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1. Intermediate filament
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2. Intermediate filament
• Intermediate filaments (IFs) are cytoskeletal structural
components found in the cells of vertebrates, and
many invertebrates.
• Homologues of the IF protein have been noted in an invertebrate,
the cephalochordate Branchiostoma.
• Intermediate filaments are composed of a family of
related proteins sharing common structural and sequence
features. Initially designated 'intermediate' because their average
diameter (10 nm) is between those of
narrower microfilaments (actin) and wider myosin filaments found
in muscle cells, the diameter of intermediate filaments is now
commonly compared to actin microfilaments (7 nm)
and microtubules (25 nm).

3. • Initially designated 'intermediate' because their average
diameter (10 nm) is between those of
narrower microfilaments (actin) and
wider myosin filaments found in muscle cells, the
diameter of intermediate filaments is now commonly
compared to actin microfilaments (7 nm)
and microtubules (25 nm).

4. • Animal intermediate filaments are subcategorized into six
types based on similarities in amino acid sequence
and protein structure .
• Most types are cytoplasmic, but one type, Type V is
a nuclear lamin. Unlike microtubules, IF distribution in
cells show no good correlation with the distribution of
either mitochondria or endoplasmic reticulum.

5. Microfiament Structure
The structure of proteins
that form intermediate
filaments (IF) was first
predicted by computerized
analysis of the amino acid
sequence of a human
epidermal keratin derived
from cloned cDNAs.

6. • 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.

7. • The central building block of an intermediate filament is a
pair of two intertwined proteins that is called a coiled-coil
structure.
• Structural analysis of a pair of keratins shows that the two
proteins that form the coiled-coil bind by hydrophobic.

8. Types
• There are about 70 different human genes coding for
various intermediate filament proteins.
• Animal IFs are subcategorized into six types based on
similarities in amino acid sequence and protein structure:
Types I and II – acidic and basic keratins
Type III
Type IV
Type V - nuclear lamins
Type VI

9. Types I and II – acidic and basic keratins
• These proteins are the most diverse
among IFs and constitute type I
(acidic) and type II (basic) IF
proteins. The many isoforms are
divided in two groups:
• epithelial keratins (about 20)
in epithelial cells (image to right)
• trichocytic keratins (about 13) (hair
keratins), which make
up hair, nails, horns and reptilian scal
es.Keratin intermediate filaments (stained red)
around epithelial cells.

10. Type III • There are four proteins classed as type III IF
proteins, which may form homo-
or heteropolymeric proteins.
• Desmin IFs are structural components of
the sarcomeres in muscle cells.
• GFAP (glial fibrillary acidic protein) is found
in astrocytes and other glia.
• Peripherin found in peripheral neurons.
• Vimentin, the most widely distributed of all IF
proteins, can be found
in fibroblasts, leukocytes, and blood
vessel endothelial cells. They support the
cellular membranes, keep some organelles in a
fixed place within the cytoplasm, and transmit
membrane receptor signals to the nucleus.
Vimentin fibers in fibroblasts.

11. Type IV
• α-Internexin
• Neurofilaments - the type IV family of intermediate
filaments that is found in high concentrations along
the axons of vertebrate neurons.
• Synemin
• Syncoilin

12. • Type V - nuclear lamins
• Lamins
• Lamins are fibrous proteins having structural function in
the cell nucleus.
• Type VI
• Beaded filaments: Filensin, Phakinin.
• Nestin (was once proposed for reclassification but due to
differences, remains as a type VI IF protein).

13. Thank you.