Cytoskeleton 2

Chapter 12, pages 511-517, 473-491

In the previous section we discussed two of the three major kinds of filaments that make up the cytoskeleton, the intermediate filaments and microtubules. We continue our examination of the cytoskeleton with a look at the role of microtubules in intracellular transport and positioning of vesicles and organelles. We will then move on to look at actin filaments and their functions.

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.






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 12.23 and 12.24).

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Copyright © 2008 Indira Rajagopal