Calmodulin as a Ca2+-Sensing Subunit of Arabidopsis Cyclic Nucleotide-Gated Channel Complexes

Ca2+ serves as a universal second messenger in eukaryotic signaling pathways, and the spatial and temporal patterns of Ca2+ concentration changes are determined by feedback and feed-forward regulation of the involved transport proteins. Cyclic nucleotide-gated channels (CNGCs) are Ca2+-permeable channels that interact with the ubiquitous Ca2+ sensor calmodulin (CaM). CNGCs interact with CaMs via diverse CaM-binding sites, including an IQ-motif, which has been identified in the C-termini of CNGC20 and CNGC12. Here we present a family-wide analysis of the IQ-motif from all 20 Arabidopsis CNGC isoforms. While most of their IQ-peptides interacted with conserved CaMs in yeast, some were unable to do so, despite high sequence conservation across the family. We showed that the CaM binding ability of the IQ-motif is highly dependent on its proximal and distal vicinity. We determined that two alanine residues positioned N-terminal to the core IQ-sequence play a significant role in CaM binding, and identified a polymorphism at this site that promoted or inhibited CaM binding in yeast. Through detailed biophysical analysis of the CNGC2 IQ-motif, we found that this polymorphism specifically affected the Ca2+-independent interactions with the C-lobe of CaM. This same polymorphism partially suppressed the induction of programmed cell death by CNGC11/12 in planta. Our work expands the model of CNGC regulation, and posits that the C-lobe of apo-CaM is permanently associated with the channel at the N-terminal part of the IQ-domain. This mode allows CaM to function as a Ca2+-sensing regulatory subunit of the channel complex, providing a mechanism by which Ca2+ signals may be fine-tuned.