R1.2 1 3 cu 300

For SEM analysis, the freeze-dried samples were placed on the stub and gold-sputter-coated under argon. The images were acquired by MIRA3 (Tescan, Brno, Czech Republic). TEM images were acquired by a JEOL JEM 1200 EX instrument. The samples were dispersed in water at the final concentration of 0.1 mg/mL; a drop of the sample suspension was then placed onto a 300-mesh nickel grid coated with carbon and let dry for 24 h at room temperature (25 °C) before analysis. Cryo-EM images were acquired by a Cryo transmission electron microscope Glacios (Thermo Fisher Scientific, Milan, Italy). The sample suspension (0.1 mg/mL) was placed on a grid (Quantifoil R 1.2/1.3 Cu 300), and the images were acquired.

Purified Pp GS was exchanged into Buffer B using a 10 kDa MWCO spin filter (Millipore). Next, Pp GS (0.8 mg/mL) was mixed with 2.5 mM Pp GlnR C-tail peptide (Genscript; sequence: KRPGQVSLIQGELSRFFNNR) and incubated at rt for 30 min. For grid preparation, Quantifoil R1.2/1.3 Cu 300 (Quantifoil) holey carbon grids were cleaned for 100 s using a PELCO easiGlow glow discharge cleaning system and 3 μL of the sample were applied at 95% humidity and 22 °C. Following a 10 s incubation period, the grids were blotted for 1.5 s and plunge-frozen into liquid ethane using a Leica EM GP2 (Leica Microsystems).