Diverse roles of assembly factors revealed by structures of late nuclear pre-60S ribosomes
Ribosome biogenesis is a highly complex process in eukaryotes, involving temporally
and spatially regulated ribosomal protein (r-protein) binding and ribosomal RNA
remodelling events in the nucleolus, nucleoplasm and cytoplasm.
Hundreds of assembly factors, organized into sequential functional groups, facilitate and guide the maturation process into productive
assembly branches in and across different cellular compartments. However, the
precise mechanisms by which these assembly factors function are largely unknown.
Here we use cryo-electron microscopy to characterize the structures of yeast
nucleoplasmic pre-60S particles affinity-purified using the epitope-tagged assembly
factor Nog2. Our data pinpoint the locations and determine the structures of over 20
assembly factors, which are enriched in two areas: an arc region extending from the
central protuberance to the polypeptide tunnel exit, and the domain including the
internal transcribed spacer 2 (ITS2) that separates 5.8S and 25S ribosomal RNAs. In
particular, two regulatory GTPases, Nog2 and Nog1, act as hub proteins to interact
with multiple, distant assembly factors and functional ribosomal RNA elements,
manifesting their critical roles in structural remodelling checkpoints and nuclear
export. Moreover, our snapshots of compositionally and structurally different
pre-60S intermediates provide essential mechanistic details for three major
remodelling events before nuclear export: rotation of the 5S ribonucleoprotein,
construction of the active centre and ITS2 removal. The rich structural information
in our structures provides a framework to dissect molecular roles of diverse
assembly factors in eukaryotic ribosome assembly.
Nature doi: 10.1038/nature17942
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