Fellows' research: Hook2, a crucial regulator of cell division in mammalian cells
20 Feb 2019
Dr Mahak Sharma, Intermediate Fellow
Indian Institute of Science Education and Research, Mohali
In a recently published study, we characterized a protein that regulates different stages of the mammalian cell cycle and division of the cell into two daughter cells. Many of the cellular proteins that regulate the cell cycle, such as the one reported in our study, are mutated in cancer cells, resulting in uncontrolled cell division.
Dynein is a highly conserved protein complex found in multiple distantly related species that functions as a molecular machine, producing force to move cellular cargo from one location to another. Dynein is also essential for cell division, as it regulates different steps of the process, including, the breakdown of the nuclear envelope, proper positioning of the chromosomes, and separation of the chromosomes into two daughter cells. Notably, mammalian dynein cannot function by itself and requires the help of an activator, dynactin. In the last two years or so, few linker proteins have been identified in mammalian cells that promote association of dynein and dynactin and therefore, regulate the force production by dynein. We characterized one such linker protein—Hook2—that binds to dynein and dynactin and regulates the function of dynein during cell division.
In this study, we investigated if Hook2 functions as a linker between dynein and dynactin. Additionally, we tried to understand the subcellular function of Hook2. The prior information on localization of Hook2 on a structure known as “centrosome”, which regulates the cell division cycle, was important in building the hypothesis about Hook2 subcellular function.
We investigated the binding of dynein and dynactin in mammalian cells that were depleted of Hook2 protein. Interestingly, these experiments revealed that Hook2 regulates dynein-dynactin association in an actively dividing mammalian cell. In collaboration with Sivaram Mylavarapu’s lab at Regional Centre for Biotechnology, Faridabad, we did live imaging of dividing cells. The images revealed that one of the major roles of Hook2 was regulating dynein-dependent centrosome anchoring to the nuclear envelope before entry into cell division.
Hook2 also regulated a key function of centrosome—generating the microtubule tracks on which the dynein-dynactin complex moves cellular cargo. As observed in dynein-depleted cells, chromosome (cargo) separation was not efficient in Hook2-depleted cells. Unexpectedly, we also found that Hook2-depleted cells failed to divide into two daughter cells. This was attributed to Hook2- and dynactin-dependent formation of a cell membrane bridge that separates the two daughter cells.
Previous studies have reported chromosomal translocations of Hook2 gene locus in cancers; however, the reasons for this were not understood until date. Our study identifies the role of Hook2 as a crucial regulator of dynein function and cell division; this explains how a mutation in Hook2 gene could result in the cell cycle abnormalities associated with cancer cells.
Striking co-localization of Hook2 (green) with dynactin (red) on microtubule tracks in HeLa cells overexpressing both proteins
The dynein adaptor Hook2 plays essential roles in mitotic progression and cytokinesis. Devashish Dwivedi, Amrita Kumari, Siddhi Rathi, Sivaram V.S. Mylavarapu, Mahak Sharma. Journal of Cell Biology. January 2019
Image Credits: Devashish Dwivedi, Department of Biological Sciences, Indian Institute of Science Education and Research, Mohali