PAICS ubiquitination recruits UBAP2 to trigger phase separation for purinosome assembly.

Purinosomes serve as metabolons to enhance de novo purine synthesis (DNPS) efficiency through compartmentalizing DNPS enzymes during stressed conditions. However, the mechanism underpinning purinosome assembly and its pathophysiological functions remains elusive. Here, we show that K6-polyubiquitination of the DNPS enzyme phosphoribosylaminoimidazole carboxylase and phosphoribosylaminoimidazolesuccinocarboxamide synthetase (PAICS) by cullin-5/ankyrin repeat and SOCS box containing 11 (Cul5/ASB11)-based ubiquitin ligase plays a driving role in purinosome assembly. Upon several purinosome-inducing cues, ASB11 is upregulated by relieving the H3K9me3/HP1α-mediated transcriptional silencing, thus stimulating PAICS polyubiquitination. The polyubiquitinated PAICS recruits ubiquitin-associated protein 2 (UBAP2), a ubiquitin-binding protein with multiple stretches of intrinsically disordered regions, thereby inducing phase separation to trigger purinosome assembly for enhancing DNPS pathway flux. In human melanoma, ASB11 is highly expressed to facilitate a constitutive purinosome formation to which melanoma cells are addicted for supporting their proliferation, viability, and tumorigenesis in a xenograft model. Our study identifies a driving mechanism for purinosome assembly in response to cellular stresses and uncovers the impact of purinosome formation on human malignancies. [Display omitted] • Stresses that increase purine demand upregulate ASB11 by epigenetic mechanisms • ASB11 promotes PAICS ubiquitination to recruit the ubiquitin-binding protein UBAP2 • UBAP2 governs multivalent interactions to drive purinosome assembly • Melanoma cells form purinosome constitutively to support their proliferation and survival In this study, Chou et al. discovered that PAICS polyubiquitination induced by cell stresses triggers the recruitment of UBAP2 to confer multivalent interactions for driving phase separation to form purinosome condensates. Melanoma cells are addicted to the constitutive purinosome formation to supply sufficient purines for their proliferation and survival. [ABSTRACT FROM AUTHOR]