RuBisCO catalyzes two reactions: the carboxylation of D-ribulose 1,5-bisphosphate, the primary event in carbon dioxide fixation, as well as the oxidative fragmentation of the pentose substrate in the photorespiration process. Both reactions occur simultaneously and in competition at the same active site. Beta-carboxysome assembly initiates when soluble RuBisCO aggregates is condensed into a liquid matrix in a pre-carboxysome by the RbcS-like domains of probably both CcmM58 and CcmM35. CcmN interacts with the N-terminus of CcmM58, and then recruits the CcmK2 major shell protein via CcmN's encapsulation peptide. Shell formation requires CcmK proteins and CcmO. CcmL caps the otherwise elongated carboxysome. Once fully encapsulated carboxysomes are formed, they migrate within the cell probably via interactions with the cytoskeleton. Heterohexadecamer of 8 large and 8 small subunits (Probable). The CcmM short form purifies from carboxysomes in complex with both RuBisCO subunits; a second complex with full-length CcmM and RuBisCO also includes carbonic anhydrase (CA, ccaA). RuBisCO-CcmM complexes are probably associated with the carboxysome shell (PubMed:17675289). Isolated reduced and oxidized SSUL1 binds holo-RuBisCO (RbcL(8)-RbcS(8)) but not either subunit octamer alone; RuBisCO has a higher affinity for reduced SSUL1 (PubMed:30675061). In the carboxysome RuBisCO is organized into a paracrystalline array (PubMed:28616951). This cyanobacterium makes beta-type carboxysomes (PubMed:22928045). Carboxysome size and components vary with growth conditions. When grown in ambient air at medium light (50 uE meter(-2) second(-1)) there are 853 RuBisCO complexes (RbcL8S8) per carboxysome, the numbers decrease under low light and high CO(2), and increase under high light (at protein level). Heterologous expression of 12 carboxysomal genes in E.coli (ccaA, ccmK2, ccmK3, ccmK4, ccmL, ccmM, ccmN, ccmO, ccmP, rbcL, rbcS, rbcX) leads to the formation of bodies that resemble carboxysomes, have densely packed paracrystalline arrays and RuBisCO activity. These structures open the door to generating carboxysomes in plant cells to increase their photosynthesis and productivity, as well as tailoring bacterial microcompartments to specific metabolic needs and molecule delivery. RuBisCO folding and assembly commences when the nascent large subunit folds with the help of chaperonin GroEL-GroES. Both RbcX and Raf1 help folded RbcL release from the chaperonin and dimerize; dimeric Raf1 binds to RbcL(2) leading to an RbcL8-Raf1(8) complex. RbcS displaces Raf1, resulting in holoenzyme formation. The basic functional RuBisCO is composed of a large chain homodimer in a 'head-to-tail' conformation. In form I RuBisCO this homodimer is arranged in a barrel-like tetramer with the small subunits forming a tetrameric 'cap' on each end of the 'barrel'. Belongs to the RuBisCO small chain family.