5.19: Microtubules in Cell Motility

Microtubules are thick hollow cylindrical proteins that help form the cytoskeleton. Microtubules have varied roles in the cell. These filaments help form cellular appendages like cilia and flagella, which are responsible for locomotion. The cilia arise from basal bodies, separated from the main body by a membrane-like structure forming the transition zone. This zone is the gate for the entry of lipids and proteins, creating a unique composition of lipids and proteins in the ciliary membrane and body. The central strand of the flagellum is called the axoneme and has a 9+2 arrangement of microtubules.

Microtubules help cells move using mechanisms like modulating actin polymerization by regulating Rho GTPase signaling pathways. During actin polymerization, with the help of +TIPs complex, microtubules sequester signaling molecules and actin assembly factors. These molecules are only released upon the disassembly of microtubules, thus regulating lamellipodia and filopodia formation.

Microtubules can also regulate directional migrations when they act as tracks for motor proteins to transport intracellular cargo and signaling molecules to the leading edge of the migrating cells. The cortical microtubules associated with the focal adhesion junctions help recycle focal adhesion proteins from the cell membrane during cell motility. They also facilitate the cross-talk between different cytoskeletal components. These microtubules undergo repeated cycles of rescue and catastrophe near the cell boundaries to regulate cell motility.

Cilia and flagella are fine hair-like structures responsible for motility in prokaryotic microorganisms, protozoans as well as in eukaryotic sperm.

The axoneme, the structural core, is made of microtubule bundles and associated proteins like nexins and dyneins. These microtubules are arranged in a 9+2 arrangement, where nine pairs of doublet microtubules form an outer ring surrounding a central pair of singlet microtubules.

The doublets have two structurally different microtubules— A and B. A has thirteen protofilaments, while B has ten. Microtubule A has an extended radial spoke that connects with the singlet microtubule pair at the center and helps regulate the activity of axonemal dyneins.

Axonemal dyneins have their stem embedded in microtubule A and their globular head and their stalk projected toward microtubule B of the neighboring microtubule doublet. These dyneins are primarily responsible for ciliary motion.

Adjacent microtubule doublets are connected by nexin, while central singlet microtubules are connected by central bridges made up of tryptophan and aspartic acid-rich protein PF20.