Actin is a fundamental and highly abundant protein found in nearly all eukaryotic cells. It forms a significant component of the cytoskeleton, the internal scaffolding that gives cells their shape and allows them to move. Understanding the intricate structure of actin is fundamental to appreciating its diverse roles in cellular processes.
The Individual Actin Unit
The fundamental building block of actin structures is the individual actin protein, known as globular actin or G-actin. This monomeric form is a relatively small protein, typically weighing around 42 kilodaltons (kDa), with a characteristic globular, somewhat pear-shaped appearance. Each G-actin molecule contains a specific binding site for either adenosine triphosphate (ATP) or adenosine diphosphate (ADP), located within a deep cleft. The presence of ATP or ADP influences the stability and polymerization potential of the G-actin monomer. G-actin is organized into four distinct subdomains, which fold together to create the central nucleotide-binding cleft.
Forming Actin Filaments
G-actin monomers polymerize, forming long, fibrous structures known as filamentous actin or F-actin. This process involves G-actin molecules assembling head-to-tail in a helical arrangement. The resulting F-actin filament is a double-stranded helix, resembling two intertwined strings of beads. Each strand of the helix contains 13 G-actin monomers for every full turn, and the filament has a diameter of about 7 nanometers.
F-actin filaments have polarity, meaning their two ends are structurally and functionally distinct. One end is the “barbed” (+) end, where G-actin monomers add more rapidly, while the opposite end is the “pointed” (-) end, where monomers add more slowly.
The Dynamic Nature of Actin
Actin filaments are not static structures but undergo continuous assembly and disassembly in a process called treadmilling. This dynamic behavior is influenced by the ATP or ADP bound within G-actin monomers. When ATP-bound G-actin incorporates into the filament, the ATP is hydrolyzed to ADP. The presence of ADP-actin within the filament destabilizes the polymer.
New ATP-bound G-actin monomers add to the rapidly growing barbed (+) end of the filament. Simultaneously, ADP-bound actin monomers dissociate from the more slowly growing pointed (-) end. This constant addition at one end and removal at the other allows cells to rapidly remodel their actin cytoskeleton, enabling quick adjustments to cellular needs.
Actin’s Essential Cellular Functions
The structural properties and dynamic nature of actin filaments enable a wide array of cellular functions. Actin networks contribute to maintaining cell shape and providing structural integrity, forming a scaffold beneath the plasma membrane.
Its dynamic polymerization is key for cell motility, enabling cells to crawl across surfaces through the extension of structures like lamellipodia and filopodia. In muscle cells, actin filaments interact with myosin motor proteins to enable muscle contraction, a fundamental process for movement. During cell division, a contractile ring composed of actin and myosin forms to pinch the parent cell into two daughter cells during cytokinesis. Actin also serves as tracks for intracellular transport, guiding the movement of vesicles and organelles within the cell.