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Protein isoform

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A protein isoform, or "protein variant",<a href="#cite_note-:4-1">[1]</a> is a member of a set of highly similar <a href="Proteins" class="mw-redirect" title="Proteins">proteins</a> that originate from a single <a href="Gene" title="Gene">gene</a> and are the result of genetic differences.<a href="#cite_note-2">[2]</a> While many perform the same or similar biological roles, some isoforms have unique functions. A set of protein isoforms may be formed from <a href="Alternative_splicing" title="Alternative splicing">alternative splicings</a>, variable <a href="Promoter_(genetics)" title="Promoter (genetics)">promoter</a> usage, or other <a href="Post-transcriptional_modification" title="Post-transcriptional modification">post-transcriptional modifications</a> of a single gene; <a href="Post-translational_modification" title="Post-translational modification">post-translational modifications</a> are generally not considered. (For that, see <a href="Proteoform" title="Proteoform">Proteoforms</a>.) Through <a href="RNA_splicing" title="RNA splicing">RNA splicing</a> mechanisms, <a href="MRNA" class="mw-redirect" title="MRNA">mRNA</a> has the ability to select different protein-coding segments (<a href="Exon" title="Exon">exons</a>) 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 <a href="Human_genome_project" class="mw-redirect" title="Human genome project">human genome project</a> and the large diversity of proteins seen in an organism: different proteins encoded by the same gene could increase the diversity of the <a href="Proteome" title="Proteome">proteome</a>. Isoforms at the RNA level are readily characterized by <a href="Complementary_DNA" title="Complementary DNA">cDNA</a> transcript studies. Many human genes possess confirmed <a href="Alternative_splicing" title="Alternative splicing">alternative splicing</a> isoforms. It has been estimated that ~100,000 expressed sequence tags (<a href="Expressed_sequence_tag" title="Expressed sequence tag">ESTs</a>) can be identified in humans.<a href="#cite_note-:4-1">[1]</a> Isoforms at the protein level can manifest in the deletion of whole domains or shorter loops, usually located on the surface of the protein.<a href="#cite_note-3">[3]</a>

<div class="mw-heading mw-heading2"><h2 id="Definition">Definition</h2></div>

One single gene has the ability to produce multiple proteins that differ both in structure and composition;<a href="#cite_note-:0-4">[4]</a><a href="#cite_note-:1-5">[5]</a> this process is regulated by the <a href="Alternative_splicing" title="Alternative splicing">alternative splicing</a> of mRNA, though it is not clear to what extent such a process affects the diversity of the human proteome, as the abundance of mRNA transcript isoforms does not necessarily correlate with the abundance of protein isoforms.<a href="#cite_note-6">[6]</a> Three-dimensional protein structure comparisons can be used to help determine which, if any, isoforms represent functional protein products, and the structure of most isoforms in the human proteome has been predicted by <a href="AlphaFold" title="AlphaFold">AlphaFold</a> and publicly released at <a rel="nofollow" class="external text" href="https://www.isoform.io">isoform.io</a>. <a href="#cite_note-7">[7]</a> The specificity of translated isoforms is derived by the protein's structure/function, as well as the cell type and developmental stage during which they are produced.<a href="#cite_note-:0-4">[4]</a><a href="#cite_note-:1-5">[5]</a> Determining specificity becomes more complicated when a protein has multiple subunits and each subunit has multiple isoforms.

For example, the <a href="AMP-activated_protein_kinase" title="AMP-activated protein kinase">5' AMP-activated protein kinase</a> (AMPK), an enzyme, which performs different roles in human cells, has 3 subunits:<a href="#cite_note-:2-8">[8]</a>

<ul><li>α, catalytic domain, has two isoforms: α1 and α2 which are encoded from <a href="PRKAA1" class="mw-redirect" title="PRKAA1">PRKAA1</a> and <a href="PRKAA2" title="PRKAA2">PRKAA2</a></li>

<li>β, regulatory domain, has two isoforms: β1 and β2 which are encoded from <a href="PRKAB1" title="PRKAB1">PRKAB1</a> and <a href="PRKAB2" title="PRKAB2">PRKAB2</a></li>

<li>γ, regulatory domain, has three isoforms: γ1, γ2, and γ3 which are encoded from <a href="PRKAG1" title="PRKAG1">PRKAG1</a>, <a href="PRKAG2" title="PRKAG2">PRKAG2</a>, and <a href="PRKAG3" title="PRKAG3">PRKAG3</a></li></ul>

In human skeletal muscle, the preferred form is α2β2γ1.<a href="#cite_note-:2-8">[8]</a> But in the human liver, the most abundant form is α1β2γ1.<a href="#cite_note-:2-8">[8]</a>

<div class="mw-heading mw-heading2"><h2 id="Mechanism">Mechanism</h2></div>

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</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="Alternative_splicing" title="Alternative splicing">Alternative splicing</a></div>

The primary mechanisms that produce protein isoforms are alternative splicing and variable promoter usage, though modifications due to genetic changes, such as <a href="Mutation" title="Mutation">mutations</a> and <a href="Polymorphism_(biology)" title="Polymorphism (biology)">polymorphisms</a> are sometimes also considered distinct isoforms.<a href="#cite_note-:5-9">[9]</a>

Alternative splicing is the main <a href="Post-transcriptional_modification" title="Post-transcriptional modification">post-transcriptional modification</a> process that produces mRNA transcript isoforms, and is a major molecular mechanism that may contribute to protein diversity.<a href="#cite_note-:1-5">[5]</a> The <a href="Spliceosome" title="Spliceosome">spliceosome</a>, a large <a href="Ribonucleoprotein" class="mw-redirect" title="Ribonucleoprotein">ribonucleoprotein</a>, is the molecular machine inside the nucleus responsible for RNA cleavage and <a href="Ligation_(molecular_biology)" title="Ligation (molecular biology)">ligation</a>, removing non-protein coding segments (<a href="Intron" title="Intron">introns</a>).<a href="#cite_note-:3-10">[10]</a>

Because splicing is a process that occurs between <a href="Transcription_(biology)" title="Transcription (biology)">transcription</a> and <a href="Translation_(biology)" title="Translation (biology)">translation</a>, its primary effects have mainly been studied through <a href="Genomics" title="Genomics">genomics</a> techniques—for example, <a href="Microarray_analysis_techniques" title="Microarray analysis techniques">microarray</a> analyses and <a href="RNA-Seq" title="RNA-Seq">RNA sequencing</a> have been used to identify alternatively spliced transcripts and measure their abundances.<a href="#cite_note-:5-9">[9]</a> Transcript abundance is often used as a proxy for the abundance of protein isoforms, though <a href="Proteomics" title="Proteomics">proteomics</a> experiments using gel electrophoresis and mass spectrometry have demonstrated that the correlation between transcript and protein counts is often low, and that one protein isoform is usually dominant.<a href="#cite_note-:6-11">[11]</a> One 2015 study states that the cause of this discrepancy likely occurs after translation, though the mechanism is essentially unknown.<a href="#cite_note-12">[12]</a> Consequently, although alternative splicing has been implicated as an important link between variation and disease, there is no conclusive evidence that it acts primarily by producing novel protein isoforms.<a href="#cite_note-:6-11">[11]</a>

Alternative splicing generally describes a tightly regulated process in which alternative transcripts are intentionally generated by the splicing machinery. However, such transcripts are also produced by splicing errors in a process called "noisy splicing," and are also potentially translated into protein isoforms. Although ~95% of multi-exonic genes are thought to be alternatively spliced, one study on noisy splicing observed that most of the different low-abundance transcripts are noise, and predicts that most alternative transcript and protein isoforms present in a cell are not functionally relevant.<a href="#cite_note-13">[13]</a>

Other transcriptional and post-transcriptional regulatory steps can also produce different protein isoforms.<a href="#cite_note-14">[14]</a> Variable promoter usage occurs when the transcriptional machinery of a cell (<a href="RNA_polymerase" title="RNA polymerase">RNA polymerase</a>, <a href="Transcription_factor" title="Transcription factor">transcription factors</a>, and other <a href="Enzyme" title="Enzyme">enzymes</a>) begin transcription at different promoters—the region of DNA near a gene that serves as an initial binding site—resulting in slightly modified transcripts and protein isoforms.

<div class="mw-heading mw-heading2"><h2 id="Characteristics">Characteristics</h2></div>

Generally, one protein isoform is labeled as the canonical sequence based on criteria such as its prevalence and similarity to <a href="Sequence_homology" title="Sequence homology">orthologous</a>—or functionally analogous—sequences in other species.<a href="#cite_note-15">[15]</a> Isoforms are assumed to have similar functional properties, as most have similar sequences, and share some to most exons with the canonical sequence. However, some isoforms show much greater divergence (for example, through <a href="Trans-splicing" title="Trans-splicing">trans-splicing</a>), and can share few to no exons with the canonical sequence. In addition, they can have different biological effects—for example, in an extreme case, the function of one isoform can promote cell survival, while another promotes cell death—or can have similar basic functions but differ in their sub-cellular localization.<a href="#cite_note-16">[16]</a> A 2016 study, however, functionally characterized all the isoforms of 1,492 genes and determined that most isoforms behave as "functional alloforms." The authors came to the conclusion that isoforms behave like distinct proteins after observing that the functional of most isoforms did not overlap.<a href="#cite_note-17">[17]</a> Because the study was conducted on cells in vitro, it is not known if the isoforms in the expressed human proteome share these characteristics. Additionally, because the function of each isoform must generally be determined separately, most identified and predicted isoforms still have unknown functions.

<div class="mw-heading mw-heading2"><h2 id="Types">Types</h2></div>

Isoforms can be categorized based on the nature of their differences into structural isoforms and sequence isoforms. Structural isoforms arise from alternative splicing events that result in different <a href="Exon" title="Exon">exon</a> compositions, including exon skipping/inclusion, alternative 5' or 3' splice sites, and intron retention. These mechanisms produce transcripts and <a href="Protein" title="Protein">proteins</a> with distinct <a href="Protein_domain" title="Protein domain">domain</a> architectures - for example, the inclusion or exclusion of entire functional domains, or the use of alternative donor/acceptor sites that add or remove partial exon sequences. In contrast, sequence isoforms typically result from single nucleotide variations, insertions, deletions, or post-translational modifications that alter the amino acid sequence without changing the overall exon structure <a href="#cite_note-18">[18]</a>.

<a href="Alternative_splicing" title="Alternative splicing">Alternative splicing</a> is the main post-transcriptional modification process that produces <a href="Messenger_RNA" title="Messenger RNA">mRNA</a> transcript isoforms, while isoforms can result in different functions, activities, or expression patterns <a href="#cite_note-19">[19]</a>. The distinction is functionally important: structural isoforms often exhibit dramatically different properties due to the presence or absence of entire protein domains, whereas sequence isoforms may show more subtle functional variations. Both mechanisms contribute significantly to proteome diversity, with structural variation through alternative splicing being particularly prevalent in higher <a href="Eukaryote" title="Eukaryote">eukaryotes</a> where it affects the majority of multi-exon genes.

<div class="mw-heading mw-heading2"><h2 id="Related_concepts">Related concepts</h2></div>

<div class="mw-heading mw-heading3"><h3 id="Glycoform">Glycoform</h3></div>

<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="Glycoprotein" title="Glycoprotein">Glycoprotein</a></div>

A glycoform is an isoform of a protein that differs only with respect to the number or type of attached <a href="Glycan" title="Glycan">glycan</a>. <a href="Glycoproteins" class="mw-redirect" title="Glycoproteins">Glycoproteins</a> often consist of a number of different glycoforms, with alterations in the attached <a href="Saccharide" class="mw-redirect" title="Saccharide">saccharide</a> or <a href="Oligosaccharide" title="Oligosaccharide">oligosaccharide</a>. These modifications may result from differences in <a href="Biosynthesis" title="Biosynthesis">biosynthesis</a> during the process of <a href="Glycosylation" title="Glycosylation">glycosylation</a>, or due to the action of <a href="Glycosidases" class="mw-redirect" title="Glycosidases">glycosidases</a> or <a href="Glycosyltransferases" class="mw-redirect" title="Glycosyltransferases">glycosyltransferases</a>. Glycoforms may be detected through detailed chemical analysis of separated glycoforms, but more conveniently detected through differential reaction with <a href="Lectins" class="mw-redirect" title="Lectins">lectins</a>, as in <a href="Lectin_affinity_chromatography" class="mw-redirect" title="Lectin affinity chromatography">lectin affinity chromatography</a> and <a href="Lectin" title="Lectin">lectin</a> <a href="Affinity_electrophoresis" title="Affinity electrophoresis">affinity electrophoresis</a>. Typical examples of glycoproteins consisting of glycoforms are the <a href="Blood_proteins" class="mw-redirect" title="Blood proteins">blood proteins</a> as <a href="Orosomucoid" title="Orosomucoid">orosomucoid</a>, <a href="Antitrypsin" class="mw-redirect" title="Antitrypsin">antitrypsin</a>, and <a href="Haptoglobin" title="Haptoglobin">haptoglobin</a>. An unusual glycoform variation is seen in <a href="Neural_cell_adhesion_molecule" title="Neural cell adhesion molecule">neuronal cell adhesion molecule, NCAM</a> involving <a href="Polysialic_acid" title="Polysialic acid">polysialic acids, PSA</a>.

<div class="mw-heading mw-heading2"><h2 id="Examples">Examples</h2></div>

<ul><li><a href="G-actin" class="mw-redirect" title="G-actin">G-actin</a>: despite its conserved nature, it has a varying number of isoforms (at least six in mammals).</li>

<li><a href="Creatine_kinase" title="Creatine kinase">Creatine kinase</a>, the presence of which in the blood can be used as an aid in the diagnosis of <a href="Myocardial_infarction" title="Myocardial infarction">myocardial infarction</a>, exists in 3 isoforms.</li>

<li><a href="Hyaluronan_synthase" title="Hyaluronan synthase">Hyaluronan synthase</a>, the enzyme responsible for the production of hyaluronan, has three isoforms in mammalian cells.</li>

<li><a href="UDP-glucuronosyltransferase" class="mw-redirect" title="UDP-glucuronosyltransferase">UDP-glucuronosyltransferase</a>, an enzyme superfamily responsible for the detoxification pathway of many drugs, environmental pollutants, and toxic endogenous compounds has 16 known isoforms encoded in the human genome.<a href="#cite_note-pmid17263731-20">[20]</a></li>

<li>G6PDA: normal ratio of active isoforms in cells of any tissue is 1:1 shared with G6PDG. This is precisely the normal isoform ratio in hyperplasia. Only one of these isoforms is found during neoplasia.<a href="#cite_note-21">[21]</a></li></ul>

<a href="Monoamine_oxidase" title="Monoamine oxidase">Monoamine oxidase</a>, a family of enzymes that catalyze the oxidation of monoamines, exists in two isoforms, MAO-A and MAO-B.

<ul><li><a href="Gene_isoform" title="Gene isoform">Gene isoform</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</ol></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

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<div class="side-box-text plainlist">Look up <a href="https://en.wiktionary.org/wiki/isoform" class="extiw external" title="wiktionary:isoform">isoform</a> in Wiktionary, the free dictionary.</div></div>

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<ul><li><a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/mesh?cmd=Retrieve&dopt=Full&list_uids=68020033">MeSH entry protein isoforms</a></li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20090508191243/http://www.ghr.nlm.nih.gov/glossary=isoforms">Definitions Isoform</a></li></ul>

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</style><div id="Proteins36" style="font-size:114%;margin:0 4em"><a href="Protein" title="Protein">Proteins</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Processes</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Protein_biosynthesis" title="Protein biosynthesis">Protein biosynthesis</a></li>

<li><a href="Post-translational_modification" title="Post-translational modification">Post-translational modification</a></li>

<li><a href="Protein_folding" title="Protein folding">Protein folding</a></li>

<li><a href="Protein_targeting" title="Protein targeting">Protein targeting</a></li>

<li><a href="Proteome" title="Proteome">Proteome</a></li>

<li><a href="Protein_methods" title="Protein methods">Protein methods</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Structures</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Protein_structure" title="Protein structure">Protein structure</a></li>

<li><a href="Protein_domain" title="Protein domain">Protein structural domains</a></li>

<li><a href="Proteasome" title="Proteasome">Proteasome</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Types</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="List_of_proteins" title="List of proteins">List of proteins</a></li>

<li><a href="Membrane_protein" title="Membrane protein">Membrane protein</a></li>

<li><a href="Globular_protein" title="Globular protein">Globular protein</a>

<ul><li><a href="Globulin" title="Globulin">Globulin</a></li>

<li><a href="Edestin" title="Edestin">Edestin</a></li>

<li><a href="Albumin" title="Albumin">Albumin</a></li></ul></li>

<li><a href="Scleroprotein" class="mw-redirect" title="Scleroprotein">Fibrous protein</a></li>

<li><a href="Chromoprotein" title="Chromoprotein">Chromoprotein</a></li>

<li><a href="Photoreceptor_protein" title="Photoreceptor protein">Photoreceptor protein</a></li>

<li><a href="Biliprotein" title="Biliprotein">Biliprotein</a>

<ul><li><a href="Phycobiliprotein" title="Phycobiliprotein">Phycobiliprotein</a></li>

<li><a href="Phytochrome" title="Phytochrome">Phytochrome</a></li>

<li><a href="Lipocalin" title="Lipocalin">Lipocalin</a></li></ul></li></ul>

</div></td></tr></tbody></table></div></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2025-07-21" href="https://en.wikipedia.org/wiki/?title=Protein_isoform&oldid=1301831276">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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