The SMAX1-LIKE (SMXL) protein family is distantly related to heat-shock proteins. However, these proteins are more likely to function as transcriptional regulators. We have used a structure-function analysis to determine what the different domains of SMXL proteins do. We now know which domains are responsible for nuclear localization, interaction with the receptors KAI2 and D14, proteolysis, interaction with TOPLESS family transcriptional co-repressors, and, putatively, interactions with other SMXL proteins. The “output” domain(s) of SMXL proteins, which presumably enables SMAX1 to control seed germination and seedling growth, or SMXL7 to control shoot branching, has remained mysterious, however.
We are investigating the output domain(s) of SMAX1, which will then be used as a foundation to identify the specific genomic targets of SMAX1 and protein partners that mediate developmental control (e.g. transcription factors). A comparison of the output domain(s) of different SMXL proteins may reveal how their ability to control specific developmental processes evolved in land plants.
For further reading, see
Khosla A, Morffy N, Li Q, Faure L, Chang SH, Yao J, Zheng J, †Cai ML, Stanga J, Flematti GR, Waters MT, Nelson DC. (2020) “Structure–Function Analysis of SMAX1 Reveals Domains That Mediate Its Karrikin-Induced Proteolysis and Interaction with the Receptor KAI2” The Plant Cell. 32(8):2639-2659.