The HBM sensor domain has been identified primarily in bacterial chemoreceptors but is also present on histidine kinases. Characteristic features of this domain are its size of approximately 250 amino acids and its location in the bacterial periplasm [1]. The McpS chemoreceptor of Pseudomonas putida KT2440 was found to possess an HBM sensor domain and its 3D structure in complex with physiologically relevant ligands has been reported [2]. This domain is composed of 2 long and 4 short helices that form two modules each composed of a 4-helix bundle. The McpS chemoreceptor mediates chemotaxis towards a number of organic acids [1,3]. Both modules of the McpS HBM domain contain a ligand binding site. Chemo-attractants binds to each of these sites and their binding was shown to trigger a chemotactic response [4]. This domain is primarily found in different proteobacteria but also in archaea. Interestingly, amino acids in both ligand binding sites showed a high degree of conservation suggesting that members of this family sense similar ligands. This domain recognises Multiple TCA cycle intermediates, citrate and alpha-ketoglutarate (Matilla et.al., FEMS Microbiology Reviews, fuab043, 45, 2021, 1. https://doi.org/10.1093/femsre/fuab043).

1: Identification of a chemoreceptor for tricarboxylic acid cycle intermediates: differential chemotactic response towards receptor ligands. Lacal J, Alfonso C, Liu X, Parales RE, Morel B, Conejero-Lara F, Rivas G, Duque E, Ramos JL, Krell T; J Biol Chem. 2010;285:23126-23136. PMID:20498372

2: Physiologically relevant divalent cations modulate citrate recognition by the McpS chemoreceptor. Lacal J, Garcia-Fontana C, Callejo-Garcia C, Ramos JL, Krell T; J Mol Recognit. 2011;24:378-385. PMID:21360620

3: Evidence for chemoreceptors with bimodular ligand-binding regions harboring two signal-binding sites. Pineda-Molina E, Reyes-Darias JA, Lacal J, Ramos JL, Garcia-Ruiz JM, Gavira JA, Krell T; Proc Natl Acad Sci U S A. 2012;109:18926-18931. PMID:23112148

4: The HBM domain: Introducing bimodularity to bacterial sensing. Ortega A, Krell T; Protein Sci. 2013; [Epub ahead of print] PMID:24347303