Deciphering the molecular rules of evolutionary diversification of Intraflagellar Transport

• Applicant: Dr. Zeynep Ökten
• Subject Area: Biochemistry ; Cell Biology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 520475795

Cilia (aka flagella) are ubiquitous organelles that project from the surface of most eukaryotic cells, including the cell types in the human body. Virtually all cilia require the non-membrane bound IntraFlagellar Transport (IFT) for their construction and function. IFT is a continuous transport of ciliary building blocks by the so-called IFT trains that are powered by the kinesin-2 and dynein-2 motors. A hallmark of IFT is the site-specific and mutually exclusive activation and deactivation of the two oppositely directed motors at the ciliary base and tip. It is becoming increasingly clear that a hierarchical assembly of the IFT trains enforces the strictly ordered events of the IFT process. At the ciliary base, large multi mega-Dalton IFT trains are assembled starting with the IFT-B complex. This IFT-B complex constitutes the backbone that scaffolds the IFT-A complex and the dynein-2 motor. In the last step of the assembly, the kinesin-2 motor is recruited to move the trains towards the ciliary tip. Whereas IFT-B and IFT-A complexes have been linked to the kinesin-2 and dynein-2 activities in vivo, their specific contributions to the regulation of IFT motors remain largely unknown at the molecular level. It also became clear that the composition of the respective IFT-B and IFT-A complexes are species-specific and highly diverse. The latter provokes the question of how the various organisms deploy the respective IFT subunits to orchestrate the orderly steps of the IFT process in vivo. Here, we investigate and contrast IFT of the Chromalveolates T. thermophila and T. pseudonana. Despite being members of the same evolutionary super group, these organisms deploy an astonishingly different set of IFT subunits and kinesin-2 motors, yet end up building essentially the same cellular ultrastructure, a motile cilium. T. pseudonana model is of particular interest as it has eliminated most IFT subunits but retained the ability to orchestrate the IFT process using only a subset of subunits from the IFT-B complex. This ‘minimal’ IFT model will likely teach us the limits of functional flexibility that can be coded into a specific set of proteins to create complex biological function. As already evident from our preliminary data, the kinesin-2 orthologs from the ‘minimal’ T. pseudonana and ‘canonical’ T. thermophila models display vastly different self-regulatory and kinetic properties. Yet all kinesin-2 motors are expected to obey the rules of the highly orderly steps of IFT. Delineating the molecular requirements of these key steps will expose the rules of linking the motors to specific IFT subunits that ultimately give rise to the strict chronology of events during the IFT process in the ‘minimal’ T. pseudonana and ‘canonical’ T. thermophila models, respectively. This proposal will bring us closer to derive the rules of establishing an orderly IFT process in a given organism based on the set of IFT subunits and the type(s) of kinesin-2 that it deploys.

• DFG Programme: Research Grants