Editing Protein isoform (section)

{{Short description|Forms of a protein produced from different genes}}
[[File:DNA alternative splicing.gif|thumb|Protein A, B and C are isoforms encoded from the same gene through [[alternative splicing]].|420x420px]]
A '''protein isoform''', or "'''protein variant'''",<ref name=":4" /> is a member of a set of highly similar [[proteins]] that originate from a single [[gene]] and are the result of genetic differences.<ref>{{cite journal | vauthors = Schlüter H, Apweiler R, Holzhütter HG, Jungblut PR | title = Finding one's way in proteomics: a protein species nomenclature | journal = Chemistry Central Journal | volume = 3 | pages = 11 | date = September 2009 | pmid = 19740416 | pmc = 2758878 | doi = 10.1186/1752-153X-3-11 | doi-access = free }}</ref> While many perform the same or similar biological roles, some isoforms have unique functions. A set of protein isoforms may be formed from [[Alternative splicing|alternative splicings]], variable [[Promoter (genetics)|promoter]] usage, or other [[Post-transcriptional modification|post-transcriptional modifications]] of a single gene; [[post-translational modification]]s are generally not considered. (For that, see [[Proteoform]]s.) Through [[RNA splicing]] mechanisms, [[mRNA]] has the ability to select different protein-coding segments ([[Exon|exons]]) of a gene, or even different parts of exons from RNA to form different mRNA sequences. Each unique sequence produces a specific form of a protein.

The discovery of isoforms could explain the discrepancy between the small number of protein coding regions of genes revealed by the [[human genome project]] and the large diversity of proteins seen in an organism: different proteins encoded by the same gene could increase the diversity of the [[proteome]]. Isoforms at the RNA level are readily characterized by [[Complementary DNA|cDNA]] transcript studies. Many human genes possess confirmed [[alternative splicing]] isoforms. It has been estimated that ~100,000 expressed sequence tags ([[Expressed sequence tag|ESTs]]) can be identified in humans.<ref name=":4">{{cite journal | vauthors = Brett D, Pospisil H, Valcárcel J, Reich J, Bork P | title = Alternative splicing and genome complexity | journal = Nature Genetics | volume = 30 | issue = 1 | pages = 29–30 | date = January 2002 | pmid = 11743582 | doi = 10.1038/ng803 | s2cid = 2724843 }}</ref> Isoforms at the protein level can manifest in the deletion of whole domains or shorter loops, usually located on the surface of the protein.<ref>{{Cite book | doi = 10.1002/9783527636778.ch54| chapter = Structure Prediction for Alternatively Spliced Proteins| title = Alternative pre-mRNA Splicing| pages = 582| year = 2012| last1 = Kozlowski | first1 = L. | last2 = Orlowski | first2 = J. | last3 = Bujnicki | first3 = J. M. | isbn = 9783527636778}}</ref>