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There are 6 types of intermediate filaments, whose members are shown below:
The 28 type I and 26 type II intermediate filament proteins are the keratins (sometimes called cytokeratins), and account for most of the intermediate filaments (i.e. 54 genes out of the total of 70 human intermediate filament genes). Keratins only assemble as heteropolymers: a type I and a type II protein forming a heterodimer. Each keratin pair is characteristic of a particular epithelial differentiation programme; some epithelial cells express more than one pair. The coexpressed pairs of type I plus type II keratins can be divided into two or three expression groups: simple keratins of one-layered epithelia (e.g. K8/K18, K20), barrier (keratinocyte) keratins of stratified epithelia (e.g. K5/K14, K1/K10, K3/K12, K4/K13, K6a/K16, K6b/K17, K19) and structural keratins which make up hard appendages like hair, nails, horns and reptilian scales. The new keratin nomenclature is used according to Schweizer et al 2006, J. of Cell Biol., Volume 174, Issue 2, 2006. Pages: 169-174.
TYPE I
TYPE II
There are four type III genes in this group: desmin intermediate filaments of muscle cells (all types); vimentin intermediate filaments in fibroblasts, lymphocytes, endothelial cells and many mesenchymal tissues; peripherin, mostly in peripheral neurons, that can coassemble with type IV proteins, and GFAP (glial fibrillary acidic protein) is found in astrocytes and glial cells. Although members of this group can make homopolymeric filaments in vitro, they may assemble as heteropolymers in tissues.
Type IV intermediate filament proteins are expressed mostly in nerve cells where they are implicated in the radial growth of the axon. The seven genes for type IV proteins encode the heavy (NF-H), medium (NF-M) and light neurofilament (NF-L) proteins, nestin and α-internexin in nerve cells, synemin α and desmuslin/synemin β (two alternative transcripts of the DMN gene) in muscle cells. Members of this group mostly preferentially coassemble as heteropolymers in tissues.
This type forms intranuclear filaments, in contrast to all the other groups which assemble in the cytoplasm. The nuclear lamin intermediate filament proteins are thought to be the oldest type of intermediate filament in terms of evolution and share sequence characteristics with cytoplasmic filaments in invertebrates. There are three lamin genes in humans, encoding up to six proteins: (i) lamin A, lamin C1 and lamin C2 – three alternate splice products of the LMNA gene, (ii) lamin B1 (LMNB1), and (iii) lamins B2 and B3, encoded by the LMNB2 gene.
This group includes the two eye lens intermediate filament, or “beaded filament”, proteins CP49 (phakinin) and filensin (CP115). These two are highly divergent from the other intermediate filament groups but comparative genomic analysis suggests they have been conserved over a long period of evolution as clearly homologous genes are present in fugu fish.
This page was last modified: April 17 2009 14:28:34. |
