Mechanism of eIF6-mediated inhibition of ribosomal subunit joining

Abstract

During the process of ribosomal assembly, the essential eukaryotic translation initiation factor 6 (eIF6) is known to act as a ribosomal anti-association factor. However, a molecular understanding of the anti-association activity of eIF6 is still missing. Here we present the cryo-electron microscopy reconstruction of a complex of the large ribosomal subunit with eukaryotic eIF6 from Saccharomyces cerevisiae. The structure reveals that the eIF6 binding site involves mainly rpL23 (L14p in Escherichia coli). Based on our structural data, we propose that the mechanism of the anti-association activity of eIF6 is based on steric hindrance of intersubunit bridge formation around the dynamic bridge B6.

FIGURE 1.

Localization of eIF6 in a cryo-EM reconstruction of the 60 S-eIF6 complex. A, the map is shown in the crown view. B, close up of the fitted eIF6 structure in EM-density (mesh). C and D, molecular environment of the eIF6 binding site on the 60 S subunit. Thumbnails show the respective orientation. Colors are: orange, eIF6; magenta, sarcin-ricin loop (SRL)); blue, rpL23; green, rpL24; gray, 60 S ribosomal subunit. Abbreviations are: CP, central protuberance; L1, L1 stalk; SB, stalk base; N/C, N and C terminus of eIF6, respectively).

FIGURE 2.

Mechanism of the anti-association activity of eIF6. A, cryo-EM map of the 60 S-eIF6 complex superimposed with the 40 S subunit (semitransparent yellow) in its 60 S-bound conformation. A close-up view of the interfering region of the 40 S subunit is indicated as yellow shadow. B, intersubunit bridges (red) in the vicinity of eIF6. The binding position of eIF6 at the 60 S subunit coincides directly with bridge B6. C, the binding of Efl1p would allow the release eIF6. Homology model of the eEF2-like GTPase Efl1p (red) in its putative ribosome-bound conformation together with eIF6. D, model of the putative Efl1p binding environment on the 60 S ribosomal subunit. Colors are: orange, eIF6; magenta, sarcin-ricin loop (SRL)); blue, rpL23; green, rpL24; gray, 60 S ribosomal subunit. Abbreviations are: SB, stalk base; CP, central protuberance; L1, L1 stalk.