Bacterial micro-compartments are primitive protein-based organelles that sequester specific metabolic pathways in bacterial cells. The prototypical bacterial microcompartment is the carboxysome shell, a bacterial polyhedral organelle which increase the efficiency of CO2 fixation by encapsulating RuBisCO and carbonic anhydrase. They can be divided into two types: alpha-type carboxysomes (alpha-cyanobacteria and proteobacteria) and beta-type carboxysomes (beta-cyanobacteria). Potential functional differences between the two types are not yet fully understood. In addition to these proteins there are several homologous shell proteins including those found in pdu organelles involved in coenzyme B12-dependent degradation of 1,2-propanediol and eut organelles involved in the cobalamin-dependent degradation of ethanolamine. Structure evidence shows that several carboxysome shell proteins and their homologs (Csos1A, CcmK1,2,4, and PduU) exist as hexamers which might further assemble into extended, tightly packed layers hypothesized to represent the flat facets of the polyhedral organelles outer shell. Although it has been suggested that other homologous proteins in this family might also form hexamers and play similar functional roles in the construction of their corresponding organelle outer shells at present no experimental evidence directly supports this view.
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