Difference electron density map (Fo-Fc) contoured at 3.0 sigma at residue 80 for all three TelSAM domains.

<p>This was calculated in the early stages of model building, with valine built at position 80 for all three TelSAM domains. The excess density indicates which of the three domains has the glutamate mutation at position 80. The first TelSAM is colored in green, the second is colored in cyan, and the third is colored in magenta. The symmetry related molecule is colored in gray.</p

Symmetry based assembly of a 2 dimensional protein lattice

<div><p>The design of proteins that self-assemble into higher order architectures is of great interest due to their potential application in nanotechnology. Specifically, the self-assembly of proteins into ordered lattices is of special interest to the field of structural biology. Here we designed a 2 dimensional (2D) protein lattice using a fusion of a tandem repeat of three TelSAM domains (TTT) to the Ferric uptake regulator (FUR) domain. We determined the structure of the designed (TTT-FUR) fusion protein to 2.3 Å by X-ray crystallographic methods. In agreement with the design, a 2D lattice composed of TelSAM fibers interdigitated by the FUR domain was observed. As expected, the fusion of a tandem repeat of three TelSAM domains formed 2₁ screw axis, and the self-assembly of the ordered oligomer was under pH control. We demonstrated that the fusion of TTT to a domain having a 2-fold symmetry, such as the FUR domain, can produce an ordered 2D lattice. The TTT-FUR system combines features from the rotational symmetry matching approach with the oligomer driven crystallization method. This TTT-FUR fusion was amenable to X-ray crystallographic methods, and is a promising crystallization chaperone.</p></div

The observed 2D crystal packing.

<p>One set of TTT domains are shown in red shades, and the second set is shown in blue shades. The n-FUR domains are shown in green and yellow. (A) Top-down view of the TelSAM oligomers. (B) Side view of the TelSAM oligomers. Within the 2D lattice, the interactions between neighboring TelSAM fibers are entirely mediated by the n-FUR domains. Additional figures for the crystal packing are included in the supplemental materials.</p

Graphic representation of the observed 2D lattice shown in Fig 8B.

<p>The n-FUR domains formed a 2₁ screw axis instead of a strict 2-fold axis, as shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0174485#pone.0174485.g001" target="_blank">Fig 1</a>. The color scheme is the same as used in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0174485#pone.0174485.g001" target="_blank">Fig 1</a>.</p

Data collection and refinement statistics^*.

<p>Data collection and refinement statistics^{<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0174485#t001fn001" target="_blank">*</a>}.</p

The observed 3 dimensional crystal packing, with a top-down view of the TelSAM oligomers.

<p>The TelSAM domains are shown in yellow, red, and blue, and the FUR domain is shown in green. The symmetry related molecules are shown in gray. The unit cell is shown in yellow. The crystal packing can be described as stacking of 2D crystals. The arrow between the red lines points out the 2D lattice. This view is looking down the Y axis. The unit cell dimensions are (131.3, 54.8, 88.5) Å, and the angles are (90.0°, 128.5°, 90.0°)</p

Precipitation curve.

<p>The effect of pH and cadmium on solubility. In the absence of cadmium (black squares), protein solubility is pH dependent. In the presence of cadmium (red circles), the protein is always oligomeric.</p

Design of the 2D lattice.

<p>The 2-fold symmetry axis for the n-FUR domain is shown to be able to adopt multiple conformations. In (A) the 2-fold axis is shown parallel to the TelSAM helical axis, and (B) shows how the n-FUR domain can adopt a different orientation. The TelSAM oligomer is shown as a blue ribbon. The n-FUR domains are shown in green and yellow, and the flexible linker is shown in red.</p

Pictures of the TTT-FUR crystals.

<p>Pictures of the TTT-FUR crystals.</p