TdfH selectively binds metal-loaded tetrameric calprotectin for zinc import

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Expression and purification of CP and TdfH

Calprotectin (CP) was once ready within the absence of calcium or zinc as in the past reported and frozen in aliquots at −80 °C^15,17. TdfH was once expressed in inclusion our bodies and refolded as described in the past with some adjustments¹⁶. In short, the tdfH gene was once subcloned into the pHIS2 plasmid and expressed in BL21(DE3) cells, generating TdfH with an N-terminal 6x His-tag, TEV web site, and twin-strep (TS) tags. The cells have been harvested, lysed and the lysate was once centrifuged at 7000 × g for 20 min at 4 °C to spin down the inclusion our bodies. The inclusion physique pellet was once washed thrice with 1x PBS supplemented with 1% Triton X-100 and 5 mM ethylenediaminetetraacetic acid (EDTA) pH 7.4, one time with 3 M urea in 1x PBS, and two occasions with 1x PBS with 5 mM EDTA pH 7.4 the use of a dounce homogenizer.

To refold TdfH, washed inclusion our bodies have been resuspended to five–10 mg/mL in 8 M urea containing 25 mM β-mercaptoethanol (BME) in a dounce homogenizer and supplemented with 0.5% sarkosyl. This was once combined for 15 min at room temperature after which centrifuged for 15 min at 32,000 × g. The supernatant was once then diluted 60% in refolding buffer [20 mM Tris-HCl pH 8.0, 200 mM NaCl, 10% glycerol, and 0.17% n-Dodecyl-B-d-Maltoside (DDM)] and dialyzed in a single day at 4 °C towards a 20x quantity of 1x PBS pH 7.4. The dialyzed pattern was once centrifuged at 32,000 × g for 15 minutes at 4 °C and extra purified the use of immobilized steel affinity chromatography the use of a Ni-NTA column connected to an Atka Air purifier (GE Healthcare). Height fractions have been verified via SDS-PAGE and the fractions containing TdfH have been blended and the His-tag got rid of. TdfH was once additional purified the use of a Superdex 200 Building up 10/300 GL column (GE Healthcare) in 1x PBS, pH 7.4, 0.02% lauryl maltose neopentyl glycol (LMNG).

Small-scale pull-down assays measuring CP binding to TdfH

To decide the optimum prerequisites for complicated formation, TdfH in 1x PBS, pH 7.4, 0.02% LMNG was once first sure to Streptavidin MagneSphere Paramagnetic Debris (Promega) and washed in 1x PBS, pH 7.4, 0.02% LMNG. CP was once then added below the next prerequisites: (i) 5 mM EDTA (ii) 200 μM CaCl₂, (iii) 10 μM ZnCl₂, (iv) incubated first with 200 μM CaCl₂, then with 10 μM ZnCl₂, (v) incubated first with 10 μM ZnCl₂, then with 200 μM CaCl₂, and (vi) concurrently with 10 μM ZnCl₂ and 200 μM CaCl₂. Finally, the debris have been washed, then the complexes have been eluted with 50 mM biotin and analyzed via SDS-PAGE.

Massive-scale complicated formation of TdfH-CP for cryoEM

For enormous-scale complicated formation, refolded TdfH in 1x PBS, pH 7.4, 0.02% LMNG was once carried out and the flowthrough reapplied to a 1 mL StrepTactinXT (IBA Lifesciences) column connected to an Akta FPLC computerized purification device (GE Healthcare). The column was once washed after which CP (preincubated with CaCl₂ and ZnCl₂) was once carried out and the flowthrough reapplied 2x, the column was once washed once more, after which the complicated was once eluted the use of pattern buffer supplemented with 3 mM desthiobiotin and the fractions analyzed via SDS-PAGE. Fractions containing the TdfH-CP complicated have been then pooled and concentrated to ~4 mg/mL. One proscribing issue of this find out about is our incapacity to totally saturate CP with zinc, one thing that become evident throughout our cryoEM research. CP has a top propensity to precipitate out of answer earlier than even achieving a 1:1 (zinc:CP) ratio; a ratio of two:1 can be had to totally saturate CP. From research of our cryoEM effects, this ends up in heterogeneity of the complicated and bounds the selection of zinc saturated TdfH-CP debris that may be analyzed.

CryoEM knowledge acquisition, research, and symbol processing

For grid preparation, the TdfH-CP complicated (1.5–3 mg/mL) was once carried out to glow-discharged (Easiglow, Pelco) Quantifoil R 3.5/1 Cu-200 mesh grids and plunge frozen the use of a ThermoScientific Vitrobox Mark IV. The cryoEM knowledge have been accumulated on a Titan Krios microscope (ThermoFisher) operated at 300 kV with a nominal magnification of 81,000x the use of a K3 direct electron detector (Gatan) operated in super-resolution counting mode the use of Leginon for computerized knowledge assortment. The photographs have been recorded at a defocus vary of −1.5 to −2.5 μm, with a calibrated bodily pixel measurement of 0.54 Å/pixel, with a complete dose of 44.75 e⁻/Å²; a complete of ~3300 motion pictures have been accumulated.

For symbol processing, movement correction of the films was once carried out at the fly the use of MotionCor2¹⁸ with a binning issue of two carried out inside of RELION-3¹⁹. The usage of Cryosparc²⁰, pictures have been filtered according to CTF-resolution are compatible, leaving ~2500 pictures for preliminary blob choosing. Preliminary templates have been ready via iterative rounds of 2D classification, which have been then used for template choosing, generating an preliminary ~2.5 million debris. Additional filtering via interactive rounds of 2D classification yielded ~120,000 debris of best and aspect perspectives, according to transparent options together with the presence of the detergent micelle. A couple of three-D categories have been ready via ab initio modeling adopted via heterogeneous refinement, with just a unmarried three-D elegance (~63,000 debris) having options in keeping with a membrane protein complicated inside of a detergent micelle. The three-D classification was once iterated to additional filter out the debris till a homogenous three-D elegance was once attained. Homogenous refinement was once then carried out, adopted via non-uniform refinement and after all, native refinement to provide the general 6.1 Å reconstruction of the TdfH-CP complicated. All through the three-D classification, a majority of the debris seemed to be TdfH-only; alternatively, makes an attempt to procedure those additional was once unsuccessful because of heterogeneity of the form of the barrel and the loss of outlined options out of doors of the micelle. We hypothesize that this can be because of the higher flexibility of the barrel and loops of TdfH within the absence of CP.

For style development, the manganese-bound CP construction (PDB ID 4GGF) and our in the past reported homology style for TdfH¹⁶ have been are compatible into the map the use of ChimeraX²¹. For TdfH, the loops have been all got rid of and manually traced into the visual density to be had. Given the decision and loss of enough density, now not all residues have been modeled, which is in keeping with different identical buildings the place most of the massive extracellular loops are anticipated to be in part or totally disordered because of flexibility. The overall style was once delicate the use of real-space refinement, similtaneously acting rigid-body refinement of all of the person chains with secondary construction restraints. All style development was once carried out the use of COOT²² and real-space refinement was once carried out the use of PHENIX²³. A better decision cryoEM construction is had to unambiguously localize all the residues of TdfH.

Small-scale pull-down assays measuring the impact of metals on CP binding to TdfH

To decide the impact of metals rather then zinc on complicated formation between CP and TdfH, TdfH in 1x PBS, pH 7.4, 0.02% LMNG was once first sure to Streptavidin MagneSphere Paramagnetic Debris (Promega) and washed in 1x PBS, pH 7.4, 0.02% LMNG. CP was once then added below the next prerequisites: (i) 5 mM EDTA, (ii) 200 μM CaCl₂ and 10 μM of CoCl₂, (ii) 200 μM CaCl₂ and 10 μM of CuCl₂, (iv) 200 μM CaCl₂ and 10 μM of MgCl₂, (v) 200 μM CaCl₂ and 10 μM of MnCl₂, and (vi) 200 μM CaCl₂ and 10 μM of ZnCl₂. Finally, the debris have been washed, then the complexes have been eluted with 50 mM biotin and analyzed via SDS-PAGE.