De novo design of pH-responsive self-assembling helical protein filaments

Computational design technique

Brief loops to attach pRO-2.3 helices right into a single chain have been designed utilizing an exhaustive database of spine samples composed of fragments spanning two helical areas as recognized by DSSP in high-resolution crystallographic constructions (as described beforehand¹⁴). Loops have been recognized on this database by way of inflexible alignment of the terminal residues of the fragment and goal utilizing an optimized superposition algorithm¹⁵. Candidates that met an alignment tolerance of 0.35 Å RMSD have been aligned to the goal spine by way of torsion–area coordinates and smooth coordinate constraints to the aligned candidate spine heavy-atom coordinates. Candidate loop sequences have been then designed beneath sequence profile constraints generated by way of alignment of the loop spine to the supply construction database. The candidates with the bottom scores have been chosen for the ultimate loop design.

Helical docking and design strategies⁷ have been utilized to the linked pRO-2.3 to generate helical filament design fashions. The next standards filtered particular person design trajectories: a discrepancy exceeding −15.0 Rosetta power models between the certain (polymeric) and unbound (monomeric) states, an interface floor space surpassing 700 Ų, a Rosetta form complementarity exceeding 0.62 and an unhappy polar residues depend beneath 5. Designs satisfying these standards underwent handbook refinement, involving single-point reversions to mutations deemed non-contributory to stabilizing the interface’s certain state. The highest-scoring design for every docked configuration was then built-in right into a finalized protein set for experimental validation.

Protein expression and purification

The artificial genes for a complete of 18 designs have been optimized for expression in Escherichia coli and purchased from IDT, then inserted into the pET29b+ vector’s a number of cloning website between NdeI and XhoI restriction websites. These constructs have been launched into BL21* (DE3) E. coli competent cells. Transformants have been cultured in 50 ml Terrific Broth medium supplemented with 200 mg l⁻¹ kanamycin. Expression, beneath the management of a T7 promoter, proceeded for twenty-four h at 37 °C utilizing Studier autoinduction¹⁶ till cultures have been harvested by centrifugation. Cell pellets have been resuspended in Tris-buffered saline (TBS) and lysed with Bugbuster detergent. The soluble fraction, clarified by centrifugation, underwent purification by way of Ni²⁺ immobilized steel affinity chromatography utilizing Ni-NTA Superflow resin. The resin with certain cell lysate was washed with ten column volumes of 40 mM imidazole and 500 mM NaCl, adopted by elution with 400 mM imidazole and 75 mM NaCl. The soluble and insoluble fractions have been subjected to SDS–polyacrylamide gel electrophoresis evaluation. Samples exhibiting protein bands on the right molecular weight have been chosen for electron microscopy screening. Chosen designs have been scaled as much as 0.5 l for additional characterization, with expression once more continuing for twenty-four h at 37 °C utilizing Studier autoinduction¹⁶ earlier than harvesting by centrifugation. Cell pellets have been resuspended in TBS and lysed by microfluidization, adopted by purification as described above.

Destructive stain EM

Soluble fractions have been concentrated in TBS (25 mM Tris buffer, 75 mM NaCl, pH 8) for electron microscopy screening. A 6 µl droplet (1 µl pattern immediately diluted with 5 µl of buffer) was utilized onto negatively glow-discharged, carbon-coated 200-mesh copper grids, washed with Milli-Q water and stained utilizing both 0.75% uranyl formate (pH 4.0) or Nano-W (pH 6.8) bought from Nanoprobes, Inc. as described beforehand¹⁷. Screening was carried out utilizing both a 100 kV Morgagni M268 transmission electron microscope (FEI) or a 120 kV Talos L120C transmission electron microscope (ThermoFisher). Photographs have been captured utilizing a bottom-mount Teitz CMOS 4k digital camera system and processed for enhanced distinction utilizing Fiji software program (model: 2.14.0/1.54f)¹⁸ for readability.

Fibre lengths have been quantified utilizing the fibre tracing algorithm in cryoSPARC⁸. This technique identifies fibres by cross-correlation to a template class and tracing contiguous fibres from the recognized particles. A template class generated from DpHF19 was used for all fibres measured. Fibres have been filtered in response to the common curvature (<0.0005 Å⁻¹) and the common normalized cross-correlation (>0.5) throughout every fibre. For DpHF18, we used 5, 2, 3, 20, 28 and 21 pictures for pH 3, 3.5, 4.2, 5, 8 and three to eight, respectively. For DpHF19, we used 7, 8, 8, 28, 4 and 5 pictures for pH 3, 3.5, 4.2, 5, 8 and three to eight, respectively. For DpHF19_9his, we used 6, 6, 8, 14, 15, 8 and 4 pictures have been used for pH 3, 3.5, 4.2, 5, 6, 8 and three to eight, respectively.

Cryo-EM

Cryo-EM samples have been ready by making use of protein to CFLAT holey-carbon grids, blotting away liquid and plunging the grids into liquid ethane utilizing a Vitrobot (ThermoFisher). For DpHF19, movies have been acquired on a Glacios microscope (ThermoFisher) outfitted with a Ok-2 Summit Direct Detect digital camera (Gatan Inc.) working in counting mode, with a pixel dimension of 1.16 Å per pixel, 50 frames and a complete electron dose of 65 Å⁻². For DpHF18 and DpHF7, movies have been acquired on a Titan Krios (ThermoFisher) outfitted with a Ok-2 Summit Direct Detect digital camera (Gatan Inc.) working in super-resolution mode, with a pixel dimension of 0.525 Å per pixel, 50 frames and a complete electron dose of 90 Å⁻². Automated information assortment was carried out utilizing Leginon¹⁹ model 3.4. Knowledge processing was carried out utilizing cryoSPARC⁸, and workflows are summarized in Supplementary Figs. 10–12. The movies have been aligned by patch movement correction, with super-resolution movies binned to a pixel dimension of 1.05 Å. Distinction switch perform (CTF) parameters have been estimated utilizing patch CTF. Template-free filament tracing was carried out on a subset of pictures, and the ensuing particles have been subjected to 2D classification. Chosen 2D lessons have been then used as templates for template-based filament tracing on full datasets. Following a number of rounds of 2D classification, chosen particles have been subjected to 3D refinement with helical symmetry imposed and non-uniform refinement enabled. For DpHF19, we imposed one-start helical symmetry relating particular person, non-contacting subunits, fairly than the two-start helical symmetry parameters. For DpHF7 and DpHF19, per-particle defocus, beam tilt and spherical aberration have been additionally refined. Density modification was carried out utilizing ResolveCryoEM in Phenix^20,21 model phenix-1.20.1. Atomic fashions for DpHF18 and DpHF19 have been refined into cryo-EM maps utilizing ISOLDE²², adopted by real-space refinement in Phenix, with rotamer and Ramachandran restraints disabled and with reference restraints imposed by the enter beginning mannequin. The elucidation of the mannequin for DpHF7 employed the de novo mannequin constructing protocol on the segmented cryo-EM uneven unit density²³. Subsequent residue incorporation and refinement have been achieved utilizing RosettaCM²⁴ model 2019.31, leveraging symmetry throughout the unsegmented cryo-EM map for optimum fit-to-density and intra-filament interfaces. A closing spherical of real-space refinement was carried out in Phenix, as described above for DpHF18 and DpHF19. Cryo-EM information assortment, refinement and validation statistics are summarized in Supplementary Desk 1.

TIRFM

Fibre meeting

To picture seeded nucleation of pH-responsive fibres, DpHF18 fibres have been labelled with two totally different maleimide-conjugated fluorophores, Oregon488 and sulfo-Cy5. Fibres have been labelled with a ten× molar extra, in PBS + 1 mM TCEP for 4 h at room temperature, earlier than buffer trade into TBS (25 mM Tris, 100 mM NaCl, pH 8.0) on a Zeba spin column and focus to 30 μM. Inexperienced fibres at 30 μM have been disassembled by way of the addition of 1 M citrate (0.6 μl of citrate to twenty μl of fibres) to cut back the pH to three.0. The answer was incubated for five min earlier than the addition of Tris (3.6 μl of 1 M inventory) to convey the pH again to eight.0; 1 μl of assembled DpHF18–Cy5 fibres at 30 μM was added to the answer. The answer was subsequently incubated at room temperature earlier than centrifugation at 13,000 g for two min in a benchtop centrifuge. Fibres have been resuspended in TBS and imaged by TIRFM.

Fibre disassembly

Quick TIRFM imaging of fibres disassembling at low pH was carried out on a custom-built TIRF system based mostly on a Nikon Ti stand outfitted with excellent focus system alongside a quick Z piezo stage (ASI), an azimuthal TIRF illuminator (iLas2, Roper France) with a {custom} prolonged discipline of view (Cairn) and a PLAN Apo 1.45 NA ×100 goal. Photographs have been acquired with a Photometrics Prime 95B back-illuminated sCMOS digital camera run in pseudo world shutter mode, synchronized with the azimuthal illumination. The system was operated by Metamorph 7.10.1.161. Sulfo-Cy5 maleimide-labelled fibres have been imaged with a 630 nm laser (150 mW Coherent OBIS mounted in a Cairn laser launch) and imaged utilizing a Chroma ET655lp filter mounted in a Cairn Optospin wheel at a body fee of 1 body each 16 ms.

Fibres have been imaged in imaging buffer (25 mM Tris pH 8.0, 100 mM NaCl) in an Ibidi movement cell mounted on clear room-grade coverslips ({custom}, 25 × 75 mm², Nexterion), and passivated with PLL-PEG (0.1 mg ml⁻¹ in 20 mM Hepes, pH 7.6; 5 min). Fibres have been allowed to deposit on the coverslip for five min earlier than unbound fibres have been eliminated with the imaging buffer. Throughout quick acquisition, the pH was diminished by flowing in low-pH buffer (25 mM Tris, 100 mM NaCl, pH 3.0).

To measure fibre disassembly in bulk resolution, pre-formed fibres in 1.5 ml Eppendorf tubes have been exchanged into citrate buffers at decrease pH to stimulate disassembly. A portion of every pH response was eliminated at varied time factors and added to a 96 nicely plate and for 10 min to permit the fibres to settle and cling to the glass substrate. For every situation and time level, 9 fields of view have been acquired on an IN Cell Analyzer 2500HS microscope (Molecular Units) utilizing a Nikon ×60 PLAN Apo 0.95 NA air goal and a 631 nm LED excitation supply, 150 ms publicity time with emission collected by way of a 684 ± 24 nm bandpass filter. Photographs have been quantified utilizing a {custom} CellProfiler script to phase fibres with the Otsu thresholding algorithm²⁵. Higher and decrease limits of the edge, in addition to the adaptive window for object ID, have been adjusted till fibres have been appropriately recognized relative to the background sign. The foremost axis size of objects recognized utilizing the CellProfiler pipeline have been plotted towards incubation time for every pH situation.

Liquid-phase AFM

Pattern preparation

We incubated 10 µl of a 0.01 wt% poly-lysine resolution on a freshly cleaved muscovite mica floor (12 mm, Ted Pella Inc.) for two min. The surplus resolution was eliminated and the floor was rinsed with water and dried with N₂ fuel⁷. Then 30 µl 10 µM protein resolution within the imaging buffer (25 mM Tris-HCl, 400 mM NaCl at pH 8) was incubated on the poly-lysine-coated mica for 30 min and washed with the picture buffer to take away extra protein. The pH of the disassembly buffer (25 mM Tris-HCl, 400 mM NaCl, pH 4.1, 4.4, 4, 5 or 4.7) was adjusted with 10 M NaOH or 1 M citric acid and filtered with 0.1 µm pore dimension PVDF filter earlier than use. For photoacid experiments, 10 µM protein resolution in 25 mM Tris-HCl pH 8 was incubated on naked mica for 30 min and washed with 25 mM Tris-HCl pH 5.5; a further deposition and rinse step was carried out if the quantity density of fibres on the floor was low. We additionally freshly ready 1 mM 2-nitrobenzaldehyde (Sigma-Aldrich) in 25 mM Tris-HCl pH 5.5 and instantly used it with out publicity to mild at any stage²⁶. Spectroscopic and pH measurements indicated that 2-nitrobenzaldehyde is activatable between wavelengths of 200 and 405 nm and lowers the pH from 5.5 to 2.7, and that larger laser depth results in quicker consumption and acidification.

Imaging

For the kinetic research at fixed composition, the protein-coated poly-lysine mica substrates have been positioned beneath the AFM liquid cell (Bruker Multimode8). Photographs have been captured within the imaging buffer utilizing a clear silicon nitride cantilever (Bruker, SNL-10, spring fixed: 0.12 N m⁻¹, UV ozoned for five min) in tapping mode at room temperature (25 °C). Earlier than flowing the disassembly buffer, the fibres have been imaged constantly for 10 min to optimize the parameters (256 scan traces, 1.5 Hz scan fee, excessive integral acquire (3–4) and 50–100 mV free amplitude). After confirming that no cantilever-induced harm occurred, the disassembly buffer was injected constantly at 25 µl min⁻¹. The flow-through set-up was optimized to supply negligible residence time and quick pH switching¹⁰.

For the photoacid research, protein-coated mica with 25 mM Tris-HCl pH 5.5 was positioned beneath the liquid cell of a Cypher VRS AFM (Asylum Analysis) outfitted with BlueDrive laser (×0.3 depth filter, 405 nm wavelength) with the vent valve open and operated in tapping mode. After confirming the excessive floor protection of the fibres, the imaging buffer was changed with 1 mM 2-nitrobenzaldehyde in 25 mM Tris-HCl pH 5.5, operated with out publicity to seen background mild and imaged once more. The cantilever was then retracted, and BlueDrive was turned on and rastered throughout pre-selected areas repeatedly utilizing the motorized optical microscope of the AFM. The whole UV publicity time throughout raster/dwell for spot and line patterns was no more than 10 min, after which the cantilever was moved again to the uncovered areas and imaged. For world pH adjustments, the quartz window of the AFM liquid cell involved with the photoacid resolution was uncovered to a handheld UV lamp (364 nm wavelength) for 7 min, after which imaged.

Photographs have been processed with Gwyddion SPM v2.62 information evaluation software program and analysed with Fiji software program v1.53s¹⁸. For kinetics, the overall fibre size was measured, and any fragments thought-about as already disassembled have been excluded from the size measurement. To measure the disassembly fee at every finish of particular person fibres (Supplementary Fig. 8), the centre of the fibre (half of the preliminary size) was assigned because the second finish for measuring size, whereas for fibre fragments, the centre of the fragment was measured because the second finish.