Signal peptides (SPs) are short peptides directing newly synthesized proteins towards the secretory pathway. SPs are usually 16~30 amino acids long and are found in the N-terminus of proteins in virtually all organisms. Signal peptidases will remove SPs after the protein translocation.
Several different protein export systems exist in bacteria. The general secretion (Sec) protein export pathway and the twin-arginine translocation (Tat) export pathway are the two main protein secretion pathways. Sec-dependent proteins are translocated to the plasma membrane either co- or post-translationally. In the co-translational export mode, precursor proteins are recognized at the ribosome by the signal recognition particle (SRP) and then targeted to the transmembrane SecYEG channel by SRP and FtsY, t
Signal peptides from different proteins show a common structure. Generally, a signal peptide is composed of three distinct domains: a positively charged n-region (1-5 residues long), a central, hydrophobic h-region (7-15 residues long) and a c-region (3-7 residues) with the cleavage site of signal peptidase. A highly conserved twin-arginine motif (SRRXFLK, where X is often, but not always, a polar amino acid residue) is located at the n/h-region boundary of Tat-specific signal peptides.