Fluorescence-activated particle sorting for condensate purification.

Condensates are receiving increasing attention because of their ability to organize subcellular space. In eukaryotes, nuclear condensates include nucleoli and paraspeckles, and cytoplasmic ones include P-bodies (PBs) and stress granules. One approach to investigate condensate biology is through analyzing their protein and RNA content. However, purifying condensates remains a challenge because of their densities being similar to various other organelles, and the absence of protein markers accumulating exclusively in them. These limitations, combined with the generally low number of condensates per cell, necessitate new approaches to tackle their purification. Here, we present a protocol describing fluorescence-activated particle sorting (FAPS) for purifying condensates. In brief, FAPS involves fluorescently labeling condensates to identify and isolate them from other cellular components via sorting. In this Protocol, we focus on PB purification, quality control and downstream characterization of PB protein and RNA contents. Although originally developed to purify PBs from human cell lines, FAPS can be adapted to various condensates across model organisms. The procedure requires knowledge in basic cell culture, molecular biology and flow cytometry and access to a fluorescence-activated cell sorter with sufficient sensitivity. It requires ~25-30 d, including a hands-on period of 15 d, to complete. In summary, FAPS allows the characterization of the content of diverse condensates across cell types and organisms.