Complex and heterogeneous fusion dynamics of protein condensates by high-speed imaging and optical trapping

Fusion is a key growth mechanism in the formation of mature membrane-less organelles. In this study, we have established a quantitative analysis framework, based on bright-field and fluorescence microscopes coupled with an optical tweezer and a high-speed camera, to characterize fusion onset, relaxation dynamics, and molecular exchange following fusion. Here, we found that a facing condensate intermediate with a molecular exchange barrier forms before fusion for three model proteins/peptide. Fusion is induced by stochastic conversion from intra- to inter-condensate interactions in the intermediate. Furthermore, subsequent fusion kinetics are heterogeneous for the two proteins but less so for the peptide, suggesting the presence of distinct intermolecular interaction regions within protein condensates. We propose a molecular model of protein condensate fusion.