Fig. 7.

NPM1 adopts a broad range of configurations to achieve biological multifunctionality. a Depending upon ionic conditions and available partners, NPM1 can adopt compact conformations, through intra-chain interactions between A- and B-tracts within its IDR and, extended conformations, and can assemble into multimers through in trans interactions between A- and B-tracts. b NPM1–NPM1 in trans interactions form the scaffold for homotypic LLPS. In a, b, and f, the NPM1 OD is represented as a green pentagon, the IDR as red and blue fuzzy lines representing the A- and B-tracts, respectively, and CTD as blue squares. Homotypic interactions (c) can be replaced by heterotypic interactions with multivalent R-motif proteins, such as SURF6 (curvy blue line) (d) or with rRNA (orange objects) (e), changing the nature of the scaffold. Note that, for visual clarity, the entire NPM1 pentamer is represented as a green pentagon in c–e. f Schematic illustration of NPM1-mediated ribosomal subunit assembly in the GC of the nucleolus. Ribosomal subunits are comprised of rRNA and ribosomal proteins (r-proteins; curvy blue lines). rRNA is synthesized at the boundary between the fibrillar center (FC) and dense fibrillar component (DFC) and diffuses from the center to the periphery of the nucleolus, while the ribosomal proteins diffuse from the nucleoplasm into the nucleolus. In this model, the type of LLPS mechanism utilized by NPM1 within the GC depends upon the availability of its binding partners: rRNA, which moves outwards from the DFC/GC boundary, and R-motif proteins, which move inwards from the GC/nucleoplasm boundary, each drive different heterotypic LLPS mechanisms, in the central region of the GC. The close spatial proximity of rRNA and r-proteins within this central GC region, which exists through mixed heterotypic NPM1-mediated LLPS, enables the “hand-off” of these ribosomal components to interact with each other to form nascent ribosomal subunits (represented as conjoined orange objects and curvy blue lines). As ribosomal subunits assemble, their components no longer interact extensively with NPM1; at the same time, however, NPM1’s propensity for self-interaction compensates, maintaining the liquid-like GC scaffold through homotypic LLPS