STI1 domains coordinate partitioning of UBQLN2 into stress-induced condensates

phase separation propensity largely correlates with puncta formation in transfected cells. Importantly, STI1-II and UBA domains are each required for baseline puncta formation in cells, but not STI1-I. In contrast, both STI1 domains are required for heat stress-induced puncta formation. Removal of STI1-II abrogates this stress response, and STI1-I deletion substantially attenuates it. Using N-terminal truncation constructs, we demonstrate that STI1-I strongly promotes both phase separation and puncta formation in the absence of the N-terminal region containing the UBL domain. Together, our findings demonstrate that the two STI1 domains of UBQLN2 have distinct roles in puncta formation and condensate partitioning, with STI1-II essential under all conditions.