Editing Intermediate filament (section)

== Types ==
There are about 70 different human genes coding for various intermediate filament proteins. However, different kinds of IFs share basic characteristics: In general, they are all polymers that measure between 9–11&nbsp;nm in diameter when fully assembled.{{cn|date=April 2025}}

Animal IFs are subcategorized into six types based on similarities in amino acid sequence and [[protein]] structure:<ref name="interfil"/>

=== Types I and II – acidic and basic keratins ===
[[File:Epithelial-cells.jpg|thumb|right|[[Keratin]] intermediate filaments (stained red) around [[epithelium|epithelial cells]]]]
{{further|Cytokeratin}}
These proteins are the most diverse among IFs and constitute [[Type I cytokeratin|type I (acidic)]] and [[Type II cytokeratin|type II (basic)]] IF proteins. The many [[isoform]]s are divided in two groups:{{cn|date=April 2025}}
* '''epithelial keratins''' (about 20) in [[epithelial]] cells (image to right)
* '''trichocytic keratins''' (about 13) ([[hair keratin]]s), which make up [[hair]], [[Nail (anatomy)|nails]], [[Horn (anatomy)|horns]] and [[reptile|reptilian]] [[Scale (zoology)|scales]].

Regardless of the group, keratins are either acidic or basic. Acidic and basic keratins bind each other to form acidic-basic heterodimers and these heterodimers then associate to make a keratin filament.<ref name="interfil"/>

[[Cytokeratin]] filaments laterally associate with each other to create a thick bundle of ~50&nbsp;nm radius. The optimal radius of such bundles is determined by the interplay between the long range electrostatic repulsion and short range hydrophobic attraction.<ref>{{cite journal | vauthors = Haimov E, Windoffer R, Leube RE, Urbakh M, Kozlov MM | title = Model for Bundling of Keratin Intermediate Filaments | journal = Biophysical Journal | volume = 119 | issue = 1 | pages = 65–74 | date = July 2020 | pmid = 32533940 | doi = 10.1016/j.bpj.2020.05.024 | pmc = 7335914 | bibcode = 2020BpJ...119...65H | doi-access = free }}</ref> Subsequently, these bundles would intersect through junctions to form a dynamic network, spanning the cytoplasm of epithelial cells.{{cn|date=April 2025}}

=== Type III ===
[[File:Vimentin.jpg|thumb|[[Vimentin]] fibers in [[fibroblast]]s]]
There are four proteins classed as type III intermediate filament proteins, which may form [[homopolymer|homo-]] or [[heteropolymer]]ic proteins.
* '''[[Desmin]]''' IFs are structural components of the [[sarcomere]]s in muscle cells and connect different cell organelles like the desmosomes with the cytoskeleton.<ref>{{cite journal | vauthors = Brodehl A, Gaertner-Rommel A, Milting H | title = Molecular insights into cardiomyopathies associated with desmin (DES) mutations | journal = Biophysical Reviews | volume = 10 | issue = 4 | pages = 983–1006 | date = August 2018 | pmid = 29926427 | pmc = 6082305 | doi = 10.1007/s12551-018-0429-0 }}</ref>
* '''[[Glial fibrillary acidic protein]]''' (GFAP) is found in [[astrocyte]]s and other [[glia]].{{cn|date=April 2025}}
* '''[[Peripherin]]''' found in peripheral neurons.{{cn|date=April 2025}}
* '''[[Vimentin]]''', the most widely distributed of all IF proteins, can be found in [[fibroblast]]s, [[leukocytes]], and blood vessel [[endothelial cell]]s. They support the cellular membranes, keep some [[organelle]]s in a fixed place within the [[cytoplasm]], and transmit membrane receptor signals to the nucleus.<ref name="interfil"/>
* '''[[Syncoilin]]''' is an atypical type III IF protein.<ref name="Uniprot">{{cite web |title=SYNC – Syncoilin – Homo sapiens (Human) – SYNC gene & protein |url=https://www.uniprot.org/uniprot/Q9H7C4 |website=www.uniprot.org |access-date=20 December 2021 |language=en}}</ref>

=== Type IV ===
* '''[[Alpha-internexin]]'''
* '''[[Neurofilament]]s''' – the type IV family of intermediate filaments that is found in high concentrations along the [[axon]]s of vertebrate neurons.{{cn|date=April 2025}}
* '''[[Synemin]]'''
* '''[[Syncoilin]]'''

=== Type V – nuclear lamins ===
* '''[[Lamin]]s'''
Lamins are fibrous proteins having structural function in the cell nucleus.{{cn|date=April 2025}}

In metazoan cells, there are A and B type lamins, which differ in their length and pI. Human cells have three differentially regulated genes.
B-type lamins are present in every cell. B type lamins, [[lamin B1]] and [[lamin B2|B2]], are expressed from the LMNB1 and LMNB2 genes on 5q23 and 19q13, respectively.
A-type lamins are only expressed following [[gastrulation]]. Lamin A and C are the most common A-type lamins and are splice variants of the LMNA gene found at 1q21.{{cn|date=April 2025}}

These proteins localize to two regions of the nuclear compartment, the nuclear lamina—a proteinaceous structure layer subjacent to the inner surface of the [[nuclear envelope]] and throughout the nucleoplasm in the [[Cell nucleus#Nuclear lamina|nucleoplasmic veil]].

Comparison of the lamins to vertebrate cytoskeletal IFs shows that lamins have an extra 42 residues (six heptads) within coil 1b. The c-terminal tail domain contains a nuclear localization signal (NLS), an Ig-fold-like domain, and in most cases a carboxy-terminal CaaX box that is isoprenylated and carboxymethylated (lamin C does not have a CAAX box). Lamin A is further processed to remove the last 15 amino acids and its farnesylated cysteine.

During mitosis, lamins are phosphorylated by MPF, which drives the disassembly of the lamina and the nuclear envelope.<ref name="interfil"/>

=== Type VI ===
* Beaded filaments: [[BFSP1|Filensin]], [[BFSP2|Phakinin]].<ref name="interfil"/>
* [[Nestin (protein)|Nestin]] (was once proposed for reclassification but due to differences, remains as a type VI IF protein)<ref>{{cite journal | vauthors = Bernal A, Arranz L | title = Nestin-expressing progenitor cells: function, identity and therapeutic implications | journal = Cellular and Molecular Life Sciences | volume = 75 | issue = 12 | pages = 2177–2195 | date = June 2018 | pmid = 29541793 | pmc = 5948302 | doi = 10.1007/s00018-018-2794-z}}</ref>

Vertebrate-only. Related to type I-IV. Used to contain other newly discovered IF proteins not yet assigned to a type.<ref name=Polyphyly/>