Regulates chlorophyll synthesis and plastid-to-nucleus signal transduction by binding both the product and the substrate of Mg-chelatase, an enzyme that produces magnesium-protoporphyrin IX (Mg-Proto). Activates also Mg-chelatase. Neither binds abscisic acid (ABA) nor is involved in ABA signaling. Interacts with CHLH, the protoporphyrin IX-binding subunit of Mg-chelatase. Monomer or extremely compact dimer. Might be bound to membranes by specific protein-protein interactions. Up-regulated by light.