intermediate filaments

• intermediate filaments: flexible polymers that provide structural support for cells (response to stretching and mechanical forces) (e.g. lamins - nucleus, keratin- hair)
• continuous network of intermediate filaments from nuclear envelope to attachments on plasma membrane (desmosomes/hemidesmosomes)

  • cell to cell contact: desmosomes
  • cell to extracellular matrix: hemidesmosomes
• intermediate filament network transmits forces to cell periphery

  • allows elasticity of skin and other tissues: mutations in intermediate filament proteins cause breakdown of tissues and skin (blistering)
• desmosomes and hemidesmosomes: specific structures linking the cytoskeleton to the cells environment
• structure of intermediate filaments are heterogeneous (protein composition and size)
• all intermediate filament proteins contain alpha helical coiled coil domain
• variable N and C terminal domains give distinct functions in specific cell types

  • different classes of intermediate filament proteins (70 genes)
  • keratin = 54 genes (cytoplasmic and extracellular)
• vitamin D forms desmosomes, holds cells together and allows them to bind to cells properly, key regulator in cell to cell, important to take every day
• • all proteins share a rod like domain with variable head and tail domains
  • rod domains have repeat region which sets up complementary electrostatic bonds for filament assembly
  • 20 highly conserved residues at end of rod domains that are essential for filament elongation through head to tail interactions between dimeric molecules
  • mutations in this region are prevalent in skin disorders (causes aggregates to form, not filaments - disrupts skin architecture)
  • N and C terminal variable domains can project from fiber and interact with other cellular components
• intermediate structure is variable:

  • some intermediate filaments are homodimers (e.g. lamins (class V) and class III intermediate filaments desmin, vimentin)
  • keratins are heterodimers of one acidic (class I) and one basic protein (class II)
  • many intermediate filaments form antiparallel dimers (tetramers) which are intermediates in polymer assembly
  • final intermediate filaments are 10 nm in diameter with up to 8 tetramers in X section (strands in a rope)
  • filaments are apolar (no difference between ends)
• monomer finds a neighbour (similar protein) and wraps around it to form a parallel dimer

  • can be homo or hetero depending on the protein
  • most lamina are homodimers
  • keratins are heterodimers
  • all dimers form in cytoplasm and then they start to assemble
  • 8 come together and form a complex
• assembly of intermediate filaments:

  • dissociated subunits of intermediate filaments spontaneously self-assemble in minutes in vitro
  • growth occurs primarily at ends of polymer(exact mechanism unclear)
  • intermediate filaments are very stable and resist solubilization (high temp, salt and detergent)
  • some intermediate filaments turn over in minutes/hours in some cells (vimentin)
  • some intermediate filaments are extremely stable (keratin)
• intermediate filaments are the critical element that hold cells together
• Phosphorylation


  Post-translational modification
• Intermediate filaments

  • Most disassemble upon phosphorylation on key residues
  • Lamins and vimentin break down upon phosphorylation by Cdk1: cyclin B kinase in mitosis
  • Keratins are phosphorylated during mitosis but do not completely break down
• Kinases that phosphorylate intermediate filament proteins

  • PKC
  • PKA
  • Ca - calmodulin dependent kinase
• The function, timing and regulation of kinase phosphorylation of intermediate filament proteins is still under investigation
• Neurofilaments (NFs) in axons and dendrites


  • Heavily phosphorylated at C terminus (stabilizes Intermediate filament)
  • Phosphorylation sites are not required for assembly
• neurofilaments are stabilized during phosphorylation
• as vimentin goes through mitosis it rounds up
• keratins: contain disulfide bonds that influence assembly and structure

  • extracellular keratins contain extra cysteines vs intracellular
  • cross link with other matrix proteins to form hair
  • at least 16 keratin associated protein genes (KAPs) exist on human chromosome 21q22.11
  • most have high sulfur (cysteine) cintent (>30/mol)
• disulfide bonds aid in its stability
• secreted keratin has a lot mroe amino acids with cysteie and thymines
• when you burn hair, it breaks the disulfide bond
• tissue specific expression of intermediate filaments:

  • most cells express one form of intermediate filament (epithelial- keratin, muscle- desmin)
  • during development, cells express different isoforms and types of intermediate filaments
• human epidermis: expresses 12 kerstin isoforms

  • dividing cells at base of epidermis express keratins 5 and 14
  • terminally differentiated cells express keratins 1 and 10
• • different keratins affect bundling of intermediate filaments (different layers of skin require more strength)
  • mutations in keratins affects skin structure (K10 in hyperkeratosis and K14 in epidermolysis bullosa form keratin aggregates)
• can lose keratin gene fur to null mutation

  • cells don't have any keratin
  • any stress of those cells leads to lysis of those cells
• intermediate filament associated proteins:

  • proteins bind and link intermediate filaments to other cell components and cytoskeleton (e.g. LAPs and lamin)
  • BPAG: link intermediate filaments to hemidesmosomes (with plectin)
• plectin:

  • large 170kD protein with coiled coil and globular domains
  • cytoplasmic protein links many cytoskeletal elements together
  • cross links intermediate filaments together
  • intermediate filaments to plasma membrane
  • intermediate filaments to microtubules
  • intermediate filaments to actin filaments
  • mutations in plectin form rare muscular dystrophy (with skin blisters)
• desmoplakin links keratin to desmosomes
• vitamin D major function is to express cadherin
• intermediate function in cells:

  • intermediate filaments link nucleus and plasma membrane and form flexible intracellular tendons that allow cells to respond to cell stress
  • possible through interactions with microtubules and actin
  • stretching of smooth muscle cell recognizes intermediate filament network from polygonal meshwork into linear strap that runs length of cell