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Figure 5.
Figure 5. Model for Interplay between the TF, SRP, and the
Nascent Chain during Translation
Schematic view of the
bottom of the ribosome, showing ribosomal proteins L23, (green),
L24 (yellow), and L29 (orange), and with the tunnel exit site
indicated with an arrow. (A) TF is the first chaperone to bind
to the bacterial ribosome as the nascent chain emerges from the
tunnel. Direct contact between L24 and the TF results in
conformational changes in the TF-BD that expose a hydrophobic
crevice located in the TF-BD, through which the polypeptide
chain can pass. (B) The SRP particle binds to the ribosome in
the presence of the TF, initially through contacts with L29. The
M domain of the SRP covers the hydrophobic cavity in the TF-BD
and monitors for the presence of a signal sequence in the
nascent chain. (C) Interaction between the signal
sequence-containing nascent chain and the M domain of the SRP
enables the M domain to establish contact with H24 of the 23S
rRNA. This interaction stabilizes the SRP particle on the
ribosome and could lead to dissociation of the TF. (D) In the
absence of a signal sequence, the M domain cannot establish
contact with the 23S rRNA, and the SRP dissociates from the
ribosome. Continued translation leads to elongation of the
nascent chain, which is channeled through the hydrophobic cavity
into the central cavity (body) of the TF. The restricted size of
the cavity due to the presence of L24 limits the possibility for
folding of entire protein domains. The PPIase domain (head) of
the TF may recognize the nascent chain as it emerges from the
head side, monitoring for proline residues that need to be
isomerized (arrowed).
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