Intermediate Filaments (Week 5-2: Cytoskeletal System II)

Intermediate Filaments

• Key component of the cytoskeleton in animal cells

• But they are absent in plant cells

• They provide mechanical strength and structural support to cells and tissues

Keratin

• Abundant intermediate filament

• Important component of structures that grow from skin in animals

• Provides toughness and protection

Key Characteristics of Intermediate Filaments

1. Most Stable Cytoskeletal Component 

• IF’s are highly resistant to mechanical stress

• Least soluble cytoskeletal component, providing long-term structural integrity 

2. Tissue-specific 

• Unlike actin and tubulin (which are universal), different cell types express different IF proteins

• IFs are classified into six major types based on amino acid composition 

3. Supports the Entire Cytoskeleton

• IFs provide a scaffold that integrates actin filaments and microtubules, maintaining cell shape and mechanical stability

Structure of Intermediate Filaments

• The fundamental subunits of IF proteins are dimers (composed of two IF proteins coiled together)

• IF proteins are fibrous, not globular (structurally different from actin and tubulin)

• Each IF protein has a homologous central rodlike domain of 310 to 318 amino acids in length

• The N-terminal (head) and C-terminal (tail) domains differ greatly among IF proteins

• These domains determine the specific properties and functions of different IF types

Plectin

 (type of spectraplakin) crosslinks intermediate filaments (IFs) to microfilaments (MFs) and microtubules (MTs)

Spectraplakins

linker proteins that connect intermediate filaments, microfilaments, and microtubules

Functional Properties of Intermediate Filaments

1. Tension-Bearing Role

• IFs are designed to withstand mechanical stress and stretching forces

• They provide structural integrity to cells and tissues.

2. Chemical Stability

• IFs are more chemically resistant than microtubules and microfilaments

• This makes IFs ideal for maintaining long-term cell structure and durability

Steps of Intermediate Filament Assembly

Step 1: Dimer Formation

• The basic unit of an IF is a dimer

• Dimer is composed of two IF polypeptides coiled together in a parallel α-helical coiled-coil structure

Step 2: Tetramer Formation

• Two dimers align laterally in an antiparallel fashion to form a tetrameric protofilament

• Since tetramers are antiparallel, IFs have no polarity, unlike actin filaments and microtubules

Step 3: Protofilament Assembly

• Tetramers associate end-to-end to form protofilaments

Step 4: Final Filament Formation

• Eight protofilaments overlap laterally to form a fully mature, rope-like intermediate filament

• This thick bundling of protofilaments gives IFs their mechanical strength

Summary: The Cytoskeleton is a Mechanically Integrated Structure 

• Microtubules resist bending when a cell is compressed

• Microfilaments serve as contractile elements that generate tension

• Intermediate filaments are elastic and can withstand tensile forces