Structural Analysis of the dHax3

Structure of dHax3

A group of scientists from Tsinghua University have recombinantly produced the central domain (residues 270 to 703) of dHax3 (an artificially engineered TAL effector from the original Hax3) and have crystallised its structure in the DNA-free and DNA-bound state (Deng et al, 2012). The crystallised structures in both states were then analysed using Multi-wavelength Anomalous Diffraction (MAD) and X-ray Crystalography molecular replacement techniques with a program called PHASER (More information on the methods).

The crystallized protein complex of DNA-free dHax3 contains 11 TAL repeats arranged in a continuous right-handed, super helical structure with a 60Å external diameter.

Figure 2.1 Showing the 11 TAL repeats of dHax3 in the DNA-free state, each in different colour.

Every TALE subunit in dHax3 is made up of roughly 34 amino acid residues. The 3^rd to the 11^th residue of the polypeptide chain forms a short α helix (helix “a”) while the 15^th to the 33^rd residues form a longer, bent α helix (helix “b”). These two α helices are connected by a random loop (RVD loop) that contains the two repeat variable diresidues (residues 12 and 13) and a highly conserved Glycine at residue 14, which allows for extensive bending of the random loop region. Helices “a” and “b” are packed close together by van der Waals interaction. 

Figure 2.2 A TALE subunit (the 3rd repeat in dHax3) with b-factor colouring.

The 11 “a” helices in the dHax3 are folded into the inner layer along the super helical axis, while all the 11 “b” helices make up the external layer. The 11 TALE repeats in the dHax3 all share a high degree of conformational similarity and have a highly conserved amino acid sequence.