Dynein teams assemble on lipid rafts to generate large forces on phagosomes

Abstract

Cells engulf micro-organisms and enclose them in a bilayer membrane compartment (the phagosome) for degradation. The phagosome undergoes programmed maturation in order to degrade the foreign organism. Intimately linked to this degradation is active transport of the phagosome along microtubules by dynein and kinesin motors. Early phagosomes move in back-and-forth manner near the cell periphery, and mature by fusing with other organelles during this period. In contrast, late phagosomes move in almost unidirectional manner towards the lysosomes at the cell centre. This early-to-late phagosome switch is important for the degradation of pathogens. Tuberculosis bacterium and Salmonella survive by aborting this switch. We have investigated the mechanism of this switch by precisely counting the number of motor proteins on individual motile phagosomes. This counting is done by using an optical trap which measures the force generated by motors on a phagosome as they move. Our results suggest that the “switch” in a phagosome's fate is because of the formation of cholesterol-rich lipid rafts on the phagosome. Dynein motors cluster into these raft-like microdomains, and by doing so are able to work cooperatively in large teams. I will discuss the biophysical evidence for this clustering, the biochemical mechanisms by which it may be effected and what it implies for phagosome biogenesis.