Figure 5: Head tilting results from tRNA interactions with the PE loop and protein S13.
From: Steric interactions lead to collective tilting motion in the ribosome during mRNA–tRNA translocation
Representative structural snapshots of (a) ap/P–pe/E, (b) HT and (c) P/P–E/E configurations. The PE loop (pink) and protein S13 (grey) are in close proximity to the P- and A-site tRNAs (red and yellow) during translocation. To elucidate the extent to which the PE loop and protein S13 contribute to tilting, perturbations were introduced to the model. (d–g) Probability distributions P(φhead, θhead) illustrate the relative influence of steric composition on tilting. When all sterics are included (d), 
(the average tilt as a function of rotation; dashed grey line) reaches a maximum value of ∼10°. When steric interactions between protein S13 and tRNA are not included (e), the maximum value of 
is reduced to ∼8°. When the sterics of S13 are present, while steric interactions between the PE loop and tRNA are not included (f), the maximum tilt is reduced to ∼6°. When both S13 and PE loop sterics are not included (g), head tilting is most strongly attenuated (
).
