bicarbonate) and products (3-phosphoglycerate) into and out of the carboxysome. Positively charged amino acids in the pores presumably help promote the diffusion of the negatively charged substrates and products. Other minor structural components of the shell that have been characterized include pentameric proteins (BMC-P proteins) which occupy the vertices of the icosahedral shell. A third building block of the carboxysome shell is a protein composed of two BMC domains in tandem (BMC-T proteins). Structurally, these are known to form trimers which are pseudohexameric. Some members of the BMC-T protein family stack in a face-to-face fashion and form tiny cages, notably both types of carboxysomes (alpha and beta, see below) contain these stacking trimers. Based on crystal structures, these protein cages have relatively large gated pores on both sides, and it has been proposed that the opening and closing of the pore could be controlled in a manner similar to an air-lock. Such an air-lock, in contrast to BMC-H proteins with constitutively open pores, has been suggested to serve as a route for larger substrates (ribulose-1,5-bisphosphate) and products (3-phosphoglycerate) that must cross the shell. Production of empty carboxysome shells in E. coli enabled the first visualization of the carboxysome shell by cryo-electron microscopy. A number of viral capsids are also icosahedral, composed of hexameric and pentameric proteins, but currently there is no evidence suggesting any evolutionary relationship between the carboxysome shell and viral capsids. Two Types of Carboxysomes There are two types of carboxysomes. Although they may seem similar in appearance, they differ in their protein composition, including the form of RuBisCO they enclose. Furthermore, studies have revealed fundamental differences in their gene organization and possibly their assembly pathway. Based on bioinformatic studies of shell proteins, it appears that the two types of carboxysomes evolved independently. Alpha-Carboxysomes Alpha-carboxysomes (aka α-carboxysomes) are also referred as the cso type of carboxysome. They contain Form IA RuBisCO; they are found in alpha-cyanobacteria, some nitrifying bacteria, some sulfur-oxidizing bacteria (for example, Halothiobacillus neapolitanus), and some purple bacteria; these are all classified as Pseudomonadota). The alpha-carboxysome was the first bacterial microcompartment to be purified and characterized. Electron microscopy studies on purified alpha-carboxysomes or cell sections containing alpha-carboxysomes revealed that they are typically 100-160 nm in diameter. Common building blocks for the shell of alpha-carboxysomes are called CsoS1A/B/C (BMC-H), CsoS4A/B (BMC-P), and CsoS1D (BMC-T). CsoS4A/B were the first BMC-P proteins to be experimentally demonstrated as minor components