Transport Proteins: Always Keeping A Safe Distance

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Transport Proteins: Always Keeping A Safe Distance

http://www.medicalnewstoday.com/medicalnews.php?newsid=55042

Scientists from the Max Planck Institute for Molecular Cell Biology

and Genetics (MPI-CBG) in Dresden, along with a colleague from the

University of Florida in the United States, have been carrying out

research into how transport proteins can move in cells without

bumping into or sticking to anything. Using an ultra-sensitive

microscopy method, the researchers have defined how the motor

protein Kinesin-1 interacts with its " transport rails " , the

microtubules, based on a nanometer scale. The motor protein stays at

a distance of 17 nanometers from the microtubules, which explains

how it manages to carry its load to its destination without

collisions (PNAS, 24 October 2006).

Cells are very similar to the Port of Hamburg: goods and commodities

arrive, have to be classified, temporarily stored and subsequently

dispatched. In both small and large cells, it is very important to

avoid backups and collisions, as these transport processes are vital

for every organism.

Motor proteins take care of transport in cells, carrying their goods

back and forth like small containers. They need transport rails to

accomplish this, otherwise they would float aimlessly in the

cytoplasm. This role is performed by microtubules, among others,

long thread-shaped structures about 25 nanometers in diameter and

several micrometers in length, which extend through the whole of the

cell. Motor proteins, of which kinesin-1 is a well-researched

example, consist of a head, middle section and tail. They are

considerably smaller than their transport rails.

Whether kinesin-1 is transporting individual cell organelles or

other cargo, its head section moves forward step by step on a

microtubule - the length of the step and the precise coordination of

such steps have already been investigated in detail. It is also

known how kinesin attaches the load to its tail. But how do the

small motor proteins make sure that nothing gets caught up during

transport? The crucial question is: how many nanometers from the

microtubule does kinesin-1 hold the load during transport? Does this

perhaps have anything to do with the previously unexplained role of

the section between head and tail?

Naturally, it is not possible to simply measure nanometers with a

ruler. For the scientists working at the MPI-CBG in Dresden,

Kerssemakers from Stefan Diez Junior Research Group and Jonathon

(MPI-CBG) and Henry Hess (University of Florida, USA),

analyzing a process in such a minute dimension presented a major

challenge, both technically and in terms of methodology.

They accomplished this feat using a highly sensitive, indirect

microscopy method, in which the researchers caused molecules to

which they had previously administered a fluorescent dye to glide

over a reflective silicon mirror. Depending on their distance from

the surface, the molecules lit up to different degrees as a result

of the interference effect (Fluorescence Interference Contrast:

FLIC). Consequently, the distance is indirectly determined by the

level of brilliance.

As large microtubule filaments are easier to perceive using this

method than individual kinesin-1 molecules, the researchers injected

them with the dye. " The whole system had to be turned upside down, "

explained Kerssemakers. Therefore the researchers did not directly

measure the distance of the motor protein from the microtubule, but

the distance that kinesin-1 holds the transport rails from the

reflecting surface. To this end, they covered the reflective surface

of the FLIC microscope with the motor proteins, to form a " kinesin

lawn " so to speak. The " kinesin stalks " held the microtubules away,

at precisely the distance at which the load otherwise glides along

the rails.

After precisely calibrating the measuring system, the scientists

finally calculated the distance. The result is 17 nanometers; the

distance at which kinesin-1 keeps the microtubules or holds its

cargo from the microtubules. This value is most expedient, as the

majority of particles that become obstacles in cells are smaller

than this gap. It would appear that this is how refined motor

proteins succeed in getting cargo to its destination without any

resistance or catching. Consequently, this finding quite literally

sheds a light into another aspect of the dark intracellular

transport channels.