The Journal of Cell Biology, Vol 104, 61-66, Copyright © 1987 by The Rockefeller University Press
Signal recognition particle causes a transient arrest in the biosynthesis of prepromelittin and mediates its translocation across mammalian endoplasmic reticulum
I Ibrahimi
The translocation of prepromelittin (pPM) across mammalian endoplasmic
reticulum was studied in both wheat germ and reticulocyte lysate. In the
wheat germ system, signal recognition particle (SRP) caused a transient
arrest in the synthesis of pPM. This was indicated by a slowdown in the
rate of synthesis of pPM in the presence of SRP. The arrest was specific,
dependent on the concentration of SRP, and more effective at early
incubation time. In a tightly synchronized translation system, SRP had no
apparent effect on the elongation of pPM, indicating that the effect of SRP
on pPM chain synthesis might be at the final stages of chain elongation and
release from the ribosome. This was reflected in a transient accumulation
of pPM as peptidyl tRNA. Because pPM is composed of only 70 amino acids,
arrest by SRP may be very close to chain termination. Arrest at this stage
of chain synthesis seems to be unstable and the nascent chain gets
terminated and released from the ribosome after a transient delay. The
translocation of pPM was shown to be dependent on both SRP and docking
protein. The difference in the translocation efficiency of pPM in
reticulocyte and wheat germ lysates may reflect a difference in the
targeting process in the two systems.
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