Cargo transport inside cells
doi:10.1038/nindia.2013.15 Published online 1 February 2013
Researchers have devised a new technique that shows how a protein molecule ferries its cargo inside a cell1. The technique also helps capture live images of such tiny cellular affairs and measures how much force the protein exerts on its cargo during transport.
The technique will shed new light on protein-based cellular transport and its role in running cellular processes.
While carrying cellular materials, tiny proteins walk in a step-like manner on a network of roadways made of protein filaments. The proteins generate forces about a million-million times smaller than those experienced in day-to-day life. Studies have tried to explore the activity of such proteins by optically trapping protein-coated plastic beads, but could not mimic the exact cellular environment.
To mimic protein-mediated cellular transport and clarify its role in cellular processes, the researchers first probed optically trapped kinesin-1 protein-coated artificial beads and then analysed kinesin-1-driven motion of rat liver lipid droplets on microtubules, a type of cellular protein filament. Sophisticated imaging techniques including a special photodiode detector and charge-coupled device camera captured live images of these microscopic events.
The study measured the force of purified kinesin-1 on beads of known size. In studies with rat liver lipid droplets, 75% of lipid droplets moved when placed on microtubules. Kinesin-1 is known to drive the movement of lipid droplets. The study consistently detected kinesin-1 on the lipid droplets. Using a kinesin-1 inhibitor reduced the motion of lipid droplets, showing that kinesin-1 drives the movement of lipid droplets.
The researchers say that this method could be used to measure force exerted by proteins on single organelles in cell extracts and throw light on various cellular processes.