Actin and Intermediate Filaments

Vocabulary flashcards based on Lecture 2 on the cytoskeleton, covering actin dynamics, regulatory proteins, and the structure and function of tissue-specific intermediate filaments.

Actin filaments

Flexible and polar polymers of globular actin subunits that associate in a head-to-tail fashion to form a helical structure.

Actin filament polarity

A structural feature where subunits have the same orientation, creating a $(+)$ (barbed) end and a $(-)$ (pointed) end visible by electron microscopy.

Treadmilling

A steady state in actin dynamics where subunits are added to the $(+)$ end and removed from the $(-)$ end at an equivalent rate, resulting in constant turnover while maintaining filament length.

ATP hydrolysis

The process where $ATP$ binds to the $(+)$ end of actin and is slowly hydrolysed to release $P_i$; $ADP$-actin is more concentrated at the $(-)$ end and is less stably associated.

Thymosin

An actin-binding protein that binds to free subunits and blocks their addition to the filament, regulating polymerisation.

Profilin

An actin-binding protein that competes with Thymosin to enhance subunit addition to the actin filament; it can be activated by phosphorylation or phospholipids.

Capping proteins

Proteins that stabilise actin filaments by blocking assembly and disassembly, maintaining structures like the muscle sarcomere.

Formins

Protein dimers located at the $(+)$ end that nucleate the formation of unbranched actin filaments.

Arp2/3 complex

$Arp2/3$ stands for Actin Related Protein; it binds to the side of an existing filament to act as a template for a new branched network at the $(-)$ end.

Rho GTPase

A small GTPase located at the plasma membrane that activates formins to produce unbranched actin filaments.

Cdc42 and Rac GTPases

Small GTPases at the plasma membrane that activate the $Arp2/3$ complex to produce branched actin filaments.

Myosin

An $ATP$-powered motor protein that binds to actin to form contractile structures such as the contractile ring in mitosis or stress fibres in migrating cells.

Integrins

Membrane-spanning adaptor proteins that directly connect the actin cytoskeleton to the extracellular matrix to facilitate cell adhesion and migration.

Intermediate Filaments (IFs)

Diverse, rope-like polymers of extended protein subunits that are non-polar, do not bind nucleotides, and provide high tensile strength and resistance to mechanical strain.

Keratins

Intermediate filaments found in epithelial cells (gut, skin, hair, and nails) formed from heterodimers of acidic and basic/neutral chains, stabilized by disulphide bonds containing $14\%$ cysteine in hair.

Desmosomes

Specialised cell structures that connect the keratin cytoplasmic network of one cell to adjacent cells.

Epidermolysis bullosa simplex

A skin blistering disease caused by mutations in Keratin $5$ and $14$ that weaken the basal layer of the epidermis, causing it to detach from the basal lamina.

Nuclear lamins

Intermediate filaments that form a meshwork called the nuclear lamina to provide structural support to the nuclear envelope and organise chromatin to regulate gene expression.

SUN and KASH

Linker proteins used by lamins to connect the nucleus to all cytoskeletal filaments, optimising structural networks within the cell.

CDK1

A mitotic kinase that phosphorylates lamins, causing the nuclear lamina to depolymerise and promoting nuclear envelope breakdown during mitosis.

Emery-Dreifuss muscular dystrophy

A disorder caused by lamin mutations that disrupt polymerisation and weaken the nuclear lamina, affecting muscle cells that must withstand high mechanical strain from contraction.