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Signal Sequence

In 1970, Gunter Blobel performed experiments on the translocation of proteins across membranes, a study that actually earned him the 1999 Nobel prize. Blobel discovered that many proteins have a signal sequence which refers to a short amino acid sequence at one end that functions like a postal code for the target organelle. The translation of mRNA into protein by a ribosome actually happens inside the cytosol. If the synthesized proteins is part of, or grouped in a different organelle they can be brought there using either of two ways, depending on the protein.

In cotranslational translocation the N-terminal signal sequence of the protein is recognized by a signal recognition particle (SRP) while the protein is still being synthesized on the ribosome. The synthesis process is paused while the ribosome-protein complex is transferred to an SRP receptor on the endoplasmic reticulum. There the nascent protein is inserted into the Sec61 translocation complex that passes through the ER membrane. The signal sequence is then cleaved from the polypeptide when it is translocated into the ER by signal peptidase in secretory proteins.

The other method is posttranslational translocation. Even though a majority of proteins are cotranslationally translocated there are some that are translated in the cytosol and then later transported to their destination. This happens only for proteins that got a mitochondrion, a chloroplast or a peroxisome.

Web Resources On Signal Sequence

Signal Sequence & Transmembrane Predictions
Gene design of signal sequence for the effective secretion of recombinant protein using insect cell

Book Resources On Signal Sequence

Biochemistry of Signal Transduction and Regulation by Gerhard Krauss
Receptor Signal Transduction Protocols by Gary B. Willars, R. A. J. Challiss

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