Methyl-accepting chemotaxis protein (MCP) is a transmembrane sensor protein of bacteria. Use of the MCP allows bacteria to detect concentrations of molecules in the extracellular matrix so that the bacteria may smooth swim or tumble accordingly. If the bacteria detects rising levels of attractants (nutrients) or declining levels of repellents (toxins), the bacteria will continue swimming forward, or smooth swimming. If the bacteria detects declining levels of attractants or rising levels of repellents, the bacteria will tumble and re-orient itself in a new direction. In this manner, a bacteria may swim towards nutrients and away from toxins.

Structure

The MCP consists of a transmembrane receptor and a helix hairpin structure in the cytoplasmic side. The helix hairpin contains a region rich in glutamate that functions as a means of adaptation by methylation or demethylation of the glutamate residues.

MCP's form dimers. Three dimers of MCP are held together by CheY to form hexamers. Hexamers are held together by CheA to form lattices.

Function

Binding a ligand causes a conformational change in the MCP receptor which translates down the hairpin structure. At the tip of the hairpin are two proteins that associate to the MCP: CheW and CheA. CheA acts as the sensor kinase. CheA has kinase activity and autophosphorylates itself on a histidyl residue with activated by the MCP. CheW is believed to be a transducer of the signal from the MCP to CheA. Phosphorylated CheA is active so it phosphorylates CheY, the activator response regulator. Phosphorylated CheY is active so it phosphorylates the basal body which is connected to the flagellum. Phosphorylation of the basal body acts as a flagellar switch and changes the direction of rotation of the flagellum. This change in direction allows for alternation between smooth swim and tumbling.