Cargo transport by molecular motor complexes in the crowded cell

The cell requires a high degree of internal organization for its survival. A set of specialized proteins known as molecular motors, are responsible for positioning large molecules and organelles in their correct spatiotemporal location. These proteins must navigate through the crowded cytoplasm as they haul their cargoes to their destination. Although the properties of the individual motors have been studied extensively in vitro, less is known about their functioning inside the cell. Of particular interest is the question of how in vivo opposing forces, e.g. cytoplasmic drag, affect cargo transport. This work presents studies of how cytoplasmic drag forces are involved in cargo transport at various length scales. First, a novel model of centrosome centering in large cells is presented. This model shows that the drag forces experienced by motor-driven cargoes are sufficient to position the large centrosome and associated microtubule aster; however, it raises the question of how these opposing forces affect the function of molecular motors. To address this issue, a combination of biophysical and biochemical tools is used to reveal the average response to drag forces of molecular motors as they haul lipid droplets in Drosophila embryos. A strikingly different response to load is found for the molecular motors kinesin-1 and cytoplasmic dynein. The results here presented validate, for the first time, the applicability of the Force-velocity curves previously measured in vitro for in vivo studies.