Functional characterization of 14-3-3 proteins in brassinosteroid signaling

14-3-3 phospho-binding proteins have been shown to be involved in signal transduction mechanisms in all life kingdoms. In Arabidopsis, thirteen 14-3-3 isoforms are expressed. They play a role in brassinosteroids (BRs) signaling pathway as well as in a plethora of different cellular mechanisms in response to various stimuli. Brassinosteroids are sev- eral related biomolecules acting as a phytohormones, mainly involved in plant growth and development. It is known that 14-3-3 proteins physically interact with several pro- teins involved in plants BR signaling. Amongst 14-3-3s interacting partners involved in BR signal transduction, BRASSINAZOLE-RESISTANT1 (BZR1) and BRI1-KINASE- INHIBITOR (BKI1) were shown to interact in vivo with 14-3-3s. However, the exact molecular interaction is still in investigation. Furthermore, the functional specificity of 14-3-3s isoforms remains an open question in BR signaling. The aim of this master thesis was firstly to characterize those interactions biochemically and structurally and secondly to decipher 14-3-3 isoforms function. Peptides of BZR1 and BKI1 have been used in this work to assess their binding affinity with 14-3-3s. I have shown that minimal BZR1 and BKI1 peptides bind to several 14-3-3 isoforms in a similar manner. Next, I presented the atomic structure of a 14-3-3 in complex with the BZR1 peptide. The comparison of this atomic structure and a human 14-3-3 bound to a ligand peptide show that the molecular 14-3-3 binding mechanisms are conserved between the two kingdoms. Finally, to assess the genetic role of 14-3-3s in BR signaling, I analyzed the phenotypes of sev- eral higher-order 14-3-3 knock out plants. Surprisingly, 14-3-3 knock outs led to either a gain-of-function or a loss-of-function phenotype in BR-dependent hypocotyl growth assays. This work have shown that some 14-3-3 isoforms differently act in the BR signaling pathway and that all 14-3-3s bind to minimal conserved motifs in a similar manner from plants to mammals