Phylogenetic convergence of phase separation and mitotic function in the disordered protein BuGZ

Intrinsically disordered proteins (IDPs) effect biological function despite their sequence-encoded lack of preference for stable three-dimensional structure. Among their many functions, IDPs form membraneless cellular compartments through liquid-liquid phase separation (LLPS), also termed biomolecular condensation. The extent to which LLPS has been evolutionarily selected remains largely unknown, as the complexities of IDP evolution hamper progress. Unlike structured proteins, rapid sequence divergence typical of IDPs confounds inference of their biophysical or biological functions from comparative sequence analyses. Here, we leverage mitosis as a universal eukaryotic feature to interrogate condensate evolutionary history. We observe that evolution has conserved the ability for six homologs of the mitotic IDP BuGZ to undergo LLPS and to serve the same mitotic function, despite low sequence conservation. We also observe that cellular context may tune LLPS. The phylogenetic correlation of LLPS and mitotic function in one protein raises the possibility of an ancient evolutionary interplay between LLPS and biological function, dating back at least 1.6 billion years to the last common ancestor of plants and animals.