Fluorescent protein and peptide tags alter condensate formation and dynamics in vivo and in vitro.

Fluorescent proteins and peptide tags are essential tools in cellular biology, but can alter the biochemical and physiological behavior of target proteins. Biomolecular condensates, which have emerged as key elements of cellular organization, are suggested to provide robustness to cells, yet they can also respond sensitively to small changes in environmental conditions including tags. Here, we investigate the effects of over twenty widely used tags on condensate formation in vitro, in cells, in various model organisms and by computational modelling. We find that tagging strongly influences condensation for some proteins, while others remain unaffected. Effects vary, with some tags enhancing and others decreasing condensation, with the outcome depending on the protein being tagged. Coarse-grained simulations suggest that the charge of the fluorescent protein tags is a critical factor modulating condensation behavior. Together, our results underscore the need to tag with caution and highlight the importance of careful experimental design and interpretation, especially in condensate studies, but also suggest that fluorescent protein tags could serve as a tool to modulate condensate properties.