Signal sequences are the sequences that help in targeting proteins to their proper cellular destinations. These sequences are present in the synthesized protein itself. They are about 20–50 amino acids in length.
These signal sequences or uptake-targeting sequences are bound by receptor proteins. These govern the specificity of targeting. After binding with the receptor, the protein chain is transferred to a translocation channel that allows the protein to pass through the membrane bilayer. The energy required for this unidirectional transfer of a protein into an organelle, without sliding back into the cytosol, is achieved by coupling translocation to ATP hydrolysis.
Some proteins are subsequently sorted further to reach a sub-compartment within the target organelle; this requires yet other signal sequences and other receptor proteins. Once translocation across the membrane is completed, specific proteases remove signal sequences from the mature protein.