Editing Protein targeting (section)

==In bacteria and archaea==
As discussed above (see [[#Protein translocation|protein translocation]]), most prokaryotic membrane-bound and secretory proteins are targeted to the plasma membrane by either a co-translation pathway that uses bacterial SRP or a post-translation pathway that requires SecA and SecB. At the plasma membrane, these two pathways deliver proteins to the SecYEG translocon for translocation. Bacteria may have a single plasma membrane ([[Gram-positive bacteria]]), or an inner membrane plus an outer membrane separated by the [[periplasm]] ([[Gram-negative bacteria]]). Besides the plasma membrane the majority of prokaryotes lack membrane-bound organelles as found in eukaryotes, but they may assemble proteins onto various types of inclusions such as gas vesicles and storage granules.

===Gram-negative bacteria===
{{main|Secretion#Secretion_in_Gram_negative_bacteria|l1=Gram-Negative Bacterial Secretion}}
In gram-negative bacteria proteins may be incorporated into the plasma membrane, the outer membrane, the periplasm or secreted into the environment. Systems for secreting proteins across the bacterial outer membrane may be quite complex and play key roles in pathogenesis. These systems may be described as type I secretion, type II secretion, etc.

===Gram-positive bacteria===
{{main|Secretion#Secretion_in_Gram_positive_bacteria|l1=Gram-Positive Bacterial Secretion}}
In most gram-positive bacteria, certain proteins are targeted for export across the plasma membrane and subsequent covalent attachment to the bacterial cell wall. A specialized enzyme, [[sortase]], cleaves the target protein at a characteristic recognition site near the protein C-terminus, such as an LPXTG motif (where X can be any amino acid), then transfers the protein onto the cell wall. Several analogous systems are found that likewise feature a signature motif on the extra-cytoplasmic face, a C-terminal transmembrane domain, and cluster of basic residues on the cytosolic face at the protein's extreme C-terminus. The PEP-CTERM/[[exosortase]] system, found in many Gram-negative bacteria, seems to be related to [[extracellular polymeric substance]] production. The PGF-CTERM/archaeosortase A system in [[archaea]] is related to [[S-layer]] production. The GlyGly-CTERM/rhombosortase system, found in the Shewanella, Vibrio, and a few other genera, seems involved in the release of proteases, nucleases, and other enzymes.