Protein Domain : CcmK-like superfamily IPR037233

Type	Homologous_superfamily
Description	Bacterial microcompartments (BMCs) are large proteinaceous structures comprised of a roughly icosahedral shell and a series of encapsulated enzymes. The shells of BMCs are made primarily of a family of proteins whose structural core is the BMC domain, and variations upon this core provide functional diversity. This domain is found in a variety of polyhedral organelle shell proteins (CcmK), including CsoS1A, CsoS1B and CsoS1C of Thiobacillus neapolitanus (Halothiobacillus neapolitanus) and their orthologues from other bacteria [ , , ].Some autotrophic and non-autotrophic organisms form polyhedral organelles, carboxysomes/enterosomes [ ]. The best studied is the carboxysome of Halothiobacillus neapolitanus, which is composed of at least 9 proteins: six shell proteins, CsoS1A, CsoS1B, CsoS1C, Cso2A, Cso2B and CsoS3 (carbonic anhydrase) [, ], one protein of unknown function and the large and small subunits of RuBisCo (CbbL and Cbbs). Carboxysomes appear to be approximately 120 nm in diameter, most often observed as regular hexagons, with a solid interior bounded by a unilamellar protein shell. The interior is filled with type I RuBisCo, which is composed of 8 large subunits and 8 small subunits; it accounts for 60% of the carboxysomal protein, which amounts to approximately 300 molecules of enzyme per carboxysome. Carboxysomes are required for autotrophic growth at low CO2concentrations and are thought to function as part of a CO 2-concentrating mechanism [ , ].Polyhedral organelles, enterosomes, from non-autotrophic organisms are involved in coenzyme B 12-dependent 1,2-propanediol utilisation (e.g., in Salmonella enterica [ ]) and ethanolamine utilisation (e.g., in Salmonella typhimurium []). Genes needed for enterosome formation are located in the 1,2-propanediol utilisation pdu[ , ] or ethanolamine utilisation eut[ , ] operons, respectively. Although enterosomes of non-autotrophic organisms are apparently related to carboxysomes structurally, a functional relationship is uncertain. A role in CO2concentration, similar to that of the carboxysome, is unlikely since there is no known association between CO 2and coenzyme B12-dependent 1,2-propanediol or ethanolamine utilisation [ ]. It seems probable that enterosomes help protect the cells from reactive aldehyde species in the degradation pathways of 1,2-propanediol and ethanolamine [].The BMC domain fold consists of three α-helices (designated A, B, and C) and four β-strands ( ). Some instances of the BMC shell protein reveal a circular permutation in which a highly similar tertiary structure is built from secondary structure elements occurring in a different order. The secondary structure elements contributed by the C-terminal region of the typical BMC fold are instead contributed by the N-terminal region of the BMC circularly permuted domain ( ) [ , , ].CcmK has a Ferredoxin-like fold with an extra C-terminal helix and forms a compact hexameric 'tiles' of hexagonal shape.
Short Name	CcmK-like_sf