Volume 25, Issue 9, Pages 1449-1454.e3 (September 2017) A Mutation Directs the Structural Switch of DNA Binding Proteins under Starvation to a Ferritin-like Protein Cage  Sunanda Margrett Williams, Anu Vijayakumari Chandran, Sunita Prakash, Mamannamana Vijayan, Dipankar Chatterji  Structure  Volume 25, Issue 9, Pages 1449-1454.e3 (September 2017) DOI: 10.1016/j.str.2017.07.006 Copyright © 2017 Elsevier Ltd Terms and Conditions

Structure 2017 25, 1449-1454.e3DOI: (10.1016/j.str.2017.07.006) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 1 Biological Assembly of the Ferritin-like Proteins MsDps1 dodecamer (A), frog M ferritin (3RBC) (B), and F47E-Dps1 24-mer (C), viewed down the ferritin-like three fold. See also Figures S2 and S3. Structure 2017 25, 1449-1454.e3DOI: (10.1016/j.str.2017.07.006) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 2 Helical Loops Near the Folding Interface (A) Interactions involving F47, AB, and BC helical loop residues. F47 is colored green, residues of AB loop yellow, and BC loop orange. Hydrogen bonding interactions are shown by dashed lines. van der Waals contacts are represented by dots. Only interactions of the residues of the AB loop with BC loop are shown for clarity. (B) The loss of interactions between AB and BC loops in the mutant structure. E47 is colored magenta, and residues of AB and BC loops are yellow and orange, respectively. (C) Superposition of the loop regions of native (green) and mutant (magenta) protein monomers. AB loop is “flexible” as opposed to “rigid” and BC loop is untethered in the mutant. The C-terminal tail residues of the mutant are disordered, and the last perceptible residue is 157. Structure 2017 25, 1449-1454.e3DOI: (10.1016/j.str.2017.07.006) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 3 Fitting a Bent AB Turn (A) Superposition of dps-like trimeric interface composed of native and mutant monomers. AB loop of the native is colored green and F47E-Dps1 magenta. A bent AB loop in the mutant clashes with residues of the neighboring helix in a trimeric arrangement and therefore is not compatible with trimer formations. (B) The mutant subunits with a “flexible” AB loop (magenta) are properly placed in a tetrameric arrangement. (C) Superposition of tetrameric interfaces made of MsDps1 and frog M ferritin monomers. A straight AB loop (green) in the native is too close to the position of E helix (blue), whereas a bent AB loop (blue) flexes away from the E helix making space for its packing in the center. The regions showing clashes are highlighted in black dotted circles and magnified. A red star indicates close contacts. The building of a trimeric interface with F47E-Dps1 subunits (A) and tetrameric interface with MsDps1 subunits (C) is hypothetical. See also Figure S4. Structure 2017 25, 1449-1454.e3DOI: (10.1016/j.str.2017.07.006) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 4 Proposed Model for Assembly of Ferritin-like Proteins Top: a four-helix monomer with a “rigid” AB loop and a C-terminus folded on itself associating via dps-like trimeric interfaces and forming a 12-mer. Bottom: monomers with a “flexible” AB loop and C-terminal tail at an anterior position folds via tetrameric interface into a 24-mer. Structure 2017 25, 1449-1454.e3DOI: (10.1016/j.str.2017.07.006) Copyright © 2017 Elsevier Ltd Terms and Conditions