quorum-sensing protein SdiA
Bacteria communicate with each other, just like we human do. One way of bacterial communication is quorum sensing. Many bacteria generate substances called autoinducer, and release it to outside the cell. As the number of bacteria increases, the concentration of autoinducer also increases. When the concentration of autoinducer exceeds a certain threshold, autoinducer receptor protein binds the autoinducer and the complex transactivate a specific region of genes. This system can be perceived as a strategy of bacteria: They do not act against the great enemy (immune system, etc.) when they are lonely; only when they gather up to some population, they express their function (releasing toxins, etc.) all at once against the enemy. The kind of autoinducers and related proteins may vary depending on the species of bacillus. For example, many gram negative bacteria adopt AHL (N-acyl-L-homoserine lactone) as autoinducer (Fig.1). Also they have LuxI family protein as generator of AHL and LuxR family protein as receptor of AHL. SdiA of Escherichia coli is LuxR family protein, and it receives C8-HSL (N-octanoyl-L-homoserine lactone) as aoutoinducer. Without C8-HSL, SdiA can not fold into the native structure. In other words, C8-HSL has a role of folding-switch which leads SdiA to the correct structure. It turns out that SdiA/C8-HSL complex has a transfer activity of the flsQAX operon of a cell division equipment gene. However, E. coli do not have autoinducer generating proteins like LuxI family. That is, it is expected that SdiA of E.coli is the protein that recognizes and receives autoinducer from heterogeneous bacteria. In fact, it was reported that homoserine lactones, such as C6-HSL, 3-oxo-C8-HSL also work as a folding switch of SdiA. SdiA is regarded as an important protein that makes possible the communication between heterogeneous bacteria.
The structure displayed here is N-terminal domain of SdiA(1-171) complex with C8-HSL. (Fig.2). It has an α/β/α-sandwich fold; five anti-parallel β strand is sandwiched by five α-helices. This is very similar to Trar protein which is the autoinducer receptor of Ti-plasmid. The fold is stabilized by hydrophobic interaction between α2 of N-terminus and α5 of C-terminus. C8-HSL resides in the hydrophobic pocket surrounded by α3, α4, β-sheet, and G1(3-10 helix), and it form core of native structure of SdiA.
Protein Data Bank (PDB)
Yao, Y. Martinez-Yamout, M.A. Dickerson, T.J. Brogan, A.P. Wright, P.E. Dyson, H.J.; "Structure of the Escherichia coli quorum sensing protein SdiA: activation of the folding switch by acyl homoserine lactones."; J.Mol.Biol.; (2006) 355:262-273 PubMed:16307757.
author: Jun-ichi Ito