Cell biology · BEGINNER LESSON
Actin polymerisation
Actin polymerisation is the process in which actin monomers join to make actin filaments, also called microfilaments. It is reversible, so filaments can also break back into monomers. [0,1]
What it is
Actin is a common cell protein that exists as single units called globular actin, or G actin. When these units join head-to-tail, they form filamentous actin, or F actin, which makes thin flexible fibers called actin filaments or microfilaments. These filaments are part of the actin cytoskeleton, the internal protein framework of the cell. [1,2]
[2] Structure and Organization of Actin Filaments - The Cell - NCBI - NIH [3] Actin, a Central Player in Cell Shape and Movement - PMC
How the process starts and grows
Actin monomers can polymerise spontaneously under physiological salt and ion conditions. The first slow step is nucleation, the unstable formation of a small cluster, usually described as a trimer made from three monomers. After that, elongation is faster because more monomers can add to filament ends. The filament is polar, meaning its two ends are not the same: the plus or barbed end grows faster than the minus or pointed end. [0,1,2]
[1] Actin and Actin-Binding Proteins [2] Structure and Organization of Actin Filaments - The Cell - NCBI - NIH [3] Actin, a Central Player in Cell Shape and Movement - PMC
What fuels and changes the filament
Actin monomers bind adenosine triphosphate, or ATP, a molecule cells use as an energy-carrying nucleotide. ATP is not required for polymerisation, but ATP-bound actin adds more readily than ADP-bound actin, where ADP is the lower-energy product after ATP hydrolysis. After actin joins a filament, hydrolysis happens faster, and the released phosphate leaves slowly, creating a long-lived intermediate. [0,1,2]
[1] Actin and Actin-Binding Proteins [2] Structure and Organization of Actin Filaments - The Cell - NCBI - NIH [3] Actin, a Central Player in Cell Shape and Movement - PMC
Why it matters in cells
Actin filaments provide mechanical support, help determine cell shape, and support movement at the cell surface. They help cells migrate, engulf particles, internalize membrane vesicles, and divide. They can also form bundles or three-dimensional networks with gel-like properties. [1,2]
[2] Structure and Organization of Actin Filaments - The Cell - NCBI - NIH [3] Actin, a Central Player in Cell Shape and Movement - PMC
Limits and control
Polymerisation is reversible, so monomers can also dissociate from filament ends. The critical concentration is the monomer level at which addition and loss balance, so the filament neither grows nor shrinks overall. Cells control this with actin-binding proteins: some help start filaments, some cap ends, some sever filaments, and cofilin increases loss from the minus end. [0,1,2]
[1] Actin and Actin-Binding Proteins [2] Structure and Organization of Actin Filaments - The Cell - NCBI - NIH [3] Actin, a Central Player in Cell Shape and Movement - PMC
SEE THE IDEA
Actin polymerisation in simple steps
Single actin units in the cell
Three unstable monomers form a first seed
More monomers add, faster at the barbed end
Proteins cap, sever, or promote growth
Supports shape, movement, and division
Monomers first form a weak seed, then grow into polar filaments, while other proteins control where and how long growth continues. [0,1,2]
Actin polymerisation is a reversible assembly process shaped by filament polarity and by proteins that help or stop growth. [0,1,2]
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.