Abstract of the PDB Structure's related Publication:
The formation of eukaryotic ribosomal subunits extends from the nucleolus to the cytoplasm and entails hundreds of assembly factors. Despite differences in the pathways of ribosome formation, high-resolution structural information has been available only from fungi. Here we present cryo-electron microscopy structures of late-stage human 40S assembly intermediates, representing one state reconstituted in vitro and five native states that range from nuclear to late cytoplasmic. The earliest particles reveal the position of the biogenesis factor RRP12 and distinct immature rRNA conformations that accompany the formation of the 40S subunit head. Molecular models of the late-acting assembly factors TSR1, RIOK1, RIOK2, ENP1, LTV1, PNO1 and NOB1 provide mechanistic details that underlie their contribution to a sequential 40S subunit assembly. The NOB1 architecture displays an inactive nuclease conformation that requires rearrangement of the PNO1-bound 3' rRNA, thereby coordinating the final rRNA folding steps with site 3 cleavage.
The tRNA methyltransferase subunit 11-2 activates rRNA, tRNA, and protein methyltransferase. Forming a heterodimer with WBSCR22, it methylates guanosine, in position 1639 of human rRNA subunit 18S, on its N7 position (cap 0) (Zorbas et al. 2015 ). Acting in concert with THUMPD3, it catalyzes the formation of N2-methylguanosine on the 6th and 10th positions of a broad range of tRNA substrates, and specifically modifying the 7th position of tRNA (Trp) (Yang et al. 2021). It, moreover, forms a heterodimer with ALKBH8 catalyzing the methylation of 5-carboxymethyl uridine to 5-methylcarboxymethyl uridine at the wobble position of the anticodon loop in target tRNA species (Fu et al. 2010). Considering other RNA types, together with METTL5, it specifically methylates rRNA 18S A1832 in its 6th position (Yu et al. 2021, Sepich-Poore. et al 2022, van Tran et al. 2019). It is Trm112 yeast's homologue.