CHARACTERIZING THE R2R3 S21 AND S23 MYB TRANSCRIPTION FACTORS IN ARABIDOPSIS THALIANA

Chase Beathard

CHARACTERIZING THE R2R3 S21 AND S23 MYB TRANSCRIPTION FACTORS IN ARABIDOPSIS THALIANA Abstract By Chase Montgomery Beathard, Ph.D. Washington State University June 2019 Chair: Hanjo A. Hellmann The ubiquitin (UBQ) proteasome pathway is a highly conserved mechanism by which plants respond to their environments. The pathway proceeds by an E1-E2-E3 enzymatic cascade that targets proteins with UBQs to signal for their degradation via the 26S proteasome. The E3 ligase component provides specificity to the pathway and physically assembles with a substrate protein. The interaction between the E3 ligase and the substrate is facilitated by a substrate adaptor protein. One type of substrate adaptor that is utilized by CULLIN3-based (CRL3) E3 ligases is BRIC-A-BRAC, TRAM-TRACK AND BROAD COMPLEX (BTB)/POX VIRUS AND ZINC FINGER (POZ)/ MEPRIN AND TUMOR NECROSIS FACTOR RECEPTOR ASSOCIATED FACTOR HOMOLOGY (MATH), or BPM protein. BPM proteins target transcription factors from families including MYELOBLASTOSIS (MYB), HOMEOBOX LEUCINE ZIPPER (HD-ZIP), and ETHYLENE RESPONSE FACTOR/APETALA2 (ERF/AP2) proteins. Two regions have been identified that facilitate a substrate’s interaction with BPMs, a SPOP binding consensus (SBC) domain and a PEST motif that is also associated with protein instability. This work demonstrates that a clade of R2R3 S21 MYB transcription factors, MYB52, MYB54, MYB56, and MYB69, interact with a BPM substrate adaptor and are instable to varying degrees. Moreover, if the MYB56 protein loses its PEST motif, it becomes more stable. We also find that the R2R3 S23 MYB transcription factors, MYB1, MYB25, and MYB109 are instable targets of the UBQ proteasome pathway. The overexpression of MYB25 and MYB109 confers hypersensitivity to salt and an associated stress hormone, abscisic acid (ABA). Transcriptomic data further support the role of MYB25 and MYB109 in salt and ABA response and also show involvement of MYB25 and MYB109 in other cellular processes including defense, light signaling, and general stress response. Collectively, these results broaden the knowledge of R2R3 MYBs that are targeted by CRL3BPM. We provide a first characterization of MYB1, MYB25, MYB52, MYB54, and MYB109 regarding their instability and interaction with BPMs, and further support the PEST motif’s role in protein stability and interaction with CRL3BPM. We show a first phenotypic characterization of MYB25 and MYB109, and provide insight into the broader transcriptomic roles that MYB25 and MYB109 play in plants.

CHARACTERIZING THE R2R3 S21 AND S23 MYB TRANSCRIPTION FACTORS IN ARABIDOPSIS THALIANA

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