Figure 5-D-2. Insertion of newly
synthesized peptide into rough endoplasmic reticulum.
(a) Before synthesis.
(b) After about 70 amino acids have been synthesized, the signal sequence
emerges from the ribosome. A signal recognition particle (SRP)
directs the new peptide (together with the ribosome) toward the ER membrane
surface, because it can bind both the new peptide and a receptor on the membrane.
(c) The signal sequence inserts itself into the ER membrane with the help
of a signal sequence binding protein. Several proteins are recruited to
form a channel for the growing peptide to pass through the membrane.
(d) SRP dissociates from the peptide-ribosome complex. The signal
sequence is cleaved and quickly degraded.
Figure 5-D-3. Prevention of protein misfolding
by the protein BiP (a chaperone).
(a) Before a peptide is completely synthesized, it may fold prematurely
since hydrophobic regions tend to aggregate.
(b) BiP can bind to unfolded, hydrophobic segments of peptides,
preventing premature aggregation.