Figure 4.
Action of elongation factor P (EF-P) on ribosomes stalled at polyproline stretches. (A) Ribosomes stall during translation of proteins containing three consecutive prolines (red stars). Binding of the peptidyl-tRNA (green) to the P site is destabilized, which (B) can lead to peptidyl-tRNA drop-off. (C) The all-trans or all-cis conformations of polyprolines of the nascent chain are not possible because of a steric clash with G2061 (gray) within the tunnel wall. Peptidyl-tRNA is destabilized and prevents accommodation of the A-site tRNA (orange) and peptide bond formation. (D) Ribosomes stalled on polyproline stretches are recognized by EF-P (pink), which binds within the E-site region and stabilizes the peptidyl-tRNA. EF-P binding is facilitated via contacts with the L1 stalk and the P-site tRNA as well as E-site codon. (E) Interaction of the ε(R)-β-lysyl-hydroxylysine with the CCA-end of P-site tRNAPro stabilizes the P-site tRNA as well as the nascent chain, by forcing the prolines to adopt an alternative conformation that passes into the ribosomal exit tunnel. (F) Thus, an optimal geometry between the nascent chain and the aminoacyl-tRNA in the A site is achieved and peptide bond formation can occur. (From Huter et al. 2017; adapted, with permission, from Elsevier © 2017.)