Carboxysome shell protein CcmP; (Carbon dioxide concentrating mechanism protein) of 213 aas and as many as 4 peaks of moderate hydrophobicity, in a 2 + 2 TMS arrangement, that could be TMSs. The crystal structure is known (see Larsson et al. 2017 and the family description for details). The structure of CcmP, a tandem bacterial microcompartment domain protein from a β-carboxysome, forms a subcompartment within a microcompartment (Cai et al. 2013). Carboxysomes, encoded by 10 genes, can be heterologously produced in E. coli. Expression of carboxysomes in E. coli resulted in the production of icosahedral complexes similar to those from the native host. In vivo, the complexes were capable of both assembling with carboxysomal proteins and fixing CO₂. Characterization of purified synthetic carboxysomes indicated that they were well formed in structure, contained the expected molecular components, and were capable of fixing CO₂ in vitro. In addition, the postulated pore-forming protein CsoS1D, may modulate function (Bonacci et al. 2012).