Unit 06 Pt4

Unit Overview

  • Unit 06: the Cytoskeleton

    • Focus: Intermediate Filaments

    • Course: BIOL 331 – Advanced Cell Biology Part 4

    • Source: MBoC (7th Ed): Chapter 16 (pages: 1007-1013)

Key Concepts

Intermediate Filament Structure

  • Lateral Bundling and Twisting of Coiled-Coils

  • Mechanical Stability: Imparted to animal cells by intermediate filaments (IFs)

  • Linker Proteins: Connect cytoskeletal filaments and bridge the nuclear envelope

  • Septins: Form filaments that contribute to subcellular organization

  • Bacterial Cell Shape: Dependent on homologs of eukaryotic cytoskeletal proteins

Intermediate Filament Characteristics

  • Present in some metazoan cells, not all

  • Diameter: 8 - 11 nm

  • Prominent in the cytoplasm of cells subject to mechanical stress

  • Relatives of nuclear lamins; more diverse than actin and tubulin

  • Create rope-like structures that hold epithelial cells together

Structure and Function of Intermediate Filaments

Coiled-Coils and Polymerization

  • All IFs have conserved α-helical domain forming extended coiled-coil between two monomers

  • Parallel dimers associate into anti-parallel staggered tetramers

  • No nucleotide binding or polarity in polymerization

  • Total of 32 individual polypeptides provides tensile strength

  • Stability: Difficult to break, can be stretched up to 3X normal length

  • Regulation of stability: Less understood, likely related to phosphorylation

Keratins: The Most Diverse Intermediate Filaments

  • Numerous types of keratins

  • Subunits: Heterodimeric (1 acidic + 1 neutral/basic)

  • Cross-linked networks via S-S bonds allow for resilience (survive cell death)

  • Expressed in carcinomas; can indicate tissue origin

  • Provide mechanical strength to epithelial cells, anchor at desmosomes (cell-cell connections) and hemidesmosomes (cell-matrix connections)

  • Accessory proteins (e.g. filaggrin) bundle keratin for toughness; mutations can lead to dry skin diseases

Importance of Keratin in Health

  • Mutations in keratin linked to inherited blistering diseases, e.g., epidermolysis bullosa

  • Can result in severe skin blistering upon minor mechanical stress

  • Cell biology behind blisters: Cell rupture due to mechanical stress; disorganized keratin filament cytoskeleton

Importance of Neurofilaments in Health

  • Neurofilaments (NFs) found in high concentrations in neuronal axons

  • Composed of heteropolymers; subunits longitudinally incorporated into axon

  • NF expression levels influence axon diameter and speed of signal transmission

  • Conditions like amyotrophic lateral sclerosis (ALS) linked to abnormal NF accumulation and assembly

Linker Proteins in Cytoskeletal Structure

  • Plakins: Cross-link intermediate filaments with other cytoskeletal components; bundles IFs, connects to actin & microtubules (MTs)

  • Attach IFs to plasma membrane adhesive structures

  • Plectin mutations: Result in disorders combining epidermolysis bullosa, muscular dystrophy, and neurodegeneration

Linking and Bridging the Nuclear Envelope

  • Plakins engage with SUN-KASH bridges

  • SUN-KASH protein complexes: Connect nucleus interior with actin and MTs

  • SUN proteins: Localized on inner nuclear membrane; interact with nuclear lamina and chromosomes

  • KASH proteins: Located on outer nuclear membrane; interact directly with actin and indirectly with MTs (via motor proteins, plakins)

  • Mutations in components linked to progerias (accelerated aging)

Septins: Filaments for Cell Polarity

  • Septins: GTP-binding proteins that assemble into nonpolar filaments, forming rings and cages

  • Function as scaffolds to compartmentalize cell membranes

  • Involved in cell division, migration, and vesicle trafficking

  • Assemble at the base of primary cilia, marking that part of the ciliary membrane

Review Questions

  • Name some important intermediate filaments and an associated disease caused by mutations in underlying proteins.

  • What are SUN-KASH bridges and how are they formed?

  • What are septins and what critical roles do they play in a cell?