Studying Processive Molecular Motors Inside Live Cells PDF Download

"Many cellular processes are driven by the collective action of motors working in teams. One such example of this type of phenomenon is intracellular transport -- the transport of cargoes carried out by teams of similar or opposite polarity motors (kinesin and dynein). In most cells, long-range transport occurs along microtubules that are oriented with their plus ends towards the cell periphery, and minus ends towards the cell body. Despite the presence of opposite polarity motors, the mechanism by which cargoes are directed to specific locations in the cell is not well understood. A set of microtubule-associated proteins (MAPs) and scaffolding molecules have been shown to have some effect on motility and cargo directionality, but the mechanism by which these molecules tune motor specific processivity is not well understood. Defects in intracellular trafficking can result in severe developmental and neurodegenerative diseases. One prominent example is Huntington’s disease (HD), which is an autosomal dominant neurodegenerative disorder caused by an extension of the CAG repeat region in the N-terminus of the huntingtin gene. Though it is clear that wild-type huntingtin plays a role in intracellular trafficking by interacting with motor and motor-associated proteins, it is unclear how post-translational modifications (PTMs) modulate the activity of motor proteins and the specific biophysical mechanism behind this activity. In HD, mutated huntingtin aggregates in neurons, disrupting cargo transport. Disruption in vesicle transport results in abnormal cell signaling and impaired clearance of damaged proteins and organelles. This suggests a mechanism where defects in transport are due to misregulation of motor protein activity by vesicle-bound huntingtin.In addition to scaffolding molecules like huntingtin, transport is also regulated by microtubule-associated proteins (MAPs) that bind along the microtubule surface. Tau is a neuronal MAP that stabilizes axonal microtubules by crosslinking them into bundles. It also indirectly modulates cargo motility by serving as a roadblock to motors. Misregulation of tau leads to a range of neurodegenerative diseases known as tauopathies, including Alzheimer’s disease (AD). In AD, tau hyperphosphorylation leads to the development of fibrillary bodies that aggregate to form neurofibrillary tangles (NFTs). Varying concentration of tau inhibits single motor and multiple motor activity. Yet, despite the presence of tau in healthy neurons, teams of motors are still able to navigate efficiently over long distances in the axon without being inhibited. Similar to tau, MAP7 (ensconsin) also binds along microtubules and stabilizes them. It organizes the microtubule cytoskeleton in mitosis and neuronal branching. MAP7 not only promotes the interaction of kinesin-1 to microtubules but also competes with tau for binding along microtubules to regulate kinesin transport. Since bidirectional motility is a hallmark of intracellular transport, understanding how MAPs regulate teams of kinesin and dynein motors will provide insight into how transport is regulated"--