Structure
The structure of proteins that form IF was first predicted by computerized analysis of the amino acid sequence of a human epidermal keratin derived from cloned cDNAs. Analysis of a second keratin sequence revealed that the two types of keratins share only about 30% amino acid sequence homology but share similar patterns of secondary structure domains. As suggested by the first model, all IF proteins appear to have a central alpha-helical rod domain that is composed of four alpha-helical segments (named as 1A, 1B, 2A and 2B) separated by three linker regions.
The N and C-termini of IF proteins are non-alpha-helical regions and show wide variation in their lengths and sequences across IF families. The basic building-block for IFs is a parallel and in-register dimer. The dimer is formed through the interaction of the rod domain to form a coiled coil. Cytoplasmic IF assemble into non-polar unit-length filaments (ULF), which then assemble into longer structures. Part of the assembly process includes a compaction step, in which ULF tighten and assume a smaller diameter. The reasons for this compaction are not well understood, and IF are routinely observed to have diameters ranging between 6 and 12 nm.
The N-terminal "head domain" binds DNA. Vimentin heads are able to alter nuclear architecture and chromatin distribution, and the liberation of heads by HIV-1 protease may play an important role in HIV-1 associated cytopathogenesis and carcinogenesis. Phosphorylation of the head region can affect filament stability. The head has been shown to interact with the rod domain of the same protein.
C-terminal "tail domain" shows extreme length variation between different IF proteins.
The anti-parallel orientation of tetramers means that, unlike microtubules and microfilaments, which have a plus end and a minus end, IFs lack polarity and cannot serve as basis for cell motility and intracellular transport.
Also, as opposed to actin or tubulin, intermediate filaments do not contain a binding site for a nucleoside triphosphate.
Cytoplasmic IF do not undergo treadmilling like microtubules and actin fibers, but they are dynamic. For a review see: .
Read more about this topic: Intermediate Filament
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