Cytoskeleton

The cytoskeleton is a network of fibrous proteins

The cytoskeleton consists of fibers of differing diameter. Microtubules are about 25 nm in diameter and microfilaments are 7-8 nm in diameter. In between these extremes are intermediate filament of 10-13 nm in diameter.

The scanning electron micrograph below depicts a digested cell showing clearly the cytoskeleton.

Intermediate filaments are not well-characterized in plants to date

In animals, these filaments are formed of polymers of keratin, vimentin, or lamin. In plants, filaments of these diameters (10-13 nm) are known, but the monomers that form the polymers are not well known. In one case where a filament was identified in plants, its monomer turned out to be a glycolysis enzyme! So we will not consider this element of the cytoskeleton any further. Much is known about microtubules and microfilaments in plants, and we will focus on those primarily.

The cytoskeleton functions in anchorage, mobility, information, and polarity

Because of the structure of the filaments and tubules and associated proteins, these fibers can anchor subcellular components to each other and can provide the framework for movement of components within the cell.

Because of the polarity of cytoskeletal fibers, they can coordinate of directed movements and provide orientation information about position of an organelle within the cell.

Microfilaments are of polymers of 42kd G-actin

Below are two concepts of how a plant microfilament might be assembled.

The filament exists as actin-chains twisted together which are 7-8 nm in diameter. Because of the chemical structure of actin itself, the filament has polarity; the polymerization of actin occurs at high rates at the + end. This polymerization requires the presence of actin monomer at high enough concentration and ATP. As the G-actin monomers join, the ATP is converted to ADP and the resulting conformational changes in the monomers link them into trimers of F-actin, which can continue to polymerize into long microfilaments.

Microtubules are a heterodimer of 55kd α- and β- tubulin

The microtubule is quite a bit larger in diameter than an actin filament, and is a polymer of α and β tubulin.

Each alpha and beta tubulin form a pair. These pairs are linked end to end to form a protofilament. These protofilaments associate to form a tubular array of about 25-nm diameter with about a 14-nm lumen. There are 11-16 protofilaments forming the walls of the tubule...the diagram above shows 13, which is often depicted in books. The polarity of the tubule is created by the alpha-beta axis, with the beta-tubulin at the + (polymerizing) end of the tubule. GTP is the required nucleotide for polymerization. If GTP is depleted, the microtubule will rapidly disassemble. This instability is critical for the functions associated with microtubules.

The actin and tubulin monomers are conserved

The genes that code for the monomers of the cytoskeleton are highly conserved in evolution. Over the range of eukaryotic organisms examined to date, 80-90% of the base sequences are identical. Even prokaryotic E. coli produces the FtsZ gene product which is homologous to tubulin. Thus, these components of the cell likely evolved originally in very primitive cells...before the eukaryotic transition. The following diagram shows the genetic divergence as "length of branch" from the origin. You will notice the usual groupings of kingdoms and domains. Please note: this is NOT a cladogram.

Cyclosis requires motor proteins

One function of the cytoskeleton is the movement of organelles within the cell. This is sometimes called cytoplasmic streaming or cyclosis. The framework of cytoskeleton is essentially stationary in this movement...the organelles move along the fiber. This movement is accomplished by the motor proteins: dynein, myosin, or kinesin. These are shown by scanning electron microscopy and protein modeling below:

These proteins obviously have a "head" and "arm" structure. Through the action of nucleotide triphosphates at the "head" end, these mechanochemical proteins can slide along a cytoskeletal fiber carrying an object anchored at the end of the "arm."

Plants have at least 2 of the 13 known types of myosin. These accessory motor proteins move items along by interaction with microfilaments composed of actin.

In this second figure, dynein slides organelles (in this case, another microtubule) toward the - end of a microtubule. Kinesin slides organelles toward the + end of a microtubule.

To see this in the "real world" I show you a light micrograph of Arabidopsis leaves which demonstrate the movement of chloroplasts in cells in response to light intensity. In the left panel, the light is dim and the chloroplasts form a monolayer in the light path. In the right panel, the light is bright and the chloroplasts have moved to the sidewalls of the cell to shade each other. These are likely myosin-mediated movements along microfilaments.

For a second example, I show you an electron micrograph of a pollen tube cell tip. At the very end (right) are large numbers of secretion vesicles that the pollen tube uses to export digestive enzymes to open a pathway to the egg cell. To get all of these vesicles to accumulate here, they are moved along microfilaments (arrows) away from the Golgi where they are produced. The associated mitochondria provide the ATP necessary for the myosin-driven movements.

Microtubules are involved in orientation of cellulose microfibrils in the cell wall

The array of microtubules inside the cytosol is strongly correlated with the orientation of the most-recently-produced cellulose microfibrils. This correlation is shown diagrammatically below. In this case, it seems that cellulose synthesis peripheral proteins associated with the inner monolayer of the cell membrane are moved along the under-lying microtubule and extrude cellulose fibrils toward the outside (through the outer monolayer of the cell membrane), resulting in the parallel orientation of the cellulose and microtubules.