Extended Data Fig. 5: Mammals and yeast employ distinct modes of NatA/E recruitment to ribosomes.

Side (a, b) and top (c, d) views of the mammalian ternary RNC_RpL4-NAC-NatA/E complex (a, c) and those of the yeast RNC-NatA/E complex (b, d; PDB# 6HD5). The ribosomes are shown in surface representation, bound factors and selected expansion segments in cartoon. The NAC heterodimer is shown in yellow and orange, mammalian NatA/E in hues of green, rRNA expansion segments coordinating NatA/E in blue, and yeast NatA/E in hues of pink. While the N-terminal helical domain of Naa15 contacts the ribosome near the exit tunnel in both mammalian and yeast systems, all other structural elements that mediate NatA recruitment and positioning on the ribosome are distinct in the two organisms. Firstly, ribosome-bound NAC captures and helps position NatA/E in mammals, whereas in yeast, the rRNA extension Es27 is proposed to act as a protein recruitment hub for NatA in place of NAC. Secondly, the second catalytic subunit Naa50 mediates an additional contact with the rRNA extionsion Es7a on the yeast ribosome, whereas Naa50 is not involved in ribosome contact and hence does not contribute to the ribosome affinity of mammalian NatA/E as shown in Fig. 1. Thirdly, the locations of the NatA/E complex at the ribosome exit site are distinct in the two organisms. Finally, the ribosome binding site of mammalian NAC heavily overlaps with that of yeast NatA/E, suggesting that yeast NAC antagonizes rather than facilitates the ribosome recruitment of NatA/E. These differences, together with the absence of the NatA regulator HYPK in S. cerevisiae, suggest that the ribosome recruitment mechanisms for protein biogenesis factors are distinct between yeast and higher eukaryotic organisms.