What is the structural form of G-actin?

G-actin, or globular actin, is characterized as a globular monomer, which is a single polypeptide chain that can polymerize into F-actin (filamentous actin). In its monomeric form, G-actin represents a spherical protein structure that is essential for various cellular processes, such as muscle contraction, cell motility, and cytoskeletal maintenance.

The distinction between G-actin and F-actin is important; while G-actin serves as the building block or monomer, F-actin is a polymerized form that has a helical structure, providing stability and support for the cell's architecture. G-actin's ability to bind to ATP and subsequently hydrolyze it is critical for the dynamic nature of the cytoskeleton, allowing for rapid assembly and disassembly, which is fundamental during cellular movement and division.

The other options refer to different forms or structures not representative of G-actin. Filamentous refers to the polymerized form of actin (F-actin), whereas reinforced polymer and composite structure suggest more complex arrangements that aren't characteristic of G-actin in its monomeric state. Thus, identifying G-actin specifically as a globular monomer accurately captures its