Phase separation of PGL-3 driven by structured domains that oligomerize and interact with RGG motifs

Phase separation (PS) of biomolecular condensates is often assumed to be driven by interactions involving nucleic acids and intrinsically disordered regions (IDRs) of proteins. PGL-3 is a component of P granules, biomolecular condensates in C. elegans, that contains two structured domains (D1-D2), an internal IDR, and a C-terminal IDR rich in RGG motifs. Theoretical and in vitro studies implicated the internal IDR and RGG motifs in driving PGL-3 PS via self-interactions and binding to RNA. Studies in cells, however, implicated the D1 and D2 domains. Here, we investigate the molecular basis of PGL-3 PS in vitro using microscopy, crosslinking mass spectrometry, and biophysical measurements. We find that D1-D2 forms oligomers and is necessary and sufficient for PS. The terminal RGG region interacts with D1-D2 in a manner that enhances PS even in the absence of RNA. In contrast, the internal IDR is neither necessary nor sufficient for PS. These findings support an alternative model for PGL-3 PS that does not require RNA and is driven by oligomerization of structured domains that interact with RGG repeats.