Bclaf1 biomolecular condensates protect nuclear PTK2 from ubiquitin-proteasome system promoting cardiomyocyte survival during oxidative stress

Abstract

PTK2, a non-receptor tyrosine kinase, plays a critical role in regulating essential cellular functions, including cell survival, by reducing p53 levels and activating the PI3K/AKT pathway. However, the mechanism underlying PTK2 stabilization during stress remains unclear. In this study, we identified Bclaf1, a multifunctional protein known to stabilize partners, as a PTK2 interactor. Using advanced microscopy techniques we identified nuclear Bclaf1 biomolecular condensates containing PTK2 in cardiomyocytes under oxidative stress. While diffuse PTK2 in the nucleus was susceptible to ubiquitination, PTK2 sequestered in the Bclaf1 condensates was protected from the ubiquitin-proteasome system (UPS). The K926 residue was identified as a ubiquitination site on PTK2, and subsequent proteasome inhibition experiments confirmed the role of the UPS in PTK2 homeostasis. Furthermore, disrupting Bclaf1 biomolecular condensates lead to PTK2 degradation, subsequently increasing p53 levels and activating apoptosis. Our findings support the role of Bclaf1 in the formation of pro-survival nuclear condensates that sequester and stabilize PTK2, promoting cardiomyocyte survival during oxidative stress. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=151 SRC="FIGDIR/small/636487v3_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@15377a9org.highwire.dtl.DTLVardef@d5325forg.highwire.dtl.DTLVardef@de0ac9org.highwire.dtl.DTLVardef@196325d_HPS_FORMAT_FIGEXP M_FIG C_FIG In BriefMoretto et al. demonstrated that Bclaf1 undergo liquid-liquid phase separation (LLPS) during doxorubicin (Dox) induced oxidative stress to generate membraneless organelles where PTK2 is sequestered and protected from the ubiquitin-proteasome system (UPS). The maintenance of PTK2 stability may favor the p53 and Mdm2 interaction, which results in p53 ubiquitination and degradation, promoting cell survival. Loss of Bclaf1 disrupts PTK2 stabilization, leading to the ubiquitination and degradation of this kinase and downstream upregulation of p53, increasing cell death. HighlightsO_LIBclaf1 undergoes LLPS to stabilize protein partners during stress. C_LIO_LIPTK2 inside the Bclaf1 biomolecular condensates is protected from the UPS to promote cardiomyocyte survival during dox-induced oxidative stress. C_LIO_LIThe PTK2 ubiquitination site, lysine 926, was identified and the action of the UPS on PTK2 proteostasis was confirmed. C_LIO_LIBclaf1 knockdown results in overall protein ubiquitination and in PTK2 ubiquitination and degradation, resulting in increased levels of p53 and PUMA. C_LI