Required for the expression of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCo) (PubMed:18506097). ATPase involved in the activation of red-type RuBisCo, which tends to form inactive complexes with its substrate ribulose 1,5-bisphosphate (RuBP) (PubMed:27872295). Catalyzes the release of RuBP from inhibited RuBisCo in an ATP-dependent manner (PubMed:27872295). Activation of RuBisCO involves the ATP-dependent carboxylation of the epsilon-amino group of lysine leading to a carbamate structure (PubMed:27872295). The nuclear-encoded subunit plays a more critical role in activase function than the plastidial-encoded subunit (PubMed:27872295). Forms homooligomers (PubMed:27872295). Forms heterohexameric rings with the nuclear-encoded Rca subunit consisting of 3 of each nuclear- and plastidial-encoded subunits that alternate in the ring (PubMed:27872295). Belongs to the CbxX/CfxQ family. Extended N-terminus.