Abscisic acid (ABA) signaling: finding novel components off the beaten track

The sesquiterpene abscisic acid (ABA) is an ancient stress response molecule. In plants, many ABA-dependent processes operate via PYR/PYL/RCAR receptor complexes, but results from several studies have suggested that not all plant responses function through this mechanism. Since the ABA-dependent processes of animals and humans also operate in the absence of such receptors, we hypothesize that plant and animal proteomes harbour proteins with undiscovered ABA-binding sites. We propose that carefully curated amino acid search motifs deduced from the binding sites of experimentally confirmed ABA-binding proteins can identify many more candidates in plant and animal proteomes. Some of these candidates show structural folds that are compatible with ABA-binding. This approach identifies plant candidates including annotated ABA downstream signaling components SnRK2.2 and SnRK2.6, and proteins involved in protein folding and RNA polyadenylation. The identified ABA-binding candidates in the human proteome affect among other processes, immune responses and tumor progression. If these candidates are eventually validated experimentally, it will imply that the regulation and tuning of ABA-dependent processes is considerably more complex than hitherto suspected. It will also help to clarify the role of this conserved signaling molecule in mammals.