Molecule of the Month: Actin Branching by Arp2/3 Complex

Our cells have a complex cytoskeleton built around three basic types of protein filaments. Microtubules are used for jobs that require heavy lifting, like transporting vesicles or entire chromosomes with the help of dyneins and kinesins. Intermediate filaments are sturdy structures that are often used to build more permanent infrastructures, like the sturdy network supporting the nuclear membrane or large structures in skin and hair. Actin, on the other hand, is more nimble, being built and demolished when needed. Actin networks are often used to control the inner structure and overall shape of cells.

Arp2/3 complex helps to pattern actin filaments as they perform their many cytoskeletal jobs. It forms a branch in the actin cytoskeleton. It binds to an actin strand (the “mother” strand) and nucleates the construction of a new strand (the “daughter” strand). The shape of Arp2/3 complex results in a characteristic 70 degree angle between the two strands. This is perfect for building tree-shaped structures with many actin filaments connected by many Arp2/3 complexes.

Arp2/3 complex, shown here from PDB entry 7tpt, is a complex of seven different proteins. Together, they hug the side of the mother strand and glue it in place. Two of the proteins, termed Arp2 and Arp3, are very similar in structure to actin. As shown below in the interactive JSmol, they are arranged similarly to two actin chains in a filament, and provide the starting point for the daughter filament.

Many other proteins work together to manage the complex actin cytoskeleton, building and controlling connections. Two examples are shown here. Profilin (PDB entry 2btf), binds to actin and blocks part of the protein involved in forming interactions with neighbors in a filament. It is used to maintain a steady supply of actin monomers for use in building new filaments when needed. Capping protein, as the name implies, caps one end of an actin filament, ensuring that it doesn’t continue growing without any controls. As seen in PDB entry 7pdz, it is composed of a dimer of proteins that covers the end of the filament.