Functional Crypto-Adenylate Cyclases Operate in Complex Plant Proteins.

Adenylyl cyclases (ACs) and their catalytic product cAMP are regulatory components of many plant responses. Here, we show that an amino acid search motif based on annotated adenylate cyclases (ACs) identifies 12 unique $\textit{Arabidopsis thaliana}$ candidate ACs, four of which have a role in the biosynthesis of the stress hormone abscisic acid (ABA). One of these, the 9-cis-epoxycarotenoid dioxygenase (NCED3 and At3g14440), was identified by sequence and structural analysis as a putative AC and then tested experimentally with two different methods. Given that the $\textit{in vitro}$ activity is low (fmoles cAMP pmol$^{-1}$ protein min$^{-1}$), but highly reproducible, we term the enzyme a crypto-AC. Our results are consistent with a role for ACs with low activities in multi-domain moonlighting proteins that have at least one other distinct molecular function, such as catalysis or ion channel activation. We propose that crypto-ACs be examined from the perspective that considers their low activities as an innate feature of regulatory ACs embedded within multi-domain moonlighting proteins. It is therefore conceivable that crypto-ACs form integral components of complex plant proteins participating in intra-molecular regulatory mechanisms, and in this case, potentially linking cAMP to ABA synthesis.