Our lab focuses on how tiny vehicles called molecular motors function inside our cells during mitosis. Mitosis, the vital process of cell division that produces two identical cells from the mother cell, is required for embryonic development, stem cell regeneration, tissue repair and to maintain general homeostasis in our body. The molecular motor “dynein” is known to play a crucial role during mitosis by ferrying mitosis-specific molecular cargoes towards the negative ends of molecular tracks in the cell called microtubules. Interestingly, during the non-dividing phase of interphase, dynein mainly carries membrane-enclosed cargoes on microtubule tracks, but switches cargoes dramatically while entering mitosis (fig.1). That makes dynein a virtual “super motor” of the cellular system.
Dynein is known to undergo a specific chemical modification named phosphorylation at the beginning of mitosis. Out of four dynein protein components, the Light Intermediate Chain 1 (LIC1) subunit undergoes the maximum amount of phosphorylation. The new phosphorylation tag is essential to erase dynein’s earlier identity of an “interphase motor” by making it shed off its interphase cargoes and bind to its new mitotic cargoes. We were very curious to understand how putting a chemical tag on a subunit confers the dynein motor the new acquired ability to bind to new cargoes.
The three-dimensional structure of the LIC1 protein consists of a compactly folded N-terminal domain, but a largely unstructured and “floppy” C-terminal region. The flexible C-terminal domain contains many phosphorylation sites; imagine a fairy light wire (LIC1 amino acid stretch) rolled into a ball at one end, while the other is loose and hangs free. The loose end of the wire has many “small switching bulbs (phosphorylation sites)” clustered in a small stretch. The location of these phospho-switches in the flexible region makes them easily accessible to molecular signals in the cell. We discovered that switching off three specific phosphorylation sites on this C-terminus of LIC1 during mitosis results in various cell division defects that are signatures of cancerous cells.
We also wanted to know whether dynein is able to bind to its mitotic cargoes in the absence of phosphorylation. A major way dynein engages with diverse cargoes is by binding to different “adaptor proteins”, each with their signature “protein binding sockets” where matching cargoes can fit for dynein to carry. We wondered how phosphorylation helps dynein choose among different adaptors in dividing cells. We discovered that one local population of phosphorylated dynein interacts with a particular adaptor named spindly present on the chromosomes, enabling this fraction of dynein to ferry a specific set of cargoes from the center of the cell to the two opposite poles of the dividing cell. However, another population of phosphorylated LIC1 attracts the interesting isomerase enzyme Pin1, which recognizes these phosphorylated regions and subsequently “twists” the shape of the protein at adjacent regions, resulting in various downstream functional consequences. In the case of dynein, this interaction changes the conformation of this motor protein and increases its affinity for a different mitotic adaptor, Hook2. Now this population of dynein is directed towards a specific cellular location (spindles or poles of dividing cells) and participates in ferrying a separate set of cargoes. In essence, the pattern and intensity of the turned-on bulbs (phospho sites) on the ferry light (LIC1 protein) directs dynein to choose the right cargo at the right time (fig.1).
Our research reveals some of the intricacies underlying the design of energy-efficient molecular super motors inside a cell. Cells do not spend extra energy in making more proteins to meet the new workload during mitosis. Rather, they temporarily add specific chemical tags (such as phosphorylation) to the existing proteins to modulate their shapes and local surface chemistries, eventually providing layers of functional regulation. Imagine each phosphorylation site as a small bulb in the long fairy light wire (amino acid stretch of protein), with thousands of such wires present in the cell. They can light up during mitosis so as to produce different lighting patterns with different intensities (waves, flickers, pulses) in space and time to produce the desired output in the cell. Isn’t that super smart! This journey certainly gave us a sense of the efficient utilization of resources, which is indispensable to every aspect of life on this planet.
