ToxT

Ligand

In this resolved structure, [1] is present and bound in the beta sheet barrel (further discussed below). This unsaturated fatty acid reduces virulence expression in Vibrio cholerae.

Other Structural Features

DNA-binding. ToxT belongs to a family of transcriptional regulators headed by and known as AraC.^[1] The AraC family is characterized by a 100 amino acid region of sequence similarity that forms a with two helix-turn-helix motifs (one on either side of the black linker). ^[3] The two HTH regions are linked by a very polar alpha helix (shown in black). The overall domain is located at the C-terminus.^[1] Assuming ToxT is similar in mechanism to other AraC proteins, helix six from HTH1 and helix nine from HTH2 become aligned with the help of helix seven (the polar linking region) to allow binding to major consecutive grooves of target DNA.^[1]. The conformation of helix seven is dependent on the ligand bound.

Ligand-binding. or "jelly-roll" with three other alpha helices (overall making up the ) contain a . This is largely made up of residues from the N-terminus (Y12, Y20, F22, L25, I27, K31, F33, L61, F69, L71, V81, and V83), and a few from the C-terminus (I226, K230, M259, V261, Y266, and M269). The pocket is highly hydrophobic, and has a known volume of 780.9 Angstroms.^[1] This pocket contains a ligand: ^[1] which appears to have a negative effect on virulence when present in vitro. The cis-palmitoleate forms with residues K31 and K230 (for detail, see Figure 1B of: [2]). This unsaturated fatty acid, like other UFAs,[3] tend to inhibit genes under the control of ToxT. The Specifically, the cis-palmitoleate appears to bind directly to ToxT, change its conformation, and thus lower the ability to bind DNA and form dimers.^[1] The presence of UFAs is associated with being in the lumen of the intestine during the bacterial infection. Here, binding of PAM brings K31 and K230 together and essentially closes off ToxT. K230 is at the end of helix seven, and binding to K31 causes pulling helix six into an unfavorable conformation that disallows DNA binding. In lower concentration of fatty acids, ie: after penetrating the intestine's mucus, the two lysine residues repel each other and give ToxT an open conformation by giving helix seven and thus six freedom, allowing DNA binding.^[1]

Dimerization. Though the structure shown is a monomer with two overall domains (N-terminal and C-terminal), ToxT tends to form a dimer.^[4] The preferred state of ToxT varies between promoters, but binding to the ctx promoter to generate cholera toxin appears to be possible only in the dimer form.^[5]ToxT binds to thirteen base pair sequences (can be single, direct, or inverted repeats) called toxboxes in order to activate their respective promoters.[4]