Fig. 1: Ordered stages of NPET assembly.

a Cryo-EM structures of preribosomes revealed that the NPET undergoes distinct stages of assembly. The Nsa1 state 2 particle (PDB: 6C0F) contains Rrp14 (sky blue) associated with an NPET precursor. The uL4 (cyan) and uL22 (bright orange) internal loops can be seen but the tunnel domain (TD) of uL4 is not yet visible. The Nsa1 state C particle (PDB: 6EM1) displays Rpf1 (teal green) in an incomplete NPET. The TD of uL4 (cyan) is now visible at this stage. The Nsa1 state E particle (PDB: 6ELZ) contains a more complete NPET. The CTD of Spb1 (dark green) can be seen toward the entrance of the NPET. In Nog2 particles (PDB: 3JCT), a nearly mature NPET, now containing eL39 (green), can be visualized with the CTD of Nog1 (dark purple) stretching almost the entire length of the NPET. The Arx1 (PDB: 5APN) and Nmd3 (PDB: 5H4P) particles reveal no further NPET rRNA maturation but show that the CTDs of Rei1 (dark pink) and Reh1 (brown), respectively, now occupy the NPET. b Five domains of rRNA secondary structure make up the NPET. In Nsa1 state C, the NPET contains mostly domains I (red) and II (orange). A small portion of domain IV (green) can also be seen and domain V (blue) has not yet folded into the NPET. In Nsa1 state E, domain III (yellow) has folded into a stable conformation to join the NPET. In Nog2 state 1, domain V has stably folded into the NPET while a portion of domain II has also folded into a near mature state. Minor rearrangements in NPET rRNA can be observed in Arx1, Nmd3, and mature particles (PDB: 4V88). For clarity, only NPET-relevant portions of proteins and rRNA are shown.