This family is a four helix bundle that operates as a ubiquitous sensory module in prokaryotic signal-transduction. The 4HB_MCP is always found between two predicted transmembrane helices indicating that it detects only extracellular signals. In many cases the domain is associated with a cytoplasmic HAMP domain suggesting that most proteins carrying the bundle might share the mechanism of transmembrane signalling which is well-characterised in E coli chemoreceptors. This domain recognises citrate and TCA cycle intermediates, cis-aconitate, boric acid, Phenanthrene, pyrene and benzopyrene (Matilla et el., FEMS Microbiology Reviews, fuab043, 45, 2021, 1. https://doi.org/10.1093/femsre/fuab043).

1: Four-helix bundle: a ubiquitous sensory module in prokaryotic signal transduction. Ulrich LE, Zhulin IB; Bioinformatics. 2005;21:iii45-iii48. PMID:16306392