Signal Recognition Particle Arrests Elongation of Nascent Secretory and Membrane Proteins at Multiple Sites in a Transient Manner

The signal recognition particle (SRP) has been shown to target nascent secretory and membrane proteins to the endoplasmic reticulum. In the wheat germ cell-free system, SRP arrests the elongation of the nascent chains until the translational complex is docked to the endoplasmic reticulum membrane where the interaction between SRP and docking protein causes a release of the nascent chain arrest. For two secretory proteins, arrested peptides of 70 amino acids have been identified (Walter, P., Ibrahimi, I., and Blobel, G. (1981) J. Cell Biol. 91, 545-550; Meyer, D. I., Krause, E., and Dobberstein, B. (1982) Nature 297, 647-650). By using an in vitro coupled transcriptiontranslation system, we have analyzed SRP arrest and the resulting peptides of the two secretory proteins lysozyme and granulocyte-macrophage colony-stimulating factor and the membrane protein invariant chain. SRP arrested the elongation of all three proteins at multiple sites, giving rise to ladders of arrested peptides. The size of the arrested peptides increased with the time of translation, resulting in mostly fulllength pre-peptides after about 40 min. This suggests that SRP arrest is transient rather than stable. Upon addition of microsomes, the SRP arrest was released, and all the blocked peptides could be chased into maturf proteins or full-length precursors