The Cytoskeleton

Chapter 12, pages 497-517, 511-517, 473-491

So far, we have looked at the major organelles in the cell and their activities. We have seen that these organelles lie within the cytoplasm of the cell and are separated from it by membrane barriers. We have also noted that much of the movement of proteins from the endoplasmic reticulum to other compartments of the cell, such as the Golgi complex, the lysosome and the plasma membrane, occur in vesicles that are budded off from one organelle and transported to the target site where they fuse and deliver their contents.

In this next section we will examine the cytoskeleton, an intricate network of protein filaments that extends throughout the cell, giving it support and shape. The cytoskeleton is also involved in the directed movement of organelles and vesicles, in the separation of chromosomes during mitosis, and in cellular movement.

Do all cells have cytoskeletons?
Till recently, only eukaryotic cells were thought to possess cytoskeletons, though some simple cytoskeletal components were known to be present in bacteria. Research published in the past couple of years, however, suggests that prokaryotes may also have structures that resemble the cytoskeletal elements of eukaryotic cells. Because relatively little is known of prokaryotic cytoskeletal structures, we will focus on the eukaryotic cytoskeleton.






What is the cytoskeleton made up of?
The cytoskeleton is made of three major kinds of protein filaments:
1. Intermediate filaments
2. Microtubules
3. Actin filaments.






What is different about these three kinds of filaments?
There are many differences among these filaments, but for a start, they are made up of different kinds of protein subunits:
Intermediate filaments are composed of a variety of fibrous proteins
Microtubules are made up of a protein called tubulin
Actin filaments are made up of a protein called actin.






What are intermediate filaments?
Intermediate filaments are rope-like fibers made up of a variety of intermediate filament proteins. The rope-like structure of intermediate filaments gives them great strength and their main function is to provide mechanical strength to cells to help them withstand stress (such as stretching and changing shape).






Where in the cell are intermediate filaments found?
These filaments form a network in the cytoplasm, surrounding the nucleus and radiating out to the plasma membrane of the cell. Intermediate filaments are also found in the nucleus, where they form the meshwork called the nuclear lamina underlying the inner membrane of the nuclear envelope.






Do intermediate filaments extend from one cell to another?
In tissues like skin, the intermediate filaments (keratins are the particular kind in skin) are connected from one cell to the next by attachment to cell-cell junctions called desmosomes. This arrangement, by which the "cables" of the intermediate filaments are joined from cell to cell in a sheet of skin, gives the entire structure great strength. People with a rare mutation in their keratin genes that prevents proper assembly of keratin filaments have skin cells that rupture from even slight pressure. This condition is called epidermolysis bullosa simplex or EBS. Yes, it's as horrible as it sounds.






How are intermediate filaments assembled?
Intermediate filaments are like ropes made up of multiple strands twisted together. The strands are made up of intermediate filament proteins.
Intermediate filament proteins, despite their variety, all have the same basic structure: they have a globular head at their N-termini, a globular tail at their C termini, and a rod-like alpha helical region in between (Figure 12.36). Two such units can wind around each other to make a "coiled coil" structure (Figure 12.37). Two of these coiled coils line up head-to-tail in a staggered fashion to form a tetramer. Numerous tetramers line up end to end to make a protofilament. Eight protofilaments are wound around each other to form the rope-like intermediate filament.






What regulates the assembly and disassembly of intermediate filaments?
The assembly and breakdown of intermediate filaments is often regulated by phosphorylation of the intermediate filament proteins. A well-studied example is the breakdown of the nuclear lamina during mitosis. The lamins, the special kind of intermediate filament proteins making up the nuclear lamina are phosphorylated prior to its disassembly.






What are some examples of intermediate filaments?
One group of intermediate filaments we have already encountered is the keratin filaments in skin and other epithelial cells. Some types of keratin are involved in making up skin, hair and nails.
A second type of intermediate filaments include vimentin and its relative desmin. Desmin is found in muscle cells where it links contractile elements.
A third kind of intermediate filaments, the neurofilaments, are found in neurons.
ALS (amyotrophic lateral sclerosis) or Lou Gehrig's disease is associated with abnormalities in neurofilament assembly in motor neurons.
There are numerous other varieties of intermediate filaments but we will not discuss these further.






What are microtubules?
Microtubules are another major component of the cytoskeleton. They are long, cylindrical, hollow rods that are made up of subunits of the protein tubulin. (Figure 12.42)






What do microtubules do?
Some of the major functions of microtubules are:
They help determine cell shape
They are involved in cell movement
They are involved in the position of organelles within the cell
They are involved in the separation of chromosomes during cell division.






Where are microtubules found in the cell?
Microtubules in animal cells originate from a structure called the centrosome, near the center of the cell. From the centrosome, these rods extend out towards the periphery of the cell, creating "tracks" along which vesicles and organelles can be moved. (Fig.12.45)






How are microtubules different from intermediate filaments?
There are many differences between the microtubules and intermediate filaments, but some obvious ones are:
Unlike intermediate filaments all microtubules are made up of a single kind of protein called tubulin.
Microtubules are assembled in such a way that they have a polarity (that is, one end is different from the other).
Microtubules are rapidly assembled and broken down many times within a short span of time, while intermediate filaments are more stable.






How are microtubules assembled?
If intermediate filaments are like ropes, microtubules can be thought of as hollow pipes. These pipes are built of 13 parallel protofilaments, each of which is a chain of tubulin subunits. Tubulin subunits are each composed of an alpha and a beta tubulin. These subunits stack on each other, with alpha and beta tubulin alternating, to form a protofilament that has a beta at one end and an alpha tubulin at the other. The beta end is called the plus end and the alpha end is called the minus end. Thirteen protofilaments of this type are arranged around a hollow core, in such a way that all of them have their plus ends on the same side. This gives the overall microtubule a directionality.






How do microtubules grow out from the centrosome?
The centrosome, from which the microtubules radiate, is the site of initiation of microtubule assembly. The minus ends of the microtubules are anchored in the centrosome, while the addition of tubulin subunits to the plus ends results in the growth of the microtubule. Assembly of microtubules starts from rings of 13 tubulin molecules (of a different type, gamma tubulin) present in the centrosome. Thus, the microtubules are all situated such that their plus ends are near the periphery of the cell.






Why are microtubules assembled and disassembled repeatedly?
The functions of microtubules in cell movement and intracellular transport of vesicles and organelles, as well as in chromosome movement, require that the microtubules be capable of quickly assembling, then breaking down and reassembling elsewhere in the cell.






Why do microtubules behave in this bizarre manner?
This alternating building and breakdown of microtubules at great speed is called dynamic instability and it has a certain cellular logic. As mentioned above, chromosome movement during mitosis, as well as cellular movement, require rapid changes in microtubule structure to allow remodeling of the cytoskeleton as needed, during these processes. The microtubules thrown out and retracted rapidly have been compared to a fisherman casting a line. If nothing is caught on the line it is pulled back and cast again. In the case of microtubules the growing microtubule can be stabilized if it reaches another cellular structure to which it attaches its plus end (see MAPs below). This prevents it from being broken down. It also sets up a relatively stable connection between the different parts of the cell, and allows organelles to be properly positioned with respect to each other.






Are there other factors that affect microtubule stability?
Yes, cellular proteins that interact with microtubules can decrease or increase the stability of microtubules. The Microtubule Associated Proteins (MAPs) are proteins that help stabilize microtubules. Interactions between MAPs and microtubules are an important mechanism by which the cell stabilizes microtubules in particular locations in cells, giving the cell its shape and organization.






What happens if microtubule assembly or disassembly are disrupted?
The crucial importance of microtubules in cell division has led to the development of drugs that disrupt microtubule function to kill rapidly dividing cancer cells. Some examples of drugs that bind to tubulin subunits and prevent microtubule assembly are colchicine, vincristine and vinblastine. These drugs can be used in cancer chemotherapy since they prevent assembly of microtubules that are necessary for cell division. Another class of drugs, of which an example is taxol, prevents microtubule breakdown. This, too, can stop cells from dividing, and it, too, is used as a cancer drug. Drugs that act by preventing mitosis in cancer cells are called antimitotics.

How do microtubules function in the intracellular transport of vesicles and organelles?
Microtubules function like tracks within the cell, on which cargoes of materials like vesicles or organelles can be transported. In this way, they can guide the movement of materials through the cell (note that actin filaments, which we will discuss later, may also function in this manner). In addition to moving vesicles and organelles, microtubules function in chromosome movement during cell division, as we have already seen.







What causes vesicles and organelles to move along these cellular "tracks"?
Two families of motor proteins, called the kinesins and dyneins, that move along the microtubules, act like tow-trucks, attaching to the cargo and pulling it along the tracks to its destination. There are many kinds of kinesins and dyneins, each of which is believed to transport a different cargo.







What provides the energy for these motor proteins to tow their cargoes?
Energy for the movement of the motor proteins and their cargoes is provided by ATP, which is broken down to ADP in the process.






What is the difference between kinesins and dyneins?
Kinesins and dyneins have similarities, but an important difference is that most kinesins travel toward the plus end of the microtubule that they are on (i.e., away from the center of the cell), while dyneins travel toward the minus end of the microtubule (towards the center of the cell). Thus, kinesins function to bring cargoes to the periphery of the cell, while dyneins function to carry cargoes to the center of the cell.
See Figure 12.51.






What do kinesins and dyneins look like?
Although there are differences in detail between kinesins and dyneins, both groups of motor proteins have these features in common:
- both have globular ATP-binding heads that function as the motor domain and interact with the microtubules.
- both have a tail domain that is involved in binding the cargo.
See Figure 12.50





How do kinesins only go away from the center of the cell, while the dyneins only move towards it?
The heads of the motor proteins have stereo-specificity, which means that they can bind to the microtubule only if they are "facing the right way". This determines the direction in which they can move.






What else do kinesin and dynein do besides delivering vesicles to their target destinations?
Kinesin and dynein are involved in keeping the organelles in the cell correctly positioned.
Kinesins are thought to be involved in keeping the ER stretched out towards the periphery of the cell
Dyneins are thought to be involved in keeping the Golgi complex near the center of the cell.



Actin Filaments

What are actin filaments?
Actin filaments are the most abundant of the three types of cytoskeletal filaments. Actin filaments are composed of the protein actin and form long, thin fibers. These fibers may sometimes be grouped together to make bundles, or crosslinked to make a three-dimensional network.






What are the functions of actin filaments?
Actin filaments are needed for cell movement, phagocytosis and cell division.
They also help in providing shape to the cell
They function as tracks for intracellular traffic, like microtubules.
They are involved in muscle contraction.






How are actin filaments assembled?
-Individual actin molecules are globular proteins, each of which can bind to two other actin molecules to make a trimer.
-These trimers can then make long fibers by the addition of more actin molecules to each end.
-Like microtubules, actin filaments have a plus end and a minus end.
-The way that actin filaments form is very similar to the way that microtubules are assembled (see below).






How is the assembly of actin filaments like that of microtubules and how is it different?
Tubulin subunits of microtubules have GTP bound to them, and this GTP is hydrolyzed to GDP soon after a subunit is added to the growing microtubule. Similarly, actin monomers have ATP bound to them, and this ATP is hydrolyzed to ADP soon after the monomer has joined a growing actin filament (to remember which is which, remember A for actin and A for ATP). Like microtubules, actin filaments are readily disassembled and reassembled. Like microtubules, actin filaments can be stabilized by the binding of specific proteins.






How do actin filaments affect cell shape and bring about cell movements?
Actin filaments are found in large amounts just inside of the plasma membrane. The network of actin and associated proteins in this region is called the cell cortex and it gives a cell its characteristic shape. When cells need to move or engulf particles, the actin network underlying the plasma membrane changes in shape by growth of the actin filaments. The change in the actin filaments leads to the production of protrusions of the cell that help the cell to "crawl" across a surface or engulf particles by phagocytosis. 


What is myosin?
Myosin is a protein originally found in skeletal muscle, but now known to be present in other cells as well. There are various kinds of myosin, but the myosin-I and myosin-II groups are the most abundant. Muscle myosin belongs to the myosin-II family (see Figure 12.25).






What does myosin look like?
Muscle myosin (myosin-II) is composed of a pair of identical myosin molecules, and has two globular heads and a coiled-coil tail. Clusters of myosin-II molecules bind to each other through their tails, forming a myosin filament .
The myosin filament is organized like a double-headed arrow, with two sets of heads that are pointing away from each other. See Figure 12.23






How are myosin filaments connected to actin filaments in muscle cells?
One set of heads on a myosin filament is associated with one set of actin filaments and the other set of heads is associated with another set of actin filaments (Figure 12.23). This arrangement permits the sliding of actin filaments past one another and contracting. When whole bundles of actin and myosin filaments move in unison in this manner, the bundles can generate a contractile force that is the basis for muscle movement (Figure 12.24)

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 Copyright © 2009 Indira Rajagopal