Understanding the role of RTL8 proteins in Ubiquilin-2 biology.

The brain expressed ubiquilins (UBQLNs) 1, 2 and 4 are a family of ubiquitin adaptor proteins that participate broadly in protein quality control (PQC) pathways, including the ubiquitin proteasome system (UPS). One family member, UBQLN2, plays a role in numerous neurodegenerative diseases including ALS/FTD and Huntington's disease. UBQLN2 typically resides in the cytoplasm but can translocate to the nucleus under proteotoxic stress and in disease to facilitate nuclear PQC. How UBQLN2 translocates to the nucleus and clears aberrant nuclear proteins, however, is not well understood. In a mass spectrometry screen to discover UBQLN2 interactors, we identified a family of small (13 kDa), highly homologous uncharacterized family of proteins, retrotransposon Gag-like 8 (RTL8). We then confirmed the interaction between UBQLN2 and a representative RTL8 family member, RTL8A, both in vitro using recombinant proteins and in vivo using mouse brain tissue. We further show that RTL8A promotes nuclear translocation of UBQLN2. The robust effect of RTL8A on nuclear translocation of UBQLN2 does not extend to the other brain-expressed ubiquilins, UBQLN1 and UBQLN4. Depletion of all RTL8 proteins in human cell lines perturbs nuclear translocation of UBQLN2 following heat shock. Finally, compared to UBQLN1 and UBQLN4, UBQLN2 preferentially stabilizes endogenous RTL8 levels in cells, supporting functional heterogeneity among UBQLNs. As novel UBQLN2 interactors that recruit UBQLN2 to specific subnuclear compartments, RTL8 proteins may regulate the role of UBQLN2 in nuclear PQC.