Eukaryotic ribosome assembly involves a plethora of factors, which ensure that a
correctly folded ribosome contains all ribosomal protein components. Among these
assembly factors, Yar1 has recently emerged as a molecular chaperone for
ribosomal protein rpS3 of the small ribosomal subunit (40S) in yeast. In complex
with its chaperone, rpS3 is imported into the nucleus and protected from
aggregation. How rpS3 and other ribosomal proteins are initially sequestered and
subsequently integrated into pre-ribosomal particles is currently poorly
understood. Here, we present the crystal structure of yeast rpS3 in complex with
its chaperone Yar1 at 2.8Å resolution. The crystal structure rationalizes how
Yar1 can protect rpS3 from aggregation while facilitating nuclear import and
suggests a mechanism for a stepwise exchange of molecular partners that
ribosomal proteins interact with during ribosome assembly.
