Protein Domain : CcmK/CsoS1, bacterial microcompartment domain IPR044872

Type	Domain
Description	Bacterial microcompartments (BMCs) are large proteinaceous structures comprised of a roughly icosahedral shell and a series of encapsulated enzymes. They are found across bacteria where they play functionally diverse roles including CO(2) fixation and the catabolism of a range of organic compounds. They function as organelles by sequestering particular metabolic processes within the cell. A shell or capsid, which is composed of a few thousand protein subunits, surrounds a series of sequentially acting enzymes and controls the diffusion of substrates and products (including toxic or volatile intermediates) into and out of the lumen. Although functionally distinct BMCs vary in their encapsulated enzymes, all are defined by homologous shell proteins. 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 [, , ]. There are three classes of constituent proteins that form a shell with icosahedral symmetry: hexamer-forming proteins containing a single BMC domain (BMC-H); trimer/pseudohexamer-forming proteins consisting of a fusion of two BMC domains (BMC-T), and pentamer-forming proteins containing a bacterial microcompartment vertex (or BMV) domain (BMC-P). The BMC-H and BMC-T proteins form the facets, and the BMC-P proteins form the vertices of the icosahedron. These three protein types form cyclic homooligomers with pores at the centre of symmetry that enable metabolite transport across the shell [ , , , , , , , ].The BMC domain fold consists of three α-helices (designated A, B, and C) and four β-strands (designated β1, β2, β3, and β4). 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 [ , , ].This entry represents the BMC domain found in CsoS1/CcmK and related proteins. CsoS1 and CcmK are the shell proteins of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, ccbL-ccbS) is sequestered [, ].
Short Name	CcmK/CsoS1_BMC