RNA stores tau reversibly in complex coacervates
(A) Representative bright-field images of tau-tRNA droplet samples at room temperature with varying [NaCl]. (B) Droplet coverage (in %) with poly(A) or tRNA in room temperature with varying [NaCl]. Δtau187:RNA in (A) and (B) were maintained at a mass ratio of 7:1 (corresponding to a charge ratio of 1.2:1) and a total mass concentration of 2 mg/ml. (C) Droplet coverage with poly(A) or tRNA with varying Δtau187: RNA charge ratios. The total mass concentrations are indicated in the legends. Samples made by mixing of 80 μM Δtau187 with 222 μg/ml poly(A)/tRNA or 161 μM Δtau187 with 444 μg/ml tRNA gave the highest droplet coverage (%), which correspond to charge ratio of 1.3–1.2 between tau and RNA. Error bars in (B) and (C) represent the standard deviation from n = 3. (D) Representative bright-field images of tau-RNA droplets as a function of incubation temperature. To record these images, the temperature was ramped from 19ºC to 25°C at 1°C per minute to acquire confocal images with bright field illumination. The samples for these images are generated from 100 μM tau mixed with poly(U) at approximately 1:1 charge ratio in the presence of 30 mM NaCl. (E) Representative bright-field images of tau-RNA droplet samples incubated at 37°C under otherwise different sample conditions. The concentration for tau, poly(U) RNA and NaCl are (i) 5 μM, 15 μg/ml, 0 mM; (ii) 5 μM, 15 μg/ml, 100 mM; (iii) 2.5 μM, 7.5 μg/ml, 100 mM; (iv) 1 μM, 3 μg/ml, 100 mM. Arrows highlight some of the droplets in images (ii) and (iii). Through Fig 5, scale bars are for 50 μm and all samples were prepared with Δtau187/322C in 20 mM ammonium acetate at pH 7.0. The numerical data used in (B) and (C) are included in S1 Data.