Resolving Late-Stage Intermediates of Eukaryotic Ribosome Assembly Erik W. Hartwick, Brian T. Wimberly  Structure  Volume 25, Issue 2, Pages 216-218 (February 2017) DOI: 10.1016/j.str.2017.01.007 Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 1 Late-Stage Pre-40S Ribosome Assembly (A) Schematic representations of the assembly factors Tsr1 (blue, PDB ID 5IW7), Rio2 (red, 4GYG), Dim1 (green, 1ZQ9), Nob1 (magenta, 2LCQ), and Pno1 (purple, 3AEV). eIF5B domains I and II (orange, 4V8Y) are also shown. Note that intact eIF5B is considerably larger than the domain I and II fragment shown here. Ltv1 and Enp1 have been omitted for clarity. (B) Schematic representations of the small (pre-40S) and large (60S) ribosomal subunits derived from coordinates of the mature subunits (4KZX and 3J7O). The prominent head, beak, platform, and body landmarks of the pre-40S (yellow) are labeled, as are ribosomal proteins S5 and S15 and 18S RNA helices 44 and 45 (cyan). The 60S (royal blue) is shown bound to the subunit joining factor eIF5B (domains I and II only; orange) (4V8Y). (C) A model for several key events in late-stage small subunit assembly. Pre-40S enters the cytosol with 7 AFs bound: Tsr1, Rio2, and Dim1 bind the subunit interface, Nob1 and Pno1 occupy the platform and head near the immature 3′ end of the pre-18S rRNA, while Ltv1 and Enp1 (not pictured) bind the beak. The AFs prevent unregulated association of eIFs, tRNAs, mRNPs, and mature 60S subunits, but allow eIF5B-mediated association of 60S to form an 80S-like quality control complex (Karbstein 2013). Conformational rearrangements (see text) of Tsr1, Rio2, Dim1, and 18S helices 44 and 45 are needed to occur to accommodate eIF5B and subsequent 60S binding. During formation of the 80S-like particle, or shortly thereafter, Rio2 dissociates. In the context of the 80S-like quality control particle (boxed), Dim1 methylates adenines in 18S helix 45. Upon the removal of Dim1, 18S helix 45 moves into its mature position against helix 44, placing the immature 3′ end in the active site of the nuclease Nob1. Nob1 cleavage then yields the mature 3′ end of the 18S RNA. The 60S subunit is later removed by other AFs resembling termination factors (not shown). For clarity, the 60S subunit is not shown in the last two schematics. Structure 2017 25, 216-218DOI: (10.1016/j.str.2017.01.007) Copyright © 2017 Elsevier Ltd Terms and Conditions