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Guardians of the Genome

“It has long been an axiom of mine that the little things are infinitely the most important.”

- Arthur Conan Doyle

A tiny cell inside a human body is an amazingly complex entity, bustling with activity. The events within a cell are coordinated with remarkable accuracy and precision. Within the cell, lies the nucleus—the abode of our DNA. If you were to extract the entire DNA from one nucleus and stretch it out, it would occupy a length of 1m. A one metre long DNA resides in the nucleus that measures only 10um in diameter! How is that possible? The trick is to package the DNA into shorter thread like structures called chromosomes.

Prof. Shikha Laloraya’s lab at the Indian Institute of Science is actively engaged in studying the structural organisation of chromosomes within the nucleus and how this affects various chromosomal transactions. During cell division, DNA must be unwound to facilitate DNA replication; the replicated DNA is then equally segregated to the daughter cells. As cells enter the cell division cycle, special proteins called SMC (Structural Maintenance of Chromosomes) proteins bring about structural reorganization of DNA—such that it gets folded into shorter compact structures that are less vulnerable to damage when the complex cellular replication and segregation machines are active.

When asked about the motivation behind her research, Prof. Laloraya says, “Chromosomes have intrigued me throughout my career. Basic research on chromosomes and cell division is very important because an error in their maintenance or regulation can present serious consequences for a cell or organism.”

Within the nucleus, surveillance systems constantly monitor, alert, repair any damage and facilitate error-free cell division. In one of their studies, the researchers found an indispensable role of a subunit of the Smc 5/6 protein complex in maintenance of chromosome stability.

The SMC protein complexes are a conserved family of proteins involved in coordinating chromosomal organisation and DNA repair. The Smc 5/6 protein complex contains Smc 5, Smc 6 and six non-smc subunits. One of the non-Smc subunits of Smc5/6: Mms21/Nse2 has the additional ability to modify other proteins. It attaches SUMO, a Small Ubiquitin related modifier polypeptide, which is conjugated covalently to lysine side-chains of target proteins. This activity is facilitated by the presence of a SUMO ligase domain on Mms21/Nse2. The SUMO ligase domain has been previously shown to be non-essential for the survival and growth of Schizosaccharomyces pombe (a yeast model organism) and other cells, under normal conditions.

A study conducted by Prof. Laloraya’s lab refutes this claim. Using the budding yeast Saccharomyces cerevisiae as a model system, the researchers found that the Mms21-SUMO ligase domain is required for maintaining chromosomal integrity and preventing DNA damage during normal growth of the yeast. In S. cerevisiae mutants lacking a functional SUMO ligase domain, the researchers observed reduced cell growth and excessive DNA damage. To further explore the members of surveillance system that were activated by DNA damage, the researchers treated Mms-21 SUMO ligase mutant cells with caffeine. Their studies on the effects of caffeine had established that it inhibits the activity of Mec1- kinase, a signalling protein involved in the DNA damage checkpoint. Addition of caffeine to the mutant cells was sufficient to cause cell death. This suggests that Mec1 kinase-dependent checkpoint pathway is the key to survival of SUMO ligase defective mms21 mutant cells. The researchers also found a requirement for Mms21-mediated sumoylation in cell cycle progression especially following conditions of stress. A functional SUMO ligase domain is therefore essential for maintenance of genomic integrity and proper progression of events in the mitotic cell cycle.

The surveillance system of the cell consists of numerous protein molecules residing in the nucleus that are vigilant during normal conditions and even more active during stress. Post-translational modifications of the proteins comprising the surveillance machinery provide a way to regulate its function rapidly by attaching tags that modify the biochemical properties of the target protein- the exact consequence of attaching SUMO to DNA repair proteins remains to be elucidated. Researchers like Prof. Laloraya are playing a key role to further our understanding on the functioning of these little tags, which can be aptly called the ‘Guardians of the genome’.

About the Lab

Shikha Laloraya lab: http://biochem.iisc.ernet.in/shikhal.php