Editing Protein targeting (section)

== Signal peptides ==<!-- This section is linked from [[Carboxypeptidase E]] -->
{{main|Signal peptide}}
Signal peptides serve as targeting signals, enabling cellular transport machinery to direct proteins to specific intracellular or extracellular locations. While no [[consensus sequence]] has been identified for signal peptides, many nonetheless possess a characteristic tripartite structure:<ref name="Nelson-2017" />

# A positively charged, hydrophilic region near the N-terminal.
# A span of 10 to 15 hydrophobic amino acids near the middle of the signal peptide.
# A slightly polar region near the C-terminal, typically favoring amino acids with smaller side chains at positions approaching the cleavage site.

After a protein has reached its destination, the signal peptide is generally cleaved by a [[signal peptidase]].<ref name="Nelson-2017" /> Consequently, most mature proteins do not contain signal peptides. While most signal peptides are found at the N-terminal, in [[peroxisome]]s the targeting sequence is located on the C-terminal extension.<ref>{{cite journal | vauthors = Wanders RJ | title = Metabolic and molecular basis of peroxisomal disorders: a review | journal = American Journal of Medical Genetics. Part A | volume = 126A | issue = 4 | pages = 355–75 | date = May 2004 | pmid = 15098234 | doi = 10.1002/ajmg.a.20661 | s2cid = 24025032 }}</ref> Unlike signal peptides, [[signal patches]] are composed by amino acid residues that are discontinuous in the [[primary sequence]] but become functional when [[protein folding|folding]] brings them together on the protein surface.<ref>{{cite journal | vauthors = Moreira IS, Fernandes PA, Ramos MJ | title = Hot spots--a review of the protein-protein interface determinant amino-acid residues | journal = Proteins | volume = 68 | issue = 4 | pages = 803–12 | date = September 2007 | pmid = 17546660 | doi = 10.1002/prot.21396 | s2cid = 18578313 | doi-access = free }}</ref> Unlike most signal sequences, signal patches are not cleaved after sorting is complete.<ref name=biosynthetic>{{cite Q|Q39664981 }}</ref> In addition to intrinsic signaling sequences, [[posttranslational modification|protein modifications]] like glycosylation can also induce targeting to specific intracellular or extracellular regions.