Studies on the interactions of AFPs and bZIP transcription factor ABI5.

AFP1 interacts with ABI5 and negatively regulates the abscisic acid (ABA) signaling by accelerating ABI5's degradation during the seed germination phase in Arabidopsis, but the underlying mechanism remains unclear. Moreover, the molecular basis of the interaction between AFP1 homologs and ABI5 has yet to be elucidated. In this study, the patterns of their interactions with ABI5 were investigated in detail. We found that AFP2/3/4 can bind two regions of ABI5, one is ABI5 and another is ABI5. However, AFP1 only interacts with the second region of ABI5, i.e. ABI5. Prior research has shown that ABI5 is related to the transcriptional activity of ABI5. Thus, our results suggest that AFPs may also modulate ABI5, by directly binding to its transcriptional activation domain, thereby influencing its transcriptional activity. Further, interactions between AFPs and ABI5 were not affected if the Ser in the ABI5-SnRK2 motif were mutated respectively to Glu or Ala. Nevertheless, interactions between AFPs and ABI5 were eliminated if the Thr and Thr of ABI5 were mutated respectively to Glu or Ala. Since the two residues of Thr and Thr were located in the phosphorylation motifs of CKII, AFPs might regulate the activities of ABI5 transcription factor through a CKII-dependent pathway.