Cell Biology

Motor proteins: Traffic on the cellular highway

Not only do the cells of the body move, for example a macrophage chasing an invading pathogen or the movement of limbs in the process of walking, but the supplies inside cells are constantly being moved around. This is carried out by fascinating machines called motor proteins, which move faster than your car if you consider the difference in dimensions!

Inside the cells of the body, there is an intricate network of highways which are the routes by which supplies are distributed to different cities as and when they are required. Just like on real highways, there is a constant construction work to improve connectivity and hence efficiency. The highways are collectively called the cytoskeleton of which there are two different types called actin filaments and microtubules. They are like tightropes and are over 1000 times thinner than a hair!

Just like how cars cannot travel along a cycle lane, different motor proteins travel along different types of cytoskeleton. Kinesin and dynein motors move along microtubules, whereas myosin motors move along actin filaments. There are a huge number of these motor proteins which have subtly different characteristics, just like the automobile market. Some move faster, whereas others can transport more material at once.

The different ends of microtubules are called plus and minus. The plus end is closed to the edges of cells, whereas the minus end is located near the nucleus (the control centre in the middle of the cell). Motor proteins can only walk in one direction; they cannot make a U-turn. Kinesin walks towards the plus end, whereas dynein walks towards the minus end. The direction that material is transported in is determined by a tug of war between these opposing motors (see video). If kinesin motors win the material is moved closer to the edges of the cell whereas if dynein motors win the material is transported toward the cell interior. In comparison, actin filaments are mostly at the cell exterior and play a key role in the shape and movement of a cell. For example, a macrophage chasing a pathogen and engulfing it is all dependent on actin filaments.  In the figure below actin filaments, microtubules and the nucleus of a cell is shown in red, green and blue, respectively.

How are motor proteins powered? They do not require petrol, but instead are fuelled by the energy currency of the cell called ATP which is produced by mitochondria, the powerhouses present in cells of the body. For every molecule of ATP used, the motor protein takes a single step forward along a cytoskeleton track (1). Motor proteins move a few millimetres per hour, which may seem slow but when considering the difference in dimensions it moves faster than your car and is also far more efficient!

Why do cells need motor proteins? If cells were dependent on material inside cells randomly floating around until reaching the correct place for them to perform their function, the chemical reactions that happen in the body would not occur on the timescale required to support life. A dramatic example is the neurons in the brain tasked with controlling movement of body parts as distant as the toes. To do this, they have long projections that can stretch up to a metre in length! If motor proteins did not transport material from one end of the neuron to the other, this control of the human body would not be possible.

What type of material can be transported by motor proteins? Any protein or fat can be packaged into sacks called transport vesicles which are dragged along by the motors (see video). In addition, motors can move whole structures inside cells. For example, mitochondria can be moved to places in the cell where the most energy is required and chromosomes, the genetic material of the cell, can be transported to opposite sides of the cell in the process of cell division. Since cancer is driven by uncontrolled cell division (read more here), understanding the motors involved in this process is relevant to disease. For example, certain motors could be blocked in cancer cells to slow the growth of tumours.

Another interesting example of the role of motor proteins is the skin colour of amphibians. The skin colour of amphibians comes from types of cells called melanocytes. These cells contain melanosomes, the movement of which by dynein and kinesin motors allows the amphibian to change skin colour (2). When melanosomes are clustered in the centre of the cell, the skin appears lighter. In comparison, when they are closer to the cell exterior the skin appears darker (see diagram).


References

(1) Vale, R. D. & Milligan, R. A. (2000) The way things move: looking under the hood of molecular motor proteins. Science (New York, N.Y.). 288 (5463), 88-95.

(2) Wasmeier, C., Hume, A. N., Bolasco, G. & Seabra, M. C. (2008) Melanosomes at a glance. Journal of Cell Science. 121 (Pt 24), 3995-3999.

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