Editing Chloroplast (section)

==== Transport proteins and membrane translocons ====

After a chloroplast [[polypeptide]] is synthesized on a [[ribosome]] in the [[cytosol]], an enzyme [[enzyme specificity|specific]] to chloroplast proteins<ref name="Waegemann-1996">{{cite journal | vauthors=Waegemann K, Soll J | title=Phosphorylation of the transit sequence of chloroplast precursor proteins | journal=The Journal of Biological Chemistry | volume=271 | issue=11 | pages=6545–54 | date=March 1996 | pmid=8626459 | doi=10.1074/jbc.271.11.6545 | doi-access=free }}</ref> [[phosphorylates]], or adds a [[phosphate group]] to many (but not all) of them in their transit sequences.<ref name="Soll-2004" />
Phosphorylation helps many proteins bind the polypeptide, keeping it from [[protein folding|folding]] prematurely.<ref name="Soll-2004" /> This is important because it prevents chloroplast proteins from assuming their active form and carrying out their chloroplast functions in the wrong place—the [[cytosol]].<ref name="May-2000" /><ref name="Jarvis-2001">{{cite journal | vauthors=Jarvis P, Soll J | title=Toc, Tic, and chloroplast protein import | journal=Biochimica et Biophysica Acta (BBA) - Molecular Cell Research | volume=1541 | issue=1–2 | pages=64–79 | date=December 2001 | pmid=11750663 | doi=10.1016/S0167-4889(01)00147-1 | doi-access=free }}</ref> At the same time, they have to keep just enough shape so that they can be recognized by the chloroplast.<ref name="May-2000">{{cite journal | vauthors=May T, Soll J | title=14-3-3 proteins form a guidance complex with chloroplast precursor proteins in plants | journal=The Plant Cell | volume=12 | issue=1 | pages=53–64 | date=January 2000 | pmid=10634907 | pmc=140214 | doi=10.1105/tpc.12.1.53 | bibcode=2000PlanC..12...53M }}</ref> These proteins also help the polypeptide get imported into the chloroplast.<ref name="Soll-2004" />

From here, chloroplast proteins bound for the stroma must pass through two protein complexes—the [[TOC complex]], or ''[[translocon|'''t'''ranslocon]] on the '''o'''uter '''c'''hloroplast membrane'', and the [[TIC translocon]], or '''''t'''ranslocon on the '''i'''nner '''c'''hloroplast membrane [[translocon]]''.<!--Yes, that's what the source says--><ref name="Soll-2004" /> Chloroplast polypeptide chains probably often travel through the two complexes at the same time, but the TIC complex can also retrieve preproteins lost in the [[chloroplast intermembrane space|intermembrane space]].<ref name="Soll-2004" />