Our story begins in the ocean...the deepest parts of the ocean. Light does not penetrate this frigid landscape of perpetual night. Miles beneath the surface of the ocean, in this inhospitable environment or crushing pressures and low temperatures, life still thrives. From fissures in the Earth's crust, molten lava and gases billow out a toxic mixture of minerals and metals in a superheated plume, reaching temperatures that can exceed 400 degrees centigrade. Bacteria called thermophiles and hyperthermophiles feed on the chemically rich effluent and are the foundation for the complex ecosystem that has evolved in this alien environment.
Image is from NOAA.
Thermophiles belong to a class of organisms (dominated by bacteria and archaebacteria) known as 'extremophiles.' These organisms live at the fringes of environments that are so extreme -- think super-hot, super-acidic, or super-salty -- that scientists were surprised to discover that life could thrive. But, like all life forms on Earth, these extremophiles are built of the same building blocks that you are -- sugars, proteins, and lipids. How is it that they can be so robust in these harsh environments? What particular mechanisms have they evolved? Above is shown an image of Deinococcus radiodurans, a polyextremophile that is able to survive radiation bombardment, cold, dehydration, vacuum, and acid.
Image is from the Public Library of Science Collection. http://tolweb.org/Deinococcus-Thermus/2288
Interactions between all the proteins in a living cell are known as the 'protein interactome,' and they provide a glimpse into the abstract architecture of molecular interactions that underly the emergent properties of life. Critical proteins can be identified that form highly connected hubs, functional networks can be mapped to specific cellular functions, and evolutionary relationships between organisms can be inferred. What can the interactomes of hyperthermophilic organisms tell us about how these unique organisms are able to adapt to such an inhospitable environment? Are the fundamental properties of the thermophilic interactions (connectivity, number of hubs, etc.) different than mesophiles?
To properly probe protein-protein interactions (PPIs) in hyperthermophiles, a biosensor for detecting these binary interactions needs to be developed that works in vivo in these organisms. To pursue this strategy, I focused on split protein switches. These class of protein-based biosensors are composed of proteins that are split into two or more fragments. In a properly designed switch, these fragments are able to reconstitute the original protein function only when they are fused to other proteins that interact, thus drawing them into proximity. In my first paper, I demonstrated that when designing such protein switches, factoring in the thermostability of the proteins should be considered. More thermostable proteins result in fragments that can spontaneously reassociate (presumably due to greater residual structure) without any need for fusion to interacting proteins.
In my second manuscript, I developed a split-adenylate kinase protein switch that functions in the highly thermophilic Thermus thermophilus HB8. I developed an adenylate kinase knock-out mutant of HB8 for these studies. The thermophilic protein switch worked perfectly. When fused to three chemotaxis proteins from Thermotoga maritima MSB8, the split protein switch results in binary protein-protein interactions of these proteins that is expected from the known conserved interactions of these three proteins.