Role of the plant-specific calcium-binding C2-DOMAIN ABSCISIC ACID-RELATED (CAR) protein family in environmental signaling

Many signaling processes rely on information decoding at the plasma membrane, and membrane-associated proteins and their complexes are fundamental for regulating this process. Still many questions exist as to how protein complexes are assembled and function at membrane sites to change identity and dynamics of membrane systems. Peripheral membrane proteins containing a calcium and phospholipid-binding C2-domain can act in membrane-related signaling by providing a tethering function so that protein complexes form. C2 domain pro-teins termed C2-DOMAIN ABSCISIC ACID-RELATED (CAR) proteins are plant-specific, and the functional rele-vance of this C2 domain protein subgroup is just emerging. The ten Arabidopsis CAR proteins CAR1 to CAR10 have a single C2 domain with a plant-specific insertion, the so-called CAR-extra-signature or also termed sig domain. Via this sig domain CAR proteins can bind signaling protein complexes of different kinds and act in biotic and abiotic stress, blue light and iron nutrition. Interestingly, CAR proteins can oligomerize in membrane microdomains, and their presence in the nucleus can be linked with nuclear protein regulation. This shows that CAR proteins may play unprecedented roles in coordinating environmental responses and assembling required protein complexes to transmit information cues between plasma membrane and nucleus. The aim of this review is to summarize structure-function characteristics of the CAR protein family and assemble findings from CAR protein interactions and physiological functions. From this comparative investigation we extract common principles about the molecular operations that CAR proteins may fulfill in the cell. We also deduce functional properties of the CAR protein family based on its evolution and gene expression profiles. We highlight open questions and suggest novel avenues to prove and understand the functional networks and roles played by this protein family in plants.

Automated summary

Many membrane-associated proteins and complexes are essential for decoding information at the plasma membrane and regulating signaling processes. The plant-specific C2-DOMAIN ABSCISIC ACID-RELATED (CAR) proteins, which contain a calcium and phospholipid-binding C2-domain, play important roles in various signaling pathways including biotic and abiotic stress, blue light and iron nutrition. These proteins can oligomerize in membrane microdomains and their presence in the nucleus is associated with nuclear protein regulation, suggesting their unique roles in coordinating environmental responses and transmitting information cues between plasma membrane and nucleus.