This article deals with protein targeting in eukaryotes except where noted.
Protein targeting or protein sorting is the mechanism by which a cell transports proteins to the appropriate positions in the cell or outside of it. Sorting targets can be the inner space of an organelle, any of several interior membranes, the cell's outer membrane, or its exterior via secretion. This delivery process is carried out based on information contained in the protein itself. Correct sorting is crucial for the cell; errors can lead to diseases.
Targeting signals
Targeting signals are the pieces of information that enable the cellular transport machinery to correctly position a protein inside or outside the cell. This information is contained in the polypeptide chain or in the folded protein. The continuous stretch of amino acid residues in the chain that enables targeting are called signal peptides or targeting peptides. There are two types of targeting peptides, the presequences and the internal targeting peptides. The presequences of the targeting peptide are often found at the N-terminal extension and is composed of between 6-136 basic and hydrophobic amino acids.In case of peroxisomes the targeting sequence is on the C-terminal extension mostly. Other signals are composed by parts which are separate in the primary sequence. To function, these components have to come together on the protein surface by folding. They are called signal patches. In addition, protein modifications like glycosylations can induce targeting.