Actin Filaments and the Cell Cortex

Actin filaments are dynamic polymers that form a network beneath the plasma membrane, the cell cortex, shaping the cell surface and driving movement.

Definition

Actin filaments are helical polymers of the protein actin that, together with regulatory and motor proteins, form the cell cortex and other structures driving cell shape and movement.

Scope

This topic covers the structure and polarity of actin filaments, their nucleation and assembly dynamics, the many proteins that regulate them, the organization of the actin-rich cell cortex, and the role of actin in protrusions, adhesion, and cell shape.

Core questions

• How do actin monomers assemble into polar filaments?
• What regulates where and when actin filaments grow?
• How is the cell cortex organized beneath the membrane?
• How does actin drive protrusions such as lamellipodia and filopodia?

Key theories

Actin assembly and treadmilling

Actin filaments are polar and grow faster at one end than the other, allowing treadmilling and regulated assembly that, with nucleators and capping proteins, generates pushing forces at the cell edge.

Mechanisms

Actin monomers polymerize into polar filaments with a fast-growing plus end and slower minus end, hydrolyzing bound ATP after incorporation. Nucleators such as the Arp2/3 complex and formins initiate filaments, while capping, severing, and crosslinking proteins shape the network. Beneath the plasma membrane, a dense cortical actin meshwork controls surface mechanics; localized assembly pushes out sheet-like lamellipodia and spike-like filopodia, driving cell crawling and shape change.

Clinical relevance

The actin cortex governs cell shape, surface dynamics, and crawling, and is central to understanding cell mechanics and migration. The treatment here is descriptive and non-prescriptive.

History

Detailed biochemical and microscopy studies through the late twentieth and early twenty-first centuries, including the work of Pollard and Borisy, established how actin polymerizes and how nucleators and regulators build the dynamic cortical network that drives motility.

Key figures

• Thomas Pollard
• Gary Borisy
• Marie-France Carlier

Seminal works

• pollard2003
• alberts2014

Frequently asked questions

What is the cell cortex?

It is the dense layer of actin filaments and associated proteins just beneath the plasma membrane that controls the shape and mechanical properties of the cell surface.

How does actin push the cell membrane outward?

New actin subunits add to filament ends pointed toward the membrane, and the growing filaments push against it to form protrusions such as lamellipodia and filopodia.