5.2 Motor proteins
The final element that is needed for a vesicle transport system is motor proteins, as indicated in Figure 9. These proteins bind to vesicles and organelles and use energy from ATP to move them along the microtubule or microfilament network. Two families of motor proteins, the kinesins and dyneins, move vesicles along microtubules, and members of the myosin family move them along microfilaments (see Box 1). The myosin family is also important in cell movement.
The direction of movement of vesicles along the cytoskeleton is absolutely dependent on the polarity of the microfilaments and microtubules. Some motor proteins move from the minus end to the plus end and others in the opposite direction. For example, of the various myosins that have been discovered throughout the animal and plant kingdoms, all but one (myosin VI) move towards the plus end of the filament.
Kinesins have a tertiary structure that is similar to myosin II, even though there is no significant similarity in the primary structure. Both molecules have two heads with motor domains formed around an ATP-binding core, and a coiled tail that binds to the cargo (Figure 31). A number of other molecules are related to kinesin, and all of them share the kinesin motor domain, but very little else. These are the kinesin-related proteins. Kinesin itself moves towards the plus end of microtubules (Figure 32), but other members of the kinesin family move to the plus or minus end depending on the protein. Some of the kinesin-related proteins are involved in moving microtubules during mitosis – in this way the motor protein and the microtubule act in an analogous way to myosin and microfilaments in cell movement.
Dyneins are unrelated to either kinesins or myosins, and they move towards the minus end of microtubules. Each is composed of two or three heavy chains, with the cytoplasmic dyneins having two chains, each of which forms a large motor domain. In nerve cells, the axonemal dyneins, which have two or three motor domains, transport vesicles along microtubule bundles in the axons.
In which direction will dyneins transport vesicles along the axons?
From the nerve terminal to the nerve body. Dyneins transport towards the minus end of microtubules, which is located in the MTOC near the nucleus.
The speed of the movement mediated by dyneins and kinesins is quite extraordinary. In vitro, kinesins can move along microtubules at 2 μm s−1 and dyneins at up to 14 μm s−1. Although these high rates of movement would not be achieved in the complex environment of a cell, they can explain, for example, how caveolar transcytosis of molecules across an endothelial cell can occur in 1–2 minutes. Movement and force generation by both classes of proteins involves ATP hydrolysis and allosteric shifts in the orientation of the motor domains, so that the proteins are thought to ‘step’ progressively down the microtubule.
Notice however that the ATP-binding site of kinesin (Figure 31) is located at the distal tip of the motor domain, whereas in myosin the equivalent site is deep within the motor domain and covered by the myosin's actin-binding site. Therefore the mechanism of stepping is different in the two molecules. In particular, the α-helical linking region connecting the two motor domains of kinesin appears to transfer allosteric changes between them to coordinate ATPase activity and hence the stepping motion of the protein. It is interesting that the motor domains of kinesin-related proteins that move to the plus end and those that move to the minus end of the microtubule are similar, but the linkage between them is quite different. Kinesin has its motor domain near the N-terminus, whereas the molecule Ncd, which moves to the minus end of the microtubule, has its motor domain located near the C-terminus. It seems that whether the protein is directed to the plus or the minus end is dependent on the configuration of domains and the coordination.
The mechanical cycle of kinesin is outlined in Figure 33. Notice that kinesin is permanently attached to the microtubule, by either one head or the other. By comparison, the myosin heads (which are arranged in bundles in myofibrils) are only in contact with actin filaments for about 5% of each movement cycle.
What advantage can you see in the stepping mechanism of kinesin when compared with myosin? Remember that the kinesin molecule acts singly, whereas myosin acts in concert with other myosin molecules in the myofibril.
It means that kinesin does not let go of the microtubule between steps, so the cargo is permanently attached and less likely to be lost from its trackway, which could lead to its misdirection.
How are different motor proteins associated with different trafficking vesicles? Figure 34 shows a basic hypothesis of how a vesicle attaches to a microtubule, but the details of this process are still largely unknown. Observation of moving caveolae suggests that they tend to track initially to a large endosomal compartment, the caveosome, located near the MTOC and then switch to move away, as they traverse the cell. However it is also clear that individual caveolae can alternately move forwards or backwards along the microtubule network.
What does this imply about the motor proteins that associate with caveolae?
Different motor proteins can associate with the same vesicle – some moving to the minus end of the tubule and others to the plus end.
It has been suggested that there is occasionally competition between kinesin and dyneins to produce this shuffling back and forth, but this is the exception. Observation of the movement of secretory vesicles shows a rapid one-way transfer from the Golgi to the plasma membrane, and the key to this transfer must be the attachment of the correct motor protein as the vesicles bud from the trans Golgi network. However, this subject is not well understood. It is possible that small GTPases, such as those that assemble the vesicle and the coat proteins, could also be responsible for recruiting the correct motor protein to the vesicle, and there is good evidence for different adaptor proteins that link the motor protein to the vesicle. GGA proteins (Figure 19) could perform such a function, but at present the mechanisms of the process are unclear.