Principles of human pre-60 biogenesis.

During the early stages of human large ribosomal subunit (60) biogenesis, an ensemble of assembly factors establishes and fine-tunes the essential RNA functional centers of pre-60 particles by an unknown mechanism. Here, we report a series of cryo-electron microscopy structures of human nucleolar and nuclear pre-60 assembly intermediates at resolutions of 2.5 to 3.2 angstroms. These structures show how protein interaction hubs tether assembly factor complexes to nucleolar particles and how guanosine triphosphatases and adenosine triphosphatase couple irreversible nucleotide hydrolysis steps to the installation of functional centers. Nuclear stages highlight how a conserved RNA-processing complex, the rixosome, couples large-scale RNA conformational changes with pre-ribosomal RNA processing by the RNA degradation machinery. Our ensemble of human pre-60 particles provides a rich foundation with which to elucidate the molecular principles of ribosome formation.