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. 2019 Oct 25;5:32. doi: 10.1038/s41522-019-0106-5

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 3 — Chemotaxis in the squid–vibrio symbiosis. a Representative methyl-accepting chemotaxis protein (MCP) topology, including the ligand-binding domain, the HAMP domain, and methylation domain. Triangles represent potential ligands. b Simplified scheme of the chemotaxis signal-transduction cascade in Vibrio spp. Changes in chemoeffector levels are detected by the transmembrane MCP chemoreceptors. The resulting signal cascade starts with the SH3-like adaptor protein CheW, and passes to the CheA histidine kinase. In response to decreased chemoeffector concentration, the chemoreceptors activate CheA autophosphorylation. Phosphorylated CheA (CheA-P) phosphorylates CheY. CheY-P binds to the flagellar motor and promotes a switch in the direction of rotation from counterclockwise to clockwise. Finally, CheZ dephosphorylates CheY-P, allowing rapid termination of the signal response and re-setting of the pathway. In the absence of chemoeffector, CheA-P favors counterclockwise rotation. c Chemoeffectors secreted by the squid tissues establish a gradient (blue) extending out from the pores.^60,77 V. fischeri cells are attracted by this gradient, and migrate through the pores to their final site of colonization, the deep crypts