Editing Protein targeting (section)

===Chloroplasts===
[[Chloroplast]]s are similar to mitochondria in that they contain their own DNA for production of some of their components. However, the majority of their proteins are obtained via post-translational translocation and arise from nuclear genes. Proteins may be targeted to several sites of the chloroplast depending on their sequences such as the outer envelope, inner envelope, stroma, thylakoid lumen, or the thylakoid membrane.<ref name="Lodish, Berk, Kaiser, Krieger, Bretscher, Ploegh, Martin, Yaffe, Amon-2021" /> Proteins are targeted to Thylakoids by mechanisms related to Bacterial Protein Translocation.<ref name="Lodish-2016">{{Cite book |last1=Lodish |first1=Harvey |title=Molecular Cell Biology |last2=Berk |first2=Arnold |last3=Kaiser |first3=Chris A |last4=Krieger |first4=Monty |last5=Bretscher |first5=Anthony |last6=Ploegh |first6=Hidde |last7=Amon |first7=Angelika |last8=Scott |first8=Matthew P |publisher=W.H. Freeman and Company |year=2016 |isbn=978-1-4641-8339-3 |edition=8th |location=New York, USA |pages=609–610 |language=en}}</ref> Proteins targeted to the envelope of chloroplasts usually lack cleavable sorting sequence and are laterally displaced via membrane sorting complexes. General import for the majority of preproteins requires translocation from the cytosol through the [[TIC/TOC complex|Toc and Tic complexes]] located within the chloroplast envelope. Where Toc is an abbreviation for the translocase of the outer chloroplast envelope and Tic is the translocase of the inner chloroplast envelope. There is a minimum of three proteins that make up the function of the Toc complex. Two of which, referred to as Toc159 and Toc34, are responsible for the docking of stromal import sequences and both contain [[GTPase]] activity. The third known as Toc 75, is the actual translocation channel that feeds the recognized preprotein by Toc159/34 into the chloroplast.<ref name="Soll J, Schleiff E-2004">{{Cite journal |last=Soll J, Schleiff E |date=2004 |title=Protein import into chloroplasts |url=https://www.nature.com/articles/nrm1333 |journal=Nature Reviews Molecular Cell Biology |volume=5 |issue=3 |pages=198–208 |doi=10.1038/nrm1333 |pmid=14991000 |s2cid=32453554 |via=Nature}}</ref>

==== Stroma ====
Targeting to the stroma requires the preprotein to have a stromal import sequence that is recognized by the Tic complex of the inner envelope upon being translocated from the outer envelope by the Toc complex. The Tic complex is composed of at least five different Tic proteins that are required to form the translocation channel across the inner envelope.<ref>{{Cite journal |last=Gutensohn M, Fan E, Frielingsdorf S, Hanner P, Hou B, Hust B, Klosgen R |date=2005 |title=Toc, Tic, Tat et al.: structure and function of protein transport machineries in chloroplasts |url=https://pubmed.ncbi.nlm.nih.gov/16386331/ |journal=Journal of Plant Physiology |volume=163 |issue=3 |pages=333–347 |doi=10.1016/j.jplph.2005.11.009 |pmid=16386331 |via=PubMed}}</ref> Upon being delivered to the stroma, the stromal import sequence is cleaved off via a signal peptidase. This delivery process to the stroma is currently known to be driven by [[ATP hydrolysis]] via stromal [[Heat shock protein|HSP]] chaperones, instead of the transmembrane [[electrochemical gradient]] that is established in mitochondria to drive protein import.<ref name="Soll J, Schleiff E-2004" /> Further intra-chloroplast sorting depends on additional target sequences such as those designated to the [[Thylakoid|thylakoid membrane]] or the [[Thylakoid|thylakoid lumen]].

==== Thylakoid lumen ====
If a protein is to be targeted to the thylakoid lumen, this may occur via four differently known routes that closely resemble bacterial protein transport mechanisms. The route that is taken depends upon the protein delivered to the stroma being in either an unfolded or metal-bound folded state. Both of which will still contain a thylakoid targeting sequence that is also cleaved upon entry to the lumen. While protein import into the stroma is ATP-driven, the pathway for metal-bound proteins in a folded state to the thylakoid lumen has been shown to be driven by a pH gradient.[[File:Protein targeting to the thylakoid membrane in chloroplasts.png|thumb|Pathways for proteins targeted to the thylakoid membrane in chloroplasts.]]

==== Thylakoid membrane ====
Proteins bound for the membrane of the thylakoid will follow up to four known routes that are illustrated in the corresponding figure shown. They may follow a co-translational insertion route that utilizes stromal ribosomes and the SecY/E transmembrane complex, the SRP-dependent pathway, the spontaneous insertion pathway, or the GET pathway. The last of the three are post-translational pathways originating from nuclear genes and therefor constitute the majority of proteins targeted to the thylakoid membrane. According to recent review articles in the journal of biochemistry and molecular biology, the exact mechanisms are not yet fully understood.