Detailed procedures for sample preparation for TEM imaging are described below: 10 μl of protein solution containing liquid-like droplets or nanofibers was directly deposited on a piece of carbon film–supported 400-mesh copper grid (Beijing Zhongjingkeyi Technology Co. Ltd.) for 10 min. Excessive solution was then quickly wicked away with pieces of filter paper and washed three times with 10 μl of deionized water. Samples were negatively stained with 10 μl of 2 wt % uranyl acetate right away and then dried under an infrared lamp for 20 min. All TEM images were collected on a Tecnai G2 F20 TEM operated at a 120-kV accelerating voltage. Data are presented in Fig. 2B and fig. S2 (B and C).