Evolution of the modern eukaryotic cell’s transport machinery


07 Oct 2016

Evolution of the modern eukaryotic cell’s transport machinery

Evolution of the modern eukaryotic cell’s transport machinery

Dr Mukund Thattai, Intermediate Fellow

National Institute of Biological Sciences (NCBS), Bangalore

By Anusha Krishnan, Research  Communications officer, NCBS, Bangalore

The inside of a present day plant or animal cell quite closely resembles a busy city. Like an urban metropolis with different districts interlinked by a traffic network, a cell has distinct compartments connected to each other by a dynamic transport system.

One set of such interlinked compartments – the Golgi complex – is essential for many cellular functions, and a question that has long puzzled scientists is: how did such a complex compartment and traffic system arise within a cell?

Scientists from the National Centre for Biological Sciences have a possible answer to this question through a mathematical approach to explore how such organisation could have evolved. Somya Mani and Mukund Thattai from the Simons Centre at NCBS have shown that the Golgi complex with its attendant traffic system can emerge spontaneously from a simple model with no need for a special selection mechanism.

Within cells, the Golgi complex is a set of compartments that is essential for processing, packaging and transporting proteins and other molecules. A key characteristic of the Golgi is its organisation as a ‘maturation chain’ with different compartments having variable molecular compositions. These compartments perform different processing and packaging functions, especially in the synthesis and transport of giant proteins like collagen.

The different compartments of the Golgi complex are connected to each other and to other cellular areas via mobile membrane-bound chambers called vesicles. Vesicles constantly bud off or fuse with compartments, forming a cell-wide transport system for different types of molecules.

In order to investigate the origins of the Golgi complex, Mani and Thattai simulated this traffic system. Built on broad and simple rules, they modelled the stream of vesicles budding out of source compartments and fusing with target compartments within a cell. These events were specified by budding and fusion matrices to create a collection of simulated cellular traffic networks that had settled into a state of equilibrium.

Now, Mani and Thattai did something unconventional – they filled up the budding and fusion matrices at random. Therefore, budding and fusion events were random, with no specific purpose guiding these events.

But then, they got an astonishing result. In roughly 25% of their simulations, the researchers came across traffic networks that had developed distinct patterns closely resembling those of a Golgi complex. This means that despite the lack of a selection mechanism for budding or fusion, a vesicular traffic network in a cell could give rise to a functional ‘maturation chain’ of compartments purely by chance.

In other words, Mani and Thattai’s work shows that the evolution of the Golgi complex is likely to have been non-adaptive – no selection system need have pushed cells to develop a Golgi complex.

The scientists are now planning to use their model to study infectious systems like tuberculosis and HIV, which are caused by intracellular parasites that hijack a cell’s vesicular traffic system for their own use.

Read the full press release on the NCBS website

 

Stacking the odds for Golgi cisternal maturation. Somya Mani and Mukund Thattai. elife