of the BMC shell (only 12 pentamers are required to cap the vertices of an icosahedron). CsoS1D is the first BMC-T which has been structurally characterized; it is also the first example of dimerization of two BMC building blocks in a face-to-face fashion to create a tiny cage. The CsoS1D cage has a gated pore at both ends, which is proposed to facilitate the transfer of large metabolites across the shell. In addition to the specific form of RuBisCO, other encapsulated proteins distinguish alpha-carboxysomes from beta-carboxysomes such as CsoS2 and CsoSCA. The CsoS2 protein has a very high pI and a unique primary structure. The primary structure of CsoS2 appears tripartite, composed of an N-terminal, a middle- and a C-terminal region. Repetitive motifs can be identified in the N-terminal and middle regions. Recently, it was proposed to be an intrinsically disordered protein with an essential role in alpha-carboxysome assembly. CsoSCA is a shell-associated beta-carbonic anhydrase. Studies in Halothiobacillus neapolitanus have shown that empty shells of normal shape and composition are assembled in carboxysomal RuBisCO-lacking mutants, suggesting that alpha-carboxysome shell biogenesis and enzyme sequestration are two independent, but functionally linked processes. Intriguingly, carboxysomes of Halothiobacillus neapolitanus have been found to accommodate chimeric and heterologous species of RuBisCO. It is the large subunit of RuBisCO which determines whether the enzyme is sequestered into carboxysomes. Beta-carboxysomes Beta-carboxysomes (aka β-carboxysomes) are found in cyanobacteria. The signature proteins of the beta-carboxysome are Form IB RuBisCO and a gamma carbonic anhydrase homolog. Beta-carboxysomes are typically larger than alpha-carboxysomes: the observed diameters vary from 200 to 400 nm. The structural proteins that are essential for beta-carboxysome formation are encoded in the conserved carboxysome locus known as the ccm locus. The ccm locus includes genes for core proteins CcmM and CcmN and the shell proteins CcmK (a BMC-H protein), CcmL (a BMC-P protein) and CcmO (a BMC-T protein). A full length CcmM protein consists of a gamma-carbonic anhydrase domain and three to five RubisCO small subunit-like domains (SSLDs) on its C-terminus. The ccmM gene contains an internal translation site that produces a short form of CcmM which only consists of SSLDs; both long and short forms of CcmM are required for beta-carboxysome assembly. CcmN contains multiple hexapeptide-repeat domains on its N-terminus and a short α-helical encapsulation peptide on the C-terminus. Other structural components of beta-carboxysomes are encoded outside of the ccm locus. CcmP is a BMC-T protein that