The paper in Nature Communications is here: https://go.nature.com/2I3Cxd0
The fungus U. maydis causes smut disease of maize, an ugly looking plant tumors that the fungus proliferates and produces massive amounts of spores. U. maydis establishes a biotrophic interaction with maize in which it relies on living host tissue to obtain nutrients for proliferation and completion of its sexual cycle. To cause disease, U. maydis secretes more than 400 proteins during colonization. Many of these are novel and contribute to virulence. These so called effectors proteins can be viewed as individual soldiers with specific tasks. At the same time they play as a team with the joint mission to conquer the host and get the resources without causing too much damage. From the position of the host in the end, it is extremely important to know the enemy and the tasks of individual soldiers to understand how they hack into the system.
To generate this knowledge, we tackle the individual effectors by tracking their location, by finding out what they are doing molecularly, and how they evolve in the arms race with their host targets. Elucidating their molecular function will provide new ideas for the prevention and control of fungal plant diseases.
The U. maydis effector Rsp3 was selected for study because of its unique repeat-domain organization and strong virulence role. The investigation was a long journey filled with repeating periods of excitement, frustration, happiness and depression. However, it payed off at the end of the journey.
Part of the frustration I remember vividly started after the maize apoplastic proteins (AFP1 and AFP2) were identified as targets of Rsp3. I tried to demonstrate the presumed antifungal activity of recombinant AFP1 proteins. The recombinant proteins heterologously produced in E. coli system in large amounts and in soluble form failed to show anti-fungal activity. My confidence level was completely down after testing unsuccessfully different forms of the fungus for susceptibility and trying out numerous purification schemes. Since this was one of the key experiments in my project I needed to find a solution. AFP1 and AFP2 proteins are cysteine-rich, one possibility for this failure was that the proteins were not properly folded. After countless frustrated days and nights, I decided to give up expressing them in E. coli and switched to a transient eukaryotic expression system, the plant Nicotiana benthamiana. Although I could purify only small amounts, the activity test gave me one of the most exciting moments in life - I finally observed antifungal activity of AFP1. The take home message is– never give up, just keep going!
There are still lots of open questions in the Rsp3 story. I would rather say it is just the beginning of the story. Rsp3 shows strong length polymorphism in the field isolates. We still have not answered why it is important to molecules of different lengths in natural fungal populations and how it evolved. Rsp3 shields fungal hyphae and protects against the anti-fungal activity of AFP proteins. Is there a specific binding substrate of Rsp3 on the fungal hyphae? What is the mechanism of AFP action? Will AFPs action require binding partners in the fungal cell wall? Based on my finding that the Rsp3 orthologue from Sporisorium reilianum can fully complement the U. maydis rsp3 mutant, the protection mechanism of Rsp3 must be conserved in smut fungi. Nevertheless, their repeat-containing domains which interact with AFP proteins share only 20% amino acid identity. This raises the intriguing question which minimal elements are recognized by AFP proteins and whether recognition involves a specific sequence or structure. It is going to take many sleepless nights to answer all of this. However, I believe that it is worth every single one for the exciting moments ahead.