Editing C-terminus (section)

==Function==
===C-terminal retention signals===
While the [[N-terminus]] of a protein often contains [[Protein targeting|targeting signals]], the C-terminus can contain retention signals for protein sorting. The most common [[ER retention]] signal is the amino acid sequence [[KDEL (amino acid sequence)|-KDEL]] ([[lysine|Lys]]-[[Aspartic acid|Asp]]-[[Glutamic acid|Glu]]-[[Leucine|Leu]]) or [[HDEL (amino acid sequence)|-HDEL]] ([[Histidine|His]]-Asp-Glu-Leu) at the C-terminus.  This keeps the protein in the [[endoplasmic reticulum]] and prevents it from entering the [[secretory pathway]].

===Peroxisomal targeting signal===
{{See also|Protein targeting#Peroxisomes|l1=Protein targeting to peroxisomes}}
The sequence -SKL (Ser-Lys-Leu) or similar near C-terminus serves as [[peroxisomal targeting signal]] 1, directing the protein into [[peroxisome]].{{cn|date=April 2024}}

===C-terminal modifications===
The C-terminus of proteins can be modified [[posttranslational modification|posttranslationally]], most commonly by the addition of a [[lipid]] anchor to the C-terminus that allows the protein to be inserted into a membrane without having a [[transmembrane domain]].

====Prenylation====
{{main|Prenylation}}
One form of C-terminal modification is [[prenylation]]. During prenylation, a [[Farnesyl pyrophosphate|farnesyl]]- or [[Geranylgeranyl pyrophosphate|geranylgeranyl]]-isoprenoid membrane anchor is added to a [[cysteine]] residue near the C-terminus. Small, membrane-bound [[G protein]]s are often modified this way.{{cn|date=April 2024}}

====GPI anchors====
{{main|Glycosylphosphatidylinositol}}
Another form of C-terminal modification is the addition of a phosphoglycan, [[glycosylphosphatidylinositol]] (GPI), as a membrane anchor. The GPI anchor is attached to the C-terminus after proteolytic cleavage of a C-terminal propeptide. The most prominent example for this type of modification is the [[prion]] protein.

====Methylation====
{{See also|Methylation#In biology|l1=Methylation|(Phosphatase 2A protein)-leucine-carboxy methyltransferase}}
C-terminal [[leucine]] is methylated at carboxyl group by enzyme [[leucine carboxyl methyltransferase 1]] in vertebrates, forming [[methyl ester]].<ref>{{cite web|url=https://www.rhea-db.org/rhea/48544|title=RHEA:48544|publisher=[[Swiss Institute of Bioinformatics]]}}</ref>

===C-terminal domain===
[[File: ARN pol II en accion.jpg| thumb|300 px|RNA POL II in action.]]

The C-terminal domain of some proteins has specialized functions. In humans, the CTD of [[RNA polymerase II]] typically consists of up to 52 [[Repeated sequence (DNA)|repeats]] of the sequence [[Tyrosine|Tyr]]-Ser-[[Proline|Pro]]-[[Threonine|Thr]]-Ser-Pro-Ser.<ref>{{cite journal |author=Meinhart A, Cramer P |title=Recognition of RNA polymerase II carboxy-terminal domain by 3'-RNA-processing factors |journal=Nature |volume=430 |issue=6996 |pages=223–6 |date=July 2004 |pmid=15241417 |doi=10.1038/nature02679 |bibcode=2004Natur.430..223M |s2cid=4418258 |hdl=11858/00-001M-0000-0015-8512-8 |url=https://resolver.sub.uni-goettingen.de/purl?gro-2/1538 |hdl-access=free }}</ref> This allows other proteins to bind to the C-terminal domain of RNA polymerase in order to activate polymerase activity. These domains are then involved in the [[DNA transcription#Initiation|initiation]] of DNA transcription, the [[capping enzyme|capping]] of the [[Messenger RNA|RNA transcript]], and attachment to the [[spliceosome]] for [[RNA splicing]].<ref>{{cite journal |author=Brickey WJ, Greenleaf AL |title=Functional studies of the carboxy-terminal repeat domain of Drosophila RNA polymerase II in vivo |journal=Genetics |volume=140 |issue=2 |pages=599–613 |date=June 1995 |doi=10.1093/genetics/140.2.599 |pmid=7498740 |pmc=1206638 }}</ref>