is absolutely conserved among organisms that form beta-carboxysomes. Two CcmP pseudohexamers stack to form a nanocompartment—an example of an air-lock forming protein. Likewise, in some cyanobacterial strains the beta-carboxysomes contain a beta-carbonic anhydrase that is not encoded in the ccm locus. Shell proteins of beta carboxysomes are relatively diverse compared to their counterparts in the alpha carboxysomes, and this has been proposed to reflect variable permeability requirements of beta carboxysomes, which are found in cyanobacteria that occupy ecophysiologically dynamic environments. The beta-carboxysome assembles from the inside out. First an enzymatic core forms that is subsequently encapsulated by the protein shell. Carboxysome assembly occurs through a series of protein-protein interactions: the enzyme RuBisCO and the two isoforms (full length and short form) of the CcmM protein interact by means of the SSLDs; in strains containing CcaA the beta-carbonic anhydrase is brought into the carboxysome core by interaction with the N-terminus of the full length CcmM. Once the procarboxysome (the carboxysome core) is formed, the N-terminus of the adapter protein CcmN interacts with the N-terminus of CcmM, while the C-terminus of CcmN recruits the shell proteins CcmK (BMC-H) and CcmO (BMC-T), utilizing a 15-20 amino acids long peptide. This encapsulation peptide forms an amphipathic a-helix that interacts with the shell components and its role is essential, given that in its absence, carboxysomes cannot be formed. The final step is the addition of the vertices formed by the BMC-P protein CcmL, which then cap the enzymatic core and facets. Elucidation of the assembly pathway of beta carboxysomes enabled the design of a single synthetic protein that replaced four other proteins in carboxysome assembly. Potential uses of the carboxysome in biotechnology As is the case with other BMCs, the carboxysome is attracting significant attention by researchers for applications in plant synthetic biology. The transfer of a genetic module coding for an alpha-carboxysome has been shown to produce carboxysome-like structures in E. coli. Bioengineering of carboxysome shells has been shown to be feasible, and beta-carboxysomes constructed with chimeric proteins or with chimeric shells have been reported. The introduction of carboxysomes into plant chloroplasts as part of a concentrating mechanism such as that found in cyanobacteria is predicted to significantly improveme net fixation and yield. Expression of beta-carboxysomal shell proteins and Form IB Rubisco-CcmM complexes in tobacco chloroplasts has been achieved, but did not result in compartments containing RuBisCO. A further advance has been the construction