Chemotactic-signal transducers respond to changes in the concentration of attractants and repellents in the environment, transduce a signal from the outside to the inside of the cell, and facilitate sensory adaptation through the variation of the level of methylation. McpS is a specific chemoreceptor for 6 tricarboxylic acid (TCA) cycle intermediates (succinate, fumarate, malate, oxaloacetate, citrate and isocitrate), butyrate and acetate. Malate, succinate, fumarate and oxaloacetate cause the strongest chemotactic response. Binding of citrate to the ligand-binding domain reduces the chemotaxis towards the strong attractants such as malate and succinate. However, in physiologically relevant niches, citrate is mostly complexed with magnesium or calcium ions, and does not bind McpS. Homodimer. Exists as a mixture of monomers and dimers in solution. Ligand binding stabilizes the dimeric form. Methylated by CheR2. Mutant fails to respond to succinate but responds to toluene. Belongs to the methyl-accepting chemotaxis (MCP) protein family.