Within every plant and animal cell is a microscopic city with a complex highway system built from the cytoskeleton, the cell’s structural framework. Microtubules are a major part of this framework, functioning as rigid, hollow girders that provide support and act as roadways for transporting materials. These structures are dynamic polymers, constantly built and disassembled to meet the cell’s changing needs, allowing them to participate in a wide array of cellular activities.
The Building Blocks of Microtubules
The fundamental unit used to construct a microtubule is a protein called tubulin. Tubulin is a dimer composed of two tightly bound protein subunits: alpha-tubulin and beta-tubulin. The consistent arrangement of these subunits creates a building block with inherent directionality.
The beta-tubulin subunit has a site that can bind to guanosine triphosphate (GTP). This site is active, as the beta-tubulin can hydrolyze the bound GTP into guanosine diphosphate (GDP). This ability to bind and alter GTP directly influences the stability and assembly of the entire microtubule. The alpha-tubulin subunit also binds to a GTP molecule, but this one is non-exchangeable and is considered an integral part of the protein’s structure.
Assembling the Hollow Tube
The creation of a microtubule begins with the end-to-end joining of tubulin dimers. These dimers link in a specific orientation, with the beta-tubulin of one dimer connecting to the alpha-tubulin of the next. This sequential addition forms a long, linear chain known as a protofilament.
To form the complete structure, multiple protofilaments, 13 in human cells, align parallel to one another. They associate laterally to form a sheet that curves and closes to create a hollow tube with an outer diameter of about 25 nanometers. The protofilaments are slightly staggered, which gives the microtubule a helical pattern.
This repeated orientation of the alpha-beta tubulin dimers gives the microtubule a distinct structural polarity. One end of the tube, terminating with beta-tubulin subunits, is the “plus end.” The opposite end, which exposes alpha-tubulin subunits, is the “minus end.” This polarity refers to the different dynamic properties of the two ends, not an electrical charge. The plus end is where assembly is most active, while the minus end is less active and often anchored.
The Dynamic Nature of the Structure
Microtubules are not permanent fixtures; their existence is defined by a process called dynamic instability. This term describes the constant switching between phases of growth (polymerization) and shrinkage (depolymerization). This behavior is most prominent at the plus end, allowing the cell to quickly remodel its cytoskeleton in response to various signals or needs, such as during cell division.
This dynamic behavior is controlled by the GTP molecule bound to the beta-tubulin subunit. When tubulin dimers are added to a growing microtubule, they are in a GTP-bound state. A collection of these GTP-tubulin dimers at the plus end forms a “GTP cap.” This cap stabilizes the straight structure of the protofilaments, promoting further assembly.
Over time, the GTP on older tubulin dimers deeper within the microtubule is hydrolyzed into GDP. GDP-bound tubulin has a slightly different shape that introduces strain into the protofilament structure. If the rate of GTP hydrolysis catches up to the rate of new tubulin addition, the stabilizing GTP cap is lost. This exposes the strained, GDP-containing core, causing the protofilaments to peel outward and leading to rapid disassembly, an event termed a “catastrophe.” Occasionally, a shrinking microtubule can be “rescued” if it begins adding GTP-tubulin dimers again, re-establishing a cap and resuming growth.
Structural Variations and Organization
While the basic microtubule is a hollow tube, cells organize these structures into more complex assemblies for specialized functions. The initial growth of most microtubules in animal cells is directed by a Microtubule-Organizing Center (MTOC). The most well-known MTOC is the centrosome, located near the cell’s nucleus. The minus ends of microtubules are embedded within the centrosome, which acts as an anchor, while the dynamic plus ends grow outward.
The centrosome contains a pair of cylindrical structures called centrioles. A centriole is built from nine sets of triplet microtubules, where three microtubules are fused, arranged in a “9×3” pattern with an empty center. These centrioles are also foundational to forming basal bodies, which anchor cilia and flagella.
The core of motile cilia and flagella is a microtubule structure called the axoneme, which displays a “9+2” pattern. This consists of nine fused pairs of microtubules, or doublets, forming an outer ring that surrounds two single, central microtubules. This stable arrangement, along with associated proteins, allows for the controlled bending motion that propels cells or moves fluid.