2g13
CsoS1A with sulfate ion
Structural highlights
FunctionCSOSA_HALNC The major shell protein of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, ccbL-ccbS) is sequestered (PubMed:7934888, PubMed:16535117). Assembles into hexamers which make sheets that form the facets of the polyhedral carboxysome (PubMed:17518518). The shell probably limits the diffusion of CO(2) into and out of the carboxysome (Probable). Molecular modeling shows the central pore of this protein is selectively permeable to anions such as HCO(3) rather than CO(2) or O(2) (Probable). There are estimated to be 2970 CsoS1A/CsoS1C proteins per carboxysome (the proteins differ by only 1 residue) (Ref.5).[1] [2] [3] [4] [5] [6] Unlike beta-carboxysomes, alpha-carboxysomes (Cb) can form without cargo protein. CsoS2 is essential for Cb formation and is also capable of targeting foreign proteins to the Cb. The Cb shell assembles with the aid of CsoS2; CsoS1A, CsoS1B and CsoS1C form the majority of the shell while CsoS4A and CsoS4B form vertices. CsoS1D forms pseudohexamers that probably control metabolite flux into and out of the shell.[7] [8] Evolutionary Conservation Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf. Publication Abstract from PubMedThe carboxysome is a bacterial organelle that functions to enhance the efficiency of CO2 fixation by encapsulating the enzymes ribulose bisphosphate carboxylase/oxygenase (RuBisCO) and carbonic anhydrase. The outer shell of the carboxysome is reminiscent of a viral capsid, being constructed from many copies of a few small proteins. Here we describe the structure of the shell protein CsoS1A from the chemoautotrophic bacterium Halothiobacillus neapolitanus. The CsoS1A protein forms hexameric units that pack tightly together to form a molecular layer, which is perforated by narrow pores. Sulfate ions, soaked into crystals of CsoS1A, are observed in the pores of the molecular layer, supporting the idea that the pores could be the conduit for negatively charged metabolites such as bicarbonate, which must cross the shell. The problem of diffusion across a semiporous protein shell is discussed, with the conclusion that the shell is sufficiently porous to allow adequate transport of small molecules. The molecular layer formed by CsoS1A is similar to the recently observed layers formed by cyanobacterial carboxysome shell proteins. This similarity supports the argument that the layers observed represent the natural structure of the facets of the carboxysome shell. Insights into carboxysome function are provided by comparisons of the carboxysome shell to viral capsids, and a comparison of its pores to the pores of transmembrane protein channels. Structural analysis of CsoS1A and the protein shell of the Halothiobacillus neapolitanus carboxysome.,Tsai Y, Sawaya MR, Cannon GC, Cai F, Williams EB, Heinhorst S, Kerfeld CA, Yeates TO PLoS Biol. 2007 Jun;5(6):e144. PMID:17518518[9] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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