PROSITE documentation PDOC00198 [for PROSITE entry PS00226]

Intermediate filaments signature


Intermediate filaments (IF) [1,2,3] are proteins which are primordial components of the cytoskeleton and the nuclear envelope. They generally form filamentous structures 8 to 14 nm wide. IF proteins are members of a very large multigene family of proteins which has been subdivided in five major subgroups:

  • Type I: Acidic cytokeratins.
  • Type II: Basic cytokeratins.
  • Type III: Vimentin, desmin, glial fibrillary acidic protein (GFAP), peripherin, and plasticin.
  • Type IV: Neurofilaments L, H and M, α-internexin and nestin.
  • Type V: Nuclear lamins A, B1, B2 and C.

All IF proteins are structurally similar in that they consist of: a central rod domain comprising some 300 to 350 residues which is arranged in coiled-coiled α-helices, with at least two short characteristic interruptions; a N-terminal non-helical domain (head) of variable length; and a C-terminal domain (tail) which is also non-helical, and which shows extreme length variation between different IF proteins.

While IF proteins are evolutionary and structurally related, they have limited sequence homologies except in several regions of the rod domain. We use, as a sequence pattern for this class of proteins, a conserved region at the C-terminal extremity of the rod domain.


In the third position of the pattern, Ala is found in type IV and V IF proteins, Thr is found in IF proteins of type I, II, III, and VI, Cys in IF from snails, and Ile in IF from worms. In the first position of the pattern Val is found in type VI, Ile is found in all other types.

Last update:

December 2004 / Pattern and text revised.

Technical section

PROSITE method (with tools and information) covered by this documentation:

IF, PS00226; Intermediate filaments signature  (PATTERN)


1AuthorsQuinlan R., Hutchison C., Lane B.
SourceProtein Prof. 2:801-952(1995).

2AuthorsSteiner P.M., Roop D.R.
SourceAnnu. Rev. Biochem. 57:593-625(1988).

3AuthorsStewart M.
TitleIntermediate filaments: structure, assembly and molecular interactions.
SourceCurr. Opin. Cell Biol. 2:91-100(1990).
PubMed ID2183847

PROSITE is copyright. It is produced by the SIB Swiss Institute Bioinformatics. There are no restrictions on its use by non-profit institutions as long as its content is in no way modified. Usage by and for commercial entities requires a license agreement. For information about the licensing scheme send an email to
Prosite License or see: prosite_license.html.


View entry in original PROSITE document format
View entry in raw text format (no links)