Peeking into a cell using an electron microscope you can see many compartments which are called organelles. Each of these organelles is a mini factory producing goods required for the happy functioning of the cell. Each one of them manufactures unique products (proteins, RNAs, sugars etc) that are distributed to others either to be utilized or to be processed further. To ensure safe and timely transport of these precious consumables, the cell uses a very intricate rail system consisting of five specially designed protein molecules, two of which, called actin filament and microtubule, act as the railroads, and the rest three as the goods carriers.
Like the local trains connecting places across the city, all the regions in the cell are interconnected by actin filaments which are randomly distributed throughout the cell. The cargo in this network is transported by the motor protein called myosin (one of the goods carriers) to their respective destinations. This line connects even places where the second type of rail road, the microtubules, is unable to access. Microtubules are like the interstate expresses connecting only selected places and unlike actin, microtubules are more neatly organized. Kinesins and dyneins use this route for delivering the cargo. Kinesin mediated transport generally is used to bring cargo to cell’s periphery and dyneins ,towards the cell’s interior. All of these motors are powered by ATP (adenosine triphosphate, an energy rich molecule produced by the cells).
Like the local trains connecting places across the city, all the regions in the cell are interconnected by actin filaments which are randomly distributed throughout the cell. The cargo in this network is transported by the motor protein called myosin (one of the goods carriers) to their respective destinations. This line connects even places where the second type of rail road, the microtubules, is unable to access. Microtubules are like the interstate expresses connecting only selected places and unlike actin, microtubules are more neatly organized. Kinesins and dyneins use this route for delivering the cargo. Kinesin mediated transport generally is used to bring cargo to cell’s periphery and dyneins ,towards the cell’s interior. All of these motors are powered by ATP (adenosine triphosphate, an energy rich molecule produced by the cells).
Each of these motors, although different in structural and functional details, share some common features. They all have heads and a tail connected by a central region called stalk. They carry their goods by their tails and hold on to the rail (actin filament or microtubules as the case may be) using their heads. On their heads is also the region where ATP can bind and gets hydrolyzed to give the energy for moving the motor.
Myosin keeps a 10nm step for every single ATP used (nearly 3.048 x 10-9 times your step size), Kinesin and dynein have a 8nm step size.(remember the late Michael Jackson doing his famous moon walk , nearly that’s how the motors look when they move along the rail). Check out these videos to watch myosin and kinesin on the move:
Among the three goods carriers, myosin and kinesin give better performance than dynein . The amount of ATP in the cell affects the working efficiency of dynein, whereas kinesin and myosin are independent of this factor. Moreover dynein’s step size also varies with the load its carrying and availability of ATP. Dynein also tends to take backsteps even in the absence of a load. But it overcomes these drawbacks and comes in par with myosin and kinesin by making use of additional proteins such as dynactin which are like the additional engines that provide the extra force and support to a goods carrier travelling in hilly regions.
It has been observed that all of these carriers work together and sometimes also help each other carry the heavy load, especially dyneins almost always teams up and work together. Given that both the railroads are such a busy network, how is the traffic on the lines regulated? It may not be such a great trouble on actin filament line as it’s only the myosin using the route and also these lines are more numerous. But when it comes to microtubule, two different goods carrier (kinesin and dynein) use the same line! To add to the problem these two carriers move in different directions. Although not much is known as to how traffic jams on a single line are avoided, it is clear that dyneins can avoid traffic by changing tracks.If a kinesin and a dynein happen to take the same line, kinesin being a lean and mean machine forces dynein to change its track.
All this said about the cell’s railway system, three important questions remain unanswered as yet; how does the goods carrier know its destination? And how are head on collisions prevented (Accidents don’t occur in healthy cells)? While functioning together how do they co-ordinate with each other (ensuring that the cargo doesn’t get lost and that ATP doesn’t get wasted)? With the amount of active research going on in this field we can hope to have the complete understanding of the cell’s railway system soon.
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