High-resolution structural insights on the sugar-recognition and fusion tag properties of a versatile β-trefoil lectin domain from the mushroom Laetiporus sulphureus

In this work, we analyzed at high resolution the sugar-binding mode of the recombinant N-terminal ricin-B domain of the hemolytic protein LSLa (LSL150) from the mushroom Laetiporus sulphureus and also provide functional in vitro evidence suggesting that, together with its putative receptor-binding role, this module may also increase the solubility of its membrane pore-forming partner. We first demonstrate that recombinant LSL150 behaves as an autonomous folding unit and an active lectin. We have determined its crystal structure at 1.47 A resolution and also that of the [LSL150:(lactose)beta, gamma)] binary complex at 1.67 A resolution. This complex reveals two lactose molecules bound to the beta and gamma sites of LSL150, respectively. Isothermal titration calorimetry indicates that LSL150 binds two lactoses in solution with highly different affinities. Also, we test the working hypothesis that LSL150 exhibits in vivo properties typical of solubility tags. With this aim, we have fused an engineered version of LSL150 (LSLt) to the N-terminal end of various recombinant proteins. All the designed LSL150-tagged fusion proteins were successfully produced at high yield, and furthermore, the target proteins were purified by a straightforward affinity procedure on agarose-based matrices due to the excellent properties of LSL150 as an affinity tag. An optimized protocol for target protein purification was devised, which involved removal of the LSL150 tag through in-column cleavage of the fusion proteins with His(6)-tagged TEV endoprotease. These results permitted to set up a novel, lectin-based system for production and purification of recombinant proteins in E. coli cells with attractive biotechnological applications.