UC Merced

In cells, multiple molecular motors work together as teams to carry cargoes such as vesicles and organelles over long distances to their destinations by stepping along a network of cytoskeletal filaments. A type of molecular motors, kinesins, are known to mechanically interfere with each other and be non-cooprative when assembled in in-vitro experiments. However, these motors transport cargo over long distances in cells. It is unclear what is causing the enhanced teamwork between motors in cells. In this dissertation, we explore the possibility of lipid membranes enclosing most intracellular cargoes, enhancing teamwork. We understand the effects of lipid membranes on team dynamics by developing a three-dimensional simulation of cargo transport along microtubules by teams of kinesin-1 motors and applying it to various physiologically relevant conditions. In this model, we accounted for cargo membrane fluidity by explicitly simulating the Brownian dynamics of motors on the cargo surface and considered both the load and ATP dependence of single motor functioning. We first apply the model to a more straightforward case of cargo transport by identical kinesin motors. These simulations show that surface fluidity could lead to the reduction of negative mechanical interference between kinesins and enhanced load sharing thereby increasing the average duration of single motors on the filament. This, along with a cooperative increase in on-rates as more motors bind leads to enhanced collective processivity. At the cargo level, surface fluidity makes more motors available for binding, which can act synergistically with the above effects to further increase transport distances though this effect is significant only at low ATP or high motor density. These results reconcile experimental obervations of cargo runlength. Additionally, the fluid surface allows for the clustering of motors at a well defined location on the surface relative to the microtubule and the fluid-coupled motors can exert more collective force per motor against loads. Then we proceed to understand cargo transport at different physiologically relevant complexities, starting with heterogenous teams of motors. In vitro experiments of membrane-bound cargo transport by teams of motors have reported that coupling motors through a lipid membrane lead to higher cargo velocity. However, the mechanisms behind this increased lipid cargo velocity are unclear. Using suitable modifications to the Brownian dynamics model, we show that underlying heterogeneity in single motor velocity is essential for increased velocity of lipid cargoes. We further explored other advantages of having heterogeneous motor velocities on lipid cargoes. Our simulations show that while runlength of both rigid and lipid cargoes increases with an increase in the motor velocity heterogeneity, lipid cargoes can travel a given distance with a lower degree of heterogeneity meaning a higher cargo velocity. Together our work explains mechanisms behind previous experimental observations and generates new experimentally testable predictions on runlength relevant for in vivo transport. Next, we discuss breakdowns in cargo transport due to intracellular complexities and possible lipid membrane-mediated rescue. The presence of different kinds of roadblocks on the microtubule lattice, such as Microtubule Associated Proteins (MAPs) like the tau protein, neurofibrillary tangles, stalled cargoes, etc, are known to disrupt cargo transport. Enrichment of such roadblocks, specifically amyloid plaques and neurofibrillary tangles are observed in brain cells of patients with neurodegenerative diseases, including Alzheimer's disease. Using Brownian dynamics simulations, we show that membrane-bound cargoes also have a higher probability of crossing roadblocks than membrane-free cargoes under specific conditions. We also find that lipid and rigid cargoes might employ qualitatively different strategies to pass certain kinds of roadblocks. Finally, we discuss the effects of having different mechanical models of motors and a three-dimensional lattice structure in our model and generate experimentally testable predictions to identify a suitable motor model for future studies. We find that this improved model might address the problem of cargo transport across large roadblocks better. More work needs to be done in this direction.

Overall our work on understanding how lipid membrane impacts cargo transport by teams of motors sheds new light on cellular processes, reconciles existing observations, and encourages new experimental validation efforts; This also suggests new ways of improving the transport of artificial cargo powered by motor teams. We believe our work may inform future research on better treatments for neurodegenerative diseases like Alzheimer's.