Kien Lam Ung
Department of Molecular Biology and Genetics, Aarhus University, Aarhus, Denmark
Catching transport in motion and unraveling the molecular mechanism of glycan biogenesis
Auxin is a major class of plant growth hormone that orchestrates almost all developmental steps and growth in plants. Auxin response is mediated by an auxin gradient across the plant tissues. The secondary transporter PIN-FORMED (PIN) proteins are at the heart of this process as they govern the auxin export from the cytosol to the extracellular space[1]. For decades, the lack of structural and biochemical data has impeded a detailed comprehension of the molecular mechanism behind PIN-mediated auxin and herbicides transport. In our discoveries in 2022, we solved a series of PINs cryo-EM structures and showed that PIN8, in the homodimer form, is highly dynamic and undergoes large conformational changes allowing the transport of auxin using an elevator mechanism. This transport is independent of ion and proton gradients and our data supports a transport mechanism driven by the charge of auxin[2,3].
Modern cryo-EM focuses on understanding the mechanism, conformational landscape, and protein dynamics during the reaction. Deciphering these molecular details can lead to the design of specific inhibitors or drugs that target conformations of interest. However, these transient conformations have a very short lifetime and are often difficult to achieve with standard cryo-EM methods and inaccessible with Alphafold. In this presentation, I will share our latest results and proof-of-concept on how to structurally capture the complete transport cycle of the transporter in a single sample. This work represents the first example of a secondary active transporter determined under fully active turnover conditions. These structural data allow us to pinpoint key structural and chemical elements of PIN and auxins that control affinity and specificity. This knowledge may be used to engineer herbicide-resistant crops and develop novel, environmentally safer herbicides for sustainable agriculture (unpublished).
References
[1] Ulrich Z. Hammes, Angus S Murphy, Claus Schwechheimer. Auxin Transporters - A biochemical view. Cold Spring Harb. Perspect. Biol. (2022)
[2] Ung KL*, Winkler M*, Schulz L, Kolb M, Janacek DP, Dedic E, Stokes DL, Hammes UZ & Pedersen BP. Structures and mechanism of the plant PIN-FORMED auxin transporter. Nature (2022)
[3] Ung KL, Schulz L, Stokes DL, Hammes UZ, and Pedersen BP. (2023). Structures and mechanism of the plant PIN-FORMED auxin transporter. Trends in Biochemical Sciences (2023)