A protein related to nitrogen PII proteins is conserved near α-carboxysome genes.
Crystal structures of CPII are elucidated with and without potential ligands.
ADP and AMP binding drive major conformational changes in the protein trimer.
Bicarbonate, a substrate of carboxysomes, enhances ADP and AMP binding.
A unique clade of PII proteins, herein named CPII, is regulated by inorganic carbon.