p62/SQSTM1 self-assembles with polyubiquitin chains to form liquid-like condensates known as "p62 bodies." These condensates serve dual functions as hubs for stress signaling and as cargos for selective autophagy. We previously established a method to purify p62 bodies from human cell lines using fluorescence-activated particle sorting (Kurusu et al., Dev Cell 2023), and analyzed their protein composition by mass spectrometry. This analysis identified members of ubiquilin (UBQLN) family—key components of the intracellular protein quality control machinery—as client proteins within p62 bodies.
Recent studies have demonstrated that UBQLN proteins themselves can undergo phase separation to form liquid-like condensates. In vitro reconstitution experiments revealed that p62, UBQLN, and polyubiquitin chains co-assemble into spherical condensates. Fluorescence recovery after photobleaching (FRAP) analysis demonstrated that UBQLN and ubiquitin chains exhibit rapid exchange within these condensates, whereas p62 displays slower dynamics. Furthermore, UBQLN2 lacking the UBA (ubiquitin-associated) domain failed to form condensates, while UBQLN2 lacking the UBL（ubiquitin-like） domain was able to co-phase-separate with ubiquitin but was unable to fuse with p62-containing condensates.
These findings suggest that the fusion of distinct ubiquitin-positive condensates is governed by domain-specific interactions between p62 and UBQLN proteins.