Converting Stochastic Assembly into an Assembly Line: Non-Equilibrium Droplet Dynamics Assists Ribosome Formation

The nucleolus is a large liquid-like membraneless organelle responsible for the majority of the processing of ribosomal components and the assembly of ribosomes, which involves hundreds of proteins. It has been suggested that one of the primary functions of the nucleolus is to concentrate these proteins with the ribosomal RNA (rRNA), thereby significantly enhancing the binding rates and enzyme reaction speed for ribosome assembly and post-translational modifications.

Here we expand on this idea by considering the non-equilibrium effects that arise from having a constant rRNA flow outward from the center of a nucleolus and an inward flow of ribosomal protein (rProtein) from the nucleoplasm. We show numerically and analytically that the binding of specific rProteins to rRNA can be localized within a well-defined radial shell inside the nucleolus instead of being homogeneously distributed. By giving the different rProteins different physical properties, the different rProteins can be confined to bind to the rRNA at different radial distances from the transcription centers within the nucleolus. Thus, as rRNA molecules diffuse outward through the nucleolus, the rProteins can be added in sequential order like an assembly line.