4ox7
Structure of Synechococcus elongatus PCC 7942 CcmK2
Structural highlights
FunctionCCMK2_SYNE7 One of the shell proteins of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. Assembles into hexamers which make sheets that form the facets of the polyhedral carboxysome. The hexamer central pore probably regulates metabolite flux.[HAMAP-Rule:MF_00854] The major shell protein of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. Hexamers make sheets that form the facets of the polyhedral carboxysome (PubMed:22928045). The shell is 4.5 nm thick, as observed for CcmK hexamers (PubMed:28616951). Required for recruitment of CcmO to the pre-carboxysome (PubMed:22928045, PubMed:24267892). In PCC 7942 there are several CcmK paralogs with presumably functional differences; replacing the central pore residues (34-37) with those of either CcmK4 from this organism (Tyr-Met-Arg-Ala) or from an alpha-type carboxysome forming cyanobacterium (CsoS1 of P.marinus strain MIT 9313, Arg-Glu-Phe-Val) allows the bacterium to make carboxysomes, but the expression level is too low to know if the carboxysome is functional for CO(2) fixation (PubMed:25117559).[1] [2] [3] [4] Beta-carboxysome assembly initiates when soluble RuBisCO is condensed into a liquid matrix in a pre-carboxysome by the RbcS-like domains of probably both CcmM58 and CcmM35. CcmN interacts with the N-terminus of CcmM58, and then recruits the CcmK2 major shell protein via CcmN's encapsulation peptide. Shell formation requires CcmK proteins and CcmO. CcmL caps the otherwise elongated carboxysome. Once fully encapsulated carboxysomes are formed, they migrate within the cell probably via interactions with the cytoskeleton.[5] Publication Abstract from PubMedBacterial microcompartments (BMCs) are self-assembling organelles composed entirely of protein. Depending on the enzymes they encapsulate, BMCs function in either inorganic carbon fixation (carboxysomes) or organic carbon utilization (metabolosomes). The hallmark feature of all BMCs is a selectively permeable shell formed by multiple paralogous proteins, each proposed to confer specific flux characteristics. Gene clusters encoding diverse BMCs are distributed broadly across bacterial phyla, providing a rich variety of building blocks with a predicted range of permeability properties. In theory, shell permeability can be engineered by modifying residues flanking the pores (symmetry axes) of hexameric shell proteins or by combining shell proteins from different types of BMCs into chimeric shells. We undertook both approaches to altering shell properties using the carboxysome as a model system. There are two types of carboxysomes, alpha and beta. In both, the predominant shell protein(s) contain a single copy of the BMC domain (pfam00936), but they are significantly different in primary structure. Indeed, phylogenetic analysis shows that the two types of carboxysome shell proteins are more similar to their counterparts in metabolosomes than to each other. We solved high resolution crystal structures of the major shell proteins, CsoS1 and CcmK2 and the presumed minor shell protein CcmK4, representing both types of cyanobacterial carboxysomes and then tested the interchangeability. The in vivo study presented here confirms that both engineering pores to mimic those of other shell proteins and the construction of chimeric shells is feasible. Engineering Bacterial Microcompartment Shells: Chimeric Shell Proteins and Chimeric Carboxysome Shells.,Cai F, Sutter M, Bernstein SL, Kinney JN, Kerfeld CA ACS Synth Biol. 2014 Aug 12. PMID:25117559[6] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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