Chapter 9 Intermediate Filaments By E Birgitte Lane

Chapter 9 Intermediate Filaments By E. Birgitte Lane

9. 1 Introduction • Intermediate filaments are major components of the nuclear and cytoplasmic cytoskeletons. • Intermediate filaments are essential to maintain correct tissue structure and function.

9. 1 Introduction • Intermediate filaments: – are between actin filaments and microtubules in diameter – form robust networks • Intermediate filaments are polymers of protein subunits.

9. 1 Introduction • Intermediate filament proteins: – are heterogeneous – re encoded by a large and complex gene superfamily • Over 50 human diseases are associated with intermediate filament mutations.

9. 2 The six intermediate filament protein groups have similar structure but different expression • Intermediate filament proteins all share a similar structure that is based on an extended central α-helical rod domain. • The intermediate filament family is divided into six sequence homology classes.

9. 2 The six intermediate filament protein groups have similar structure but different expression • Different kinds of intermediate filaments have different tissue expression patterns. • Antibodies to individual intermediate filaments are important tools for monitoring cell differentiation and pathology.

9. 3 The two largest intermediate filament groups are type I and type II keratins • Most of the intermediate filament proteins in mammals are keratins. • Keratins are obligate heteropolymers of type I and type II proteins.

9. 3 The two largest intermediate filament groups are type I and type II keratins • Paired keratin expression is predictive of epithelial differentiation and proliferative status. • Simple keratins K 8 and K 18 are the least specialized keratins.

9. 3 The two largest intermediate filament groups are type I and type II keratins • Barrier keratins have the most complex and varied expression of all intermediate filaments. • Structural keratins of hard appendages: – are distinct from other keratins – may be the latest–evolving mammalian keratins

9. 4 Mutations in keratins cause epithelial cell fragility • Mutations in K 5 or K 14 cause the skin blistering disorder epidermolysis bullosa simplex. • Severe EBS mutations are associated with accumulated nonfilamentous keratin.

9. 4 Mutations in keratins cause epithelial cell fragility • Many tissue fragility disorders with diverse clinical phenotypes are caused by structurally similar mutations in other keratin genes. • Cell fragility disorders provide clear evidence of a tissue-reinforcing function for keratin intermediate filaments.

9. 5 Intermediate filaments of nerve, muscle, and connective tissue often show overlapping expression • Some type III and type IV intermediate filament proteins have overlapping expression ranges. • Many type III and type IV proteins can coassemble with each other.

9. 5 Intermediate filaments of nerve, muscle, and connective tissue often show overlapping expression • Coexpression of multiple types of intermediate filament proteins may obscure the effect of a mutation in one type of protein. • Desmin is an essential muscle protein. • Vimentin is often expressed in solitary cells. • Mutations in type III or type IV genes are usually associated with muscular or neurological degenerative disorders.

9. 6 Lamin intermediate filaments reinforce the nuclear envelope • Lamins are intranuclear, forming the lamina that lines the nuclear envelope. • Membrane anchorage sites are generated by posttranslational modifications of lamins.

9. 6 Lamin intermediate filaments reinforce the nuclear envelope • Upon phosphorylation by Cdk 1, lamin filaments depolymerize. – This allows disassembly of the nuclear envelope during mitosis. • Lamin genes undergo alternative splicing.

9. 7 Even the divergent lens filament proteins are conserved in evolution • The eye lens contains two highly unusual intermediate filament proteins, CP 49 and filensin. – These constitute the type VI sequence homology group. • These unusual intermediate filament proteins are conserved in evolution of vertebrates.

9. 8 Intermediate filament subunits assemble with high affinity into strainresistant structures • In vitro, intermediate filament assembly is rapid and requires no additional factors. • The central portion of all intermediate filament proteins is a long α-helical rod domain that forms dimers.

9. 8 Intermediate filament subunits assemble with high affinity into strain-resistant structures • Assembly from antiparallel tetramers determines the apolar nature of cytoplasmic intermediate filaments. • Intermediate filament networks: – are stronger than actin filaments or microtubules – exhibit strain hardening under stress

9. 9 Posttranslational modifications regulate the configuration of intermediate filament proteins • Intermediate filaments: – are dynamic – show periodic rapid remodeling • Several posttranslational modifications affect the head and tail domains.

9. 9 Posttranslational modifications regulate the configuration of intermediate filament proteins • Phosphorylation is the main mechanism for intermediate filament remodeling in cells. • Proteolytic degradation: – modulates protein quantity – facilitates apoptosis

9. 10 Proteins that associate with intermediate filaments are facultative rather than essential • Intermediate filament proteins do not need associated proteins for their assembly. • Specific intermediate filamentassociated proteins include: – cell-cell and cell-matrix junction proteins – terminal differentiation matrix proteins of keratinocytes

9. 10 Proteins that associate with intermediate filaments are facultative rather than essential • Transiently associated proteins include the plakin family of diverse, multifunctional cytoskeletal linkers.

9. 11 Intermediate filament genes are present throughout metazoan evolution • Intermediate filament genes are present in all metazoan genomes that have been analyzed. • The intermediate filament gene family evolved by: – duplication and translocation – followed by further duplication events

9. 11 Intermediate filament genes are present throughout metazoan evolution • Humans have 70 genes encoding intermediate filament proteins. • Human keratin genes are clustered. – But nonkeratin intermediate filament genes are dispersed.