The Cytoskeleton
The cytoskeleton is a network of different protein fibers that provides many functions: it maintains or changes the shape of the cell; it secures some organelles in specific positions; it enables movement of cytoplasm and vesicles within the cell; and it enables the cell to move in response to stimuli.
There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules. Some of the cytoskeletal fibers work in conjunction with molecular motors which move along the fibers within the cell to carry out a diverse set of functions.
There are two main families of cytoskeletally-associated molecular motors: dyneines and kinesins.
Microfilaments
Actin
Microfilaments are cytoskeleton fibers composed of actin subunits. Actin is one of the most abundant proteins in eukaryotic cells and comprises 20% of total cellular protein by weight in muscle cells.
The actin amino acid sequence is highly conserved in eukaryotic cells, meaning that the protein amino acid sequence, and therefore its final 3-D shape, has changed little over the course of evolution, maintaining more than 80% similarity between algae and humans.
Actin can be present as either a free monomer called G-actin (globular) or as part of a polymer microfilament called F-actin ("F" for filamentous).
Actin must be bound to ATP in order to assemble into its filamentous form and maintain the structural integrity of the filament. The actin filament itself has structural polarity.
This term "polarity", in reference to a cytoskeleton filament, does not mean what it did when we discussed polar functional groups earlier in this course.
Polarity here refers to the fact that there are two distinct ends to the filament. These ends are called the "(-)" end and the "(+)" end. At the "(+)" end, actin subunits are adding onto the elongating filament and at the "(-)" end, actin subunits are disassembling or falling off of the filament. This process of assembly and disassembly is controlled by the ATP to ADP ratio in the cytoplasm.
Actin participates in many cellular processes, including muscle contraction, cell motility, cytokinesis during cell division, vesicle and organelle movement, and the maintenance of cell shape. Actin filaments serve as a track for the movement of a family of motor proteins called myosins discussed in more detail in a section below.
Intermediate filaments
Intermediate filaments are made of several strands of fibrous proteins that are wound together. These elements of the cytoskeleton get their name from the fact that their diameter, eight to ten nm, is between those of the smaller microfilaments and the larger microtubules.
The intermediate filaments are the most diverse group of cytoskeletal elements. Several types of fibrous proteins are found in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the epidermis of the skin.
Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the shape of the cell, and anchor the nucleus and other organelles in place. The figure above shows how intermediate filaments create a cable-like supportive scaffolding inside the cell.
Microtubules
Microtubules are the largest component of the cytoskeleton and are found throughout the cytoplasm. These polymers are made up of globular protein subunits called α-tubulin and β-tubulin. Microtubules are found not only in eukaryotic cells but in some bacteria as well.
Both the α-tubulin and β-tubulin subunits bind to GTP. When bound to GTP, the formation of the microtubule can begin, this is called the nucleation event. As more GTP tubulin dimers assemble onto the filament, GTP is slowly hydrolyzed by β-tubulin to form GDP.
Tubulin bound to GDP is less structurally robust and can lead to disassembly of the microtubule.Much like the actin filaments discussed above, microtubules also have a distinct polarity that is critical for their biological function.
Tubulin polymerizes end to end, with the β-subunits of one tubulin dimer contacting the α-subunits of the next dimer.
These differences lead to different subunits being exposed on the two ends of the filament. The ends are designated the "(−)" and "(+)" ends. Unlike actin filaments, microtubules can elongate at both the "(+)" and "(-)" ends, but elongation is significantly more rapid at the "(+)" end.
Microtubules help the cell resist compression, provide a track along which vesicles move through the cell, pull replicated chromosomes to opposite ends of a dividing cell, and are the structural elements of flagella, cilia, and centrioles (the latter are the two perpendicular bodies of the centrosome).
In fact, in animal cells, the centrosome is the microtubule organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in bacteria, discussed below.