Chapter 9

Intermediate FilamentsBy

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.

• 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.

9.1 Introduction

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.

• Different kinds of intermediate filaments have different tissue expression patterns.

• Antibodies to individual intermediate filaments are important tools for monitoring cell differentiation and pathology.

9.2 The six intermediate filament protein groups have similar structure but different expression

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.

• Paired keratin expression is predictive of epithelial differentiation and proliferative status.

• Simple keratins K8 and K18 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

9.3 The two largest intermediate filament groups are type I and type II keratins

9.4 Mutations in keratins cause epithelial cell fragility

• Mutations in K5 or K14 cause the skin blistering disorder epidermolysis bullosa simplex.

• Severe EBS mutations are associated with accumulated nonfilamentous keratin.

• 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.

9.4 Mutations in keratins cause epithelial cell fragility

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.

• 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.

9.5 Intermediate filaments of nerve, muscle, and connective tissue often show overlapping expression

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.

• Upon phosphorylation by Cdk1, lamin filaments depolymerize.– This allows disassembly of the nuclear

envelope during mitosis.

• Lamin genes undergo alternative splicing.

9.6 Lamin intermediate filaments reinforce the nuclear envelope

9.7 Even the divergent lens filament proteins are conserved in evolution

• The eye lens contains two highly unusual intermediate filament proteins, CP49 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

strain-resistant 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.

• 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

9.8 Intermediate filament subunits assemble with high affinity into strain-resistant structures

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.

• Phosphorylation is the main mechanism for intermediate filament remodeling in cells.

• Proteolytic degradation:– modulates protein quantity – facilitates apoptosis

9.9 Posttranslational modifications regulate the configuration of intermediate filament proteins

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 filament-associated proteins include:– cell-cell and cell-matrix junction proteins – terminal differentiation matrix proteins

of keratinocytes

• Transiently associated proteins include the plakin family of diverse, multifunctional cytoskeletal linkers.

9.10 Proteins that associate with intermediate filaments are facultative rather than essential

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

• Humans have 70 genes encoding intermediate filament proteins.

• Human keratin genes are clustered.– But nonkeratin intermediate filament

genes are dispersed.

9.11 Intermediate filament genes are present throughout metazoan evolution