Abstract
Eukaryotic ribosome biogenesis requires the nuclear import of ∼80 nascent ribosomal proteins and the elimination of excess amounts by the cellular degradation machinery. Assembly chaperones recognize nascent unassembled ribosomal proteins and transport them together with karyopherins to their nuclear destination. We report the crystal structure of ribosomal protein L4 (RpL4) bound to its dedicated assembly chaperone of L4 (Acl4), revealing extensive interactions sequestering 70 exposed residues of the extended RpL4 loop. The observed molecular recognition fundamentally differs from canonical promiscuous chaperone–substrate interactions. We demonstrate that the eukaryote-specific RpL4 extension harbours overlapping binding sites for Acl4 and the nuclear transport factor Kap104, facilitating its continuous protection from the cellular degradation machinery. Thus, Acl4 serves a dual function to facilitate nuclear import and simultaneously protect unassembled RpL4 from the cellular degradation machinery.
Highlights
Eukaryotic ribosome biogenesis requires the nuclear import of B80 nascent ribosomal proteins and the elimination of excess amounts by the cellular degradation machinery
We have previously shown that the dedicated assembly chaperone Acl[4] recognizes nascent ribosomal protein L4 (RpL4), facilitates its nuclear import, and releases RpL4 on engaging RpL18 at the pre-ribosome[10]
We set out to identify the molecular basis of assembly chaperone of L4 (Acl4)-dependent RpL4 sequestration and protection
Summary
Crystals of the Chaetomium thermophilum Acl4RpL4 complex, which included the Acl[4] TPR domain (residues 28 to 361) and the globular core of RpL4 (RpL4CORE) and the entire elongated loop (RpL4LOOP), diffracted to 2.4-Å resolution (Fig. 1a). Acl4-binding induces the formation of an a-helix within RpL4LOOP (a3, residues 89 to 97), which is entirely devoid of secondary structure elements in the context of the intact ribosome Both Acl[4] and RpL4 undergo dramatic conformational changes on complex formation. Despite limited sequence conservation, C. thermophilum Acl[4] is capable of forming a chimeric complex with S. cerevisiae RpL4 (Fig. 2e) These results suggest that the Glu266-mediated interactions constitute an intrinsic weak spot that is critical for Acl4RpL4 complex disassembly. RpL4LOOP residue Lys[56] along with RpL4EXT residues Lys[310] and Lys[340] were identified as Tom[1]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
