Editing Signal peptide (section)

==Structure==
The core of the signal peptide contains a long stretch of hydrophobic amino acids (about 5–16 residues long)<ref>{{cite journal | vauthors = Käll L, Krogh A, Sonnhammer EL | title = A combined transmembrane topology and signal peptide prediction method | journal = Journal of Molecular Biology | volume = 338 | issue = 5 | pages = 1027–36 | date = May 2004 | pmid = 15111065 | doi = 10.1016/j.jmb.2004.03.016 }}</ref> that has a tendency to form a single alpha-helix and is also referred to as the "h-region". In addition, many signal peptides begin with a short positively charged stretch of amino acids, which may help to enforce proper topology of the polypeptide during translocation by what is known as the '''positive-inside rule'''.<ref name="von Heijne-1988">{{cite journal | vauthors = von Heijne G, Gavel Y | title = Topogenic signals in integral membrane proteins | journal = European Journal of Biochemistry | volume = 174 | issue = 4 | pages = 671–8 | date = July 1988 | pmid = 3134198 | doi = 10.1111/j.1432-1033.1988.tb14150.x | doi-access = free | author-link1 = Gunnar von Heijne }}</ref> Because of its close location to the [[N-terminus]] it is called the "n-region". At the end of the signal peptide there is typically a stretch of amino acids that is recognized and cleaved by [[signal peptidase]] and therefore named cleavage site. This cleavage site is absent from transmembrane-domains that serve as signal peptides, which are sometimes referred to as signal anchor sequences. Signal peptidase may cleave either during or after completion of translocation to generate a free signal peptide and a mature protein. The free signal peptides are then digested by specific proteases.
Moreover, different target locations are aimed by different types of signal peptides. For example, the structure of a target peptide aiming for the mitochondrial environment differs in terms of length and shows an alternating pattern of small positively charged and hydrophobic stretches. Nucleus aiming signal peptides can be found at both the N-terminus and the C-terminus of a protein and are in most cases retained in the mature protein.