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Intermediate filaments

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Definition

noun

plural: intermediate filaments

A type of cytoskeleton characterized by having a diameter ranging from 8 to 12 nm


Details

Overview

Cytoskeleton is a cytoplasmic structure composed of protein filaments and microtubules in the cytoplasm, and has a role in controlling cell shape, maintaining intracellular organization, and in cell movement. In eukaryotes, there are three major types of cytoskeleton, namely (1) microfilaments, (2) microtubules, and (3) intermediate filaments. For an overview of the differences between them, see table below.


type of cytoskeleton Features Functions
Microfilaments helical polymer of actin sub-units

(e.g. actin)

Cell shape

Cell locomotion (via filopodia, pseudopodia, or lamellipodia) Intracellular movement or transport Cytokinesis (by aiding centrosomes at opposite poles) Muscle contraction (with myosin filaments) Cytoplasmic streaming

Microtubules tubular structure with a diameter of 25nm and length ranging from 200nm to 25μm; exhibits polarity; in cilia and flagella, 9+2 microtubular arrangement

(e.g. alpha-tubulin and beta-tubulin)

Intracellular shape

Cell locomotion (as axoneme of cilia and flagella) Intracellular transport of organelles (e.g. mitochondria) via dyneins and kinesins Spindle fiber formation

Intermediate filaments two anti-parallel helices or dimers of varying protein sub-units with diameters ranging from 8 to 12 nm

[e.g. vimentin (mesenchyme), glial fibrillary acidic protein (glial cells), neurofilament proteins (neuronal processes), keratins (epithelial cells), and nuclear lamins]

Cell shape (by bearing tension)

"Scaffolding" for cell and nucleus Nuclear lamina formation Anchor organelles Cell-cell connections (when with proteins and desmosomes)


Features

The intermediate filaments are the type of cytoskeleton whose diameter is intermediate of the other two types. Its diameter is about 10 nm (or ranges from 8 to 12 nm) as opposed to the narrower diameter of microfilaments (i.e. about 7 nm) and the larger diameter of microtubules (i.e. 25 nm). An intermediate filament is comprised of two anti-parallel helices or dimers of varying protein sub-units. It may be composed of any of a number of different proteins and form a ring around the cell nucleus. Intermediate filaments are stretchable. They can be extended manifold their initial length. With the exception of nuclear lamin, the intermediate filaments are cytoplasmic.


Unlike microfilaments and microtubules, the intermediate filaments do not exhibit polarsity. This means that they do not have a minus (-) end and a (+) end. Also, the intermediate filaments do not have a binding site for a nucleoside triphosphate.


Types

There are six types of intermediate filaments based on the similarities in amino acid sequence and protein structure:

  • Type I – acidic keratins
  • Type II – basic keratins
(N.B. type I and type II intermediate filaments bind to each other forming acidic-basic heterodimers, which associate to form, in turn, a keratin filament.)
  • Type III – includes desmin (structural components of sarcomeres), glial fibrillary acidic protein (GFAP, in astrocytes and certain glial cells), peripherin (in peripheral neurons), and vimentin (in fibroblasts, leukocytes, endothelial cells)
  • Type IV - includes alpha-internexin, neurofilament proteins (in neuronal processes), synemin, syncoilin
  • Type V – includes nuclear lamins
  • Type VI – includes nestin

Other intermediate filaments that have yet to be classified are filensin and phakinin.



Common biological reactions

Assembly

Cytoplasmic intermediate filaments assemble into non-polar unit-length filaments (ULF). ULFs that are identical associate laterally forming antiparallel and staggered tetramers. The tetramers associate head-to-tail into protofilaments that pair up laterally into protofibrils. Four protofibrils wind together to form an intermediate filament.[1] During the compaction step, the ULF tighten and thereby causing the diameter to be narrower, about 8 to 12 nm.


Biological functions

The intermediate filaments are involved in providing cell shape by bearing tension. It also acts as scaffolding" for cell and nucleus. It anchors organelles. It is associated as well with nuclear lamina formation. It enables cell-cell adhesion particularly as keratins that interact with desmosomes, and cell-matrix adhesion when interacting with hemisdesmosomes (via adapter proteins).


Supplementary

Abbreviation

  • IF


Further reading

Compare

See also


Reference

  1. Wikipedia Contributors. (2019, July 9). Intermediate filament. Retrieved from Wikipedia website: https://en.wikipedia.org/wiki/Intermediate_filament



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