The transcriptional repressor activity of ASYMMETRIC LEAVES1 is inhibited by direct interaction with calmodulin in Arabidopsis

Calmodulin (CaM), a key Ca2+ sensor, regulates diverse cellular processes by modulating the activity of a variety of enzymes and proteins. However, little is known about the biological function of CaM in plant development. In this study, an ASYMMETRIC LEAVES1 (AS1) transcription factor was isolated as a CaM-binding protein. AS1 contains two putative CaM-binding domains (CaMBDs) at the N-terminus. Using domain mapping analysis, both predicted domains were identified as authentic Ca2+-dependent CaMBDs. We identified three hydrophobic amino acid residues for CaM binding, Trp49 in CaMBDI, and Trp81 and Phe103 in CaMBDII. The interactions of AS1 with CaM were verified in yeast and plant cells. Based on electrophoretic mobility shift assays, CaM inhibited the DNA-binding activity of the AS1/AS2 complex to two cis-regulatory motifs in the KNAT1 promoter. Furthermore, CaM relieved the suppression of KNAT1 transcription by AS1 not only in transient expression assays of protoplasts but also by the overexpression of a CaM-binding negative form of AS1 in as1 mutant plant. Our study suggests that CaM, a calcium sensor, can be involved in the transcriptional control of meristem cell-specific genes by the inhibition of AS1 under the condition of higher levels of Ca2+ in plants.