Eukaryotic ribosome biogenesis requires the action of approximately 200 trans-acting factors and the incorporation of 79 ribosomal proteins (RPs). The delivery of RPs to pre-ribosomes is a major challenge for the cell because RPs are often highly basic and contain intrinsically disordered regions prone to nonspecific interactions and aggregation. To counteract this, eukaryotes developed dedicated chaperones for certain RPs that promote their solubility and expression, often by binding

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... -specific extensions of the RPs. Rps2 (uS5) is a universally conserved RP that assembles into nuclear pre-40S subunits. However, a chaperone for Rps2 had not been identified. Our lab previously characterized Tsr4 as a 40S biogenesis factor of unknown function. Here, we report that Tsr4 co-translationally associates with Rps2. Rps2 harbors a eukaryotic-specific N-terminal extension that was critical for its interaction with Tsr4. Moreover, Tsr4 perturbation resulted in decreased Rps2 levels and phenocopied Rps2 depletion. Despite Rps2 joining nuclear pre-40S particles, Tsr4 appeared to be restricted to the cytoplasmic. Thus, we conclude that Tsr4 is a cytoplasmic chaperone dedicated to Rps2.