Blocky proline/glutamine patterns in the SFPQ intrinsically disordered region dictate paraspeckle formation as a distinct membraneless organelle

Abstract

Membraneless organelles (MLOs) formed through phase separation play crucial roles in various cellular processes. Many MLOs remain spatially compartmentalized, avoiding fusion or engulfment. MLOs are formed by dynamic multivalent interactions, often mediated by proteins with intrinsically disordered regions (IDRs). However, the molecular principles behind how IDRs maintain MLO independence remain poorly understood. Here, we investigated the proline/glutamine (P/Q)-rich IDR of SFPQ, a protein identified as a key factor in segregating paraspeckles from nuclear speckles. Paraspeckle segregation analyses, using SFPQ mutants tethered to NEAT1_2 long noncoding RNA, revealed that P/Q residues within the SFPQ IDR, conserved from humans to zebrafish, are crucial for its segregation activity. Beyond amino acid composition, the blocky patterns of P/Q residues are required for the segregation from nuclear speckles. Among human IDRs exhibiting PQ-block patterns, BRD4 IDR shows strong sequence similarity to the SFPQ IDR, and exhibits comparable segregation activity. Molecular dynamics simulation suggests that the PQ-blocky patterns required for the paraspeckle segregation do not correlate with the IDR characteristics necessary for self-assembly. Thus, these data suggest that the PQ-blocky patterns in IDRs represent a previously uncharacterized property that contributes to MLO independence, possibly through a mechanism distinct from the conventional phase separation-promoting function of IDRs.