Molecular motors are mechanochemical enzymes that use chemical energy to carry out various tasks inside the cell. Motors are involved in management of cytoskeleton shape, cell migration, and cell division. One of their most interesting and essential function is intracellular transport of cargo. These motors (dynein and kinesin) use microtubules as their tracks and transport various kinds of cargoes (organelles, RNA, etc). Dynein and kinesin walk in the opposite direction: dynein walks toward the minus end, while kinesin walks toward the plus end of microtubules. My previous work in Dr. Roop Mallik’s lab at Tata Institute of Fundamental Research, India, elegantly demonstrated how mechanochemical properties of kinesin and dynein motors relate to their ensemble functions on cellular cargo critical for bidirectional transport, receptor recycling and endosome maturation. Transport of cargo is regulated by tug-of-war between kinesin and dynein. Multiple dyneins are required against a single kinesin in this tug-of-war mechanism.
Kinesin motors are classified into 14-15 superfamilies. Motor domain of kinesin is highly conserved across these superfamilies. Motor domain binds to microtubules and generates force via hydrolysis of ATP, which drives the movement of kinesin along the microtubule track. On the other hand, tail domain is involved in dimerization and binding to cargo.
Why kinesin-3 Motors?
Kinesin-3 is one of the largest superfamilies consisting of KIF1, KIF13, KIF14, KIF16, and KIF28 subfamilies. Defects in kinesin-3 motors have implications in various illnesses, such as Alzheimer's, Huntington's, and also cancer diseases. and are responsible for wide variety of intracellular transport functions. Based on studies of one kinesin-3 motor, KIF1A in mammals and its homolog UNC-104 in elegans, three contradicting models have been put forth to describe the molecular mechanisms of kinesin-3 motor regulation and motility.
First, work on KIF1A led to the model that kinesin-3 motors are monomeric motors that drive cargo transport by diffusive motion along the microtubule surface. Despite recent evidence in favour of two alternative models, cargo-induced dimerization and dimeric motor, the model that kinesin-3 motors function as monomeric motors is still widely prevalent (see e.g. Hirokawa et al. Nat Rev Mol Cell Biol. 2009 10:877-884). My recent work from Prof. Kristen Verhey Lab on three kinesin-3 subfamilies (KIF1, KIF13, and KIF16) at the cellular and single molecule levels, we show that kinesin-3 motors do not function as monomeric motors. Importantly, we provide a unifying theme for the regulation and motility mechanism of kinesin-3 motors. Specifically, we provide the following significant advances for the field:
Kinesin-3 motors are the marathon runners of the motor world. We demonstrate for the first time that active kinesin-3 motors are highly processive with average run lengths of ~ 9 mm (kinesin-1 by comparison has a run length of ~ 1 mm). This processivity is higher than any other motor to date and suggests that kinesin-3 motors function as the marathon runners of the motor world, a feature that is particularly relevant for their function in long-distance fast axonal transport. This work opens up important new research avenues on the basic molecular and mechanical features that contribute to motor processivity.
The family-specific K-loop functions to increase motor-microtubule affinity and does not contribute to processivity. Although the kinesin motor domain is highly conserved, different kinesin families contain unique surface features whose functional outputs are largely unexamined. We show that the K-loop, a positively charged segment on the kinesin-3 motor surface, does not contribute to the processivity of dimeric kinesin-3 motors but rather provides the motor with a high affinity for the microtubule in its weakly bound state. These results clarify past results on the K-loop and provide one of the first systematic explorations of kinesin surface features.
A new mechanism of regulation for kinesin motors. Kinesin motors activity must be tightly regulated in cells to prevent futile ATP hydrolysis and clogging of microtubule tracks. Most kinesin motors studied to date are kept inactive by autoinhibition in the dimeric state. We show that kinesin-3 motors are regulated in a unique way that prevents dimerization (and thus processive motion) and we define two different mechanisms for preventing dimerization. First, we show that KIF1 and KIF16 motors utilize an intra-molecular interaction between the neck coil (NC) and coiled-coil 1 (CC1) segments to prevent motor dimerization. Second, we show that KIF13 motors contain a Proline-induced helical shift between the NC and CC1 segments that prevents motor dimerization.
Current Research in the Lab
Kinesin-3 family members are the major kinesins in neurons and their defects in transport system relate to various human diseases. In our lab, we continue to investigate kinesin-3 motors to address following scientifically broad and physiologically important questions in the field:
Determining the molecular adaptations that enable kinesin-3 motors superprocessivity.
Investigating the molecular mechanism that regulates kinesin-3 motor activity when motor not bound to cargo.
Gain fundamental insights into how the structural and mechanochemical features of kinesin-3 motors relate to their neuronal transport and functions.
How defects in cargo transport play a causal role in neurodegenerative, developmental, and cancer diseases in higher organisms? This will in turn aid in the development of new drugs and therapies to cure these diseases.