Mechanistic insight into eukaryotic 60S ribosomal subunit biogenesis by cryo-electron microscopy

Abstract

Eukaryotic ribosomes, the protein-producing factories of the cell, are composed of four ribosomal RNA molecules and roughly 80 proteins. Their biogenesis is a complex process that involves more than 200 biogenesis factors that facilitate the production, modification, and assembly of ribosomal components and the structural transitions along the maturation pathways of the preribosomal particles. Here, I review recent structural and mechanistic insights into the biogenesis of the large ribosomal subunit that were furthered by cryo-electron microscopy of natively purified pre-60S particles and in vitro reconstituted ribosome assembly factor complexes. Combined with biochemical, genetic, and previous structural data, these structures have provided detailed insights into the assembly and maturation of the central protuberance of the 60S subunit, the network of biogenesis factors near the ribosomal tunnel exit, and the functional activation of the large ribosomal subunit during cytoplasmic maturation.

A Puzzle of Life: Crafting Ribosomal Subunits

“…Interestingly, as shown by high-resolution cryo-EM, Rei1 and the closely related Reh1 probe the exit tunnel in a manner similar to Nog1 [25,28,30]. During P-stalk assembly, recruitment of uL10 is coupled with the release of Yvh1, which has previously displaced Mrt4 from pre-60S particles [103,104]. Release of the export adaptor Nmd3 is promoted by the GTPase Lsg1 and connected to the stable incorporation of uL16 [40,103,107] (Figure 2Q).…”

“…Nuclear Export and Cytoplasmic Maturation of Pre-60S Ribosomes Several export-mediating factors interact with different surfaces of pre-60S ribosomes to mediate their efficient translocation across the NPC [5,10,14,103] (Figures 2O and 5). Following the release of Nog2, the export adaptor Nmd3 binds in the A-, P-, and E-tRNA-binding sites of the LSU and recruits the exportin Crm1/Xpo1 via its essential NES sequences [10,14,30,32,101].…”

“…A recent study has demonstrated that recruitment and release of Mex67-Mtr2 is also intimately coupled to maturation of the emerging P stalk [104]. Besides these, several additional factors contribute to the export process [5,10,14,103], most likely by shielding the charged ribosomal surface against the hydrophobic environment within the NPC channel.…”

“…After export, removal of the remaining biogenesis factors is coordinated with the incorporation of the missing r-proteins [5,6,14,30,103] (Figure 2P). First, the AAA-ATPase Drg1 releases the GTPase Nog1 and its binding partner Rlp24, thereby enabling recruitment of the r-protein eL24 [99,106].…”

“…First, the AAA-ATPase Drg1 releases the GTPase Nog1 and its binding partner Rlp24, thereby enabling recruitment of the r-protein eL24 [99,106]. Subsequently, Arx1 is dissociated by Jjj1 and Rei1 [103]. Interestingly, as shown by high-resolution cryo-EM, Rei1 and the closely related Reh1 probe the exit tunnel in a manner similar to Nog1 [25,28,30].…”