Peptide signal sequences, also known as signal peptides or targeting peptides, are short amino acid sequences found at the N-terminus of proteins, responsible for targeting and directing them to their proper cellular compartments during protein synthesis.
When proteins are synthesized in a cell, they are initially produced as pre-proteins, which include the signal sequence. Peptide signal sequences play a crucial role in the sorting and localization of proteins, acting as a specific targeting signal for different cellular destinations, such as the endoplasmic reticulum (ER), mitochondria, peroxisomes, or the secretory pathway.
The signal sequence is recognized and bound by specific signal recognition particles (SRPs) or signal recognition particles receptors (SRPRs) in the cytosol or on the ER surface. This recognition initiates the process of co-translational protein translocation, where the ribosome translating the mRNA and the growing peptide chain are directed to the appropriate organelle for protein synthesis and eventual folding. Once the protein is properly targeted, the signal sequence is generally cleaved off by signal peptidases, leaving behind the mature protein.
Due to the diverse localization and function of different proteins within a cell, peptide signal sequences are highly variable and specific to their target organelle, allowing for precise and efficient protein sorting. The presence or absence of certain amino acid residues within these signal sequences determines the recognition and binding by the corresponding signal recognition particle/receptor complex. The study of peptide signal sequences is crucial for understanding protein trafficking and localization, as well as for genetic engineering and the design of recombinant proteins with desired cellular destinations.
