The N-terminal Extension of Ribosomal Protein Rps2 Shapes Its Early Fate through Chaperone-Importin Competition and Arginine Methylation.

Efficient nuclear import of ribosomal proteins is essential for the timely assembly of ribosomal subunit precursors in the nucleus and progression of ribosome biogenesis. Several ribosomal proteins are escorted to the nucleus by dedicated chaperones, which shield their interaction surfaces and assist their delivery. Here, we characterize the N-terminal extension of the small subunit ribosomal protein Rps2 as a regulatory hub that integrates binding of its dedicated chaperone Tsr4, recognition by its importin Pse1, and arginine methylation. By mapping Pse1's interaction interface on Rps2's N-terminal extension, we identified arginine 11 (R11), a known methylation site, as a critical residue. We demonstrate that Rps2 is already methylated while associated with Tsr4, indicating that methylation occurs at an early stage of the Rps2 assembly path prior to nuclear import. We further show that the chaperone Tsr4 and the importin Pse1 compete for binding to the Rps2 N-terminal extension. While Tsr4 binds this region with higher affinity than Pse1, Pse1 also contacts additional regions within full-length Rps2. Arginine methylation of Rps2 modestly reduces Pse1 binding to the Rps2 N-terminal extension, while having little effect on Tsr4 binding, suggesting that this modification can influence importin recognition without disrupting chaperone association. Finally, we find that Rps2 methylation increases at low temperature and that loss of Hmt1-mediated methylation exacerbates translational fidelity defects in an rps2 mutant background. Together, our findings reveal how a ribosomal protein N-terminal extension coordinates sequential interactions and post-translational modification events that shape the early fate of Rps2 and support accurate ribosome function.