [24] Lysed E. coli (RP437) EMD-4991, EMD-4992, EMD-4993 [33] In vitro reconstitution of chemotaxis 2D array EMD-10050, PDB-6S1K [18] E. coli minicells (WM4196) EMD-10160 [28] T. denticola (Td) ATCC 35405 EMD-11381, EMD-11384, EMD-11385, EMD-11386 [25] V. cholerae (C6706, PM6, PM7), P. aeruginosa (PAO1), S. oneidensis (MR-1), and M. alcaliphilum (20Z R ) [51] E. coli minicell (WM4196) [37] which has a kinase-ON conformation; and EEEE, which has a kinase-OFF conformation [17]. Using cryoET and subtomogram averaging (STA), where repeating subvolumes, such as the CSUs within an array, are aligned and averaged, a 3D density map of the TarCF-QEQE CSU was reconstructed at 11.3 Å resolution [17], a significant improvement from previous attempts [19][20][21].…”

“…This assembling was also reported in in vitro construction of 2D monolayer arrays and lysed E. coli cells [ 17 ]. Recently, an alternative P2 packing with a unit cell containing a single CSU has been reported from native Treponema denticola [ 25 ] and an E. coli minicell [ 37 ] (see Figure 3E , right). Although the average map of CSU from the alternative packing was not reported, its basic structure appears to be conserved.…”

“…Some bacterial cells are naturally thin and suitable for cryoET analysis on their own. Spirochetes, such as Leptospira, Treponema, and Borrelia , have a slim and elongated body shape with 200–300 nm diameter, making them good candidates for structural studies of chemosensory arrays by cryoET [ 25 , 41 ]. However, the chemotaxis machinery in spirochetes is more complicated and less-well studied than that of E. coli .…”

“…In recent years, high-resolution cryo-electron microscopy (cryoEM), especially cryo-electron tomography (cryoET) have provided potent approaches to study the organization of the chemosensory array both in vitro and in vivo. The structure and function of chemoreceptors and signaling arrays have been studied in a variety of systems (Table 1), including nanodiscs [7,15,16], lipid monolayer arrays [17,18], bacterial minicells [19,20], lysed bacterial ghosts [21,22] and intact native bacterial cells [1,20,[23][24][25]. Great strides have been made in understanding the architecture of the chemosensory array and the detailed structure of the CSU.…”