Purification of Histidine-tagged Proteins

The twenty-four single recombinant proteins were expressed as previously described (Table S2). All recombinant proteins contain a cluster of six histidine residues at the N- and C-termini. The proteins were purified using one-step metal affinity chromatography with Ni²⁺ bound to iminodiacetic acid-agarose (Merck, KGaA, Darmstadt, Germany). The purification resulted in electrophoretically homogeneous protein preparations with a purity above 90% (Figure S7). The concentration of purified proteins was determined using Bradford reagent (Merck, KGaA, Darmstadt, Germany) according to the manufacturer’s recommendation.