3kwc
Oxidized, active structure of the beta-carboxysomal gamma-Carbonic Anhydrase, CcmM
Structural highlights
FunctionCCMM_THEVB Functions as a scaffold protein for the assembly of beta-carboxysomes, initiates carboxysome assembly by coalescing RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) (Probable). Produced as a full-length and a shorter form; both forms are required for correct carboxysome assembly and growth (By similarity).[UniProtKB:Q03513][1] A carbonic anhydrase, catalyzes the reversible hydration of carbon dioxide. Essential to photosynthetic carbon dioxide fixation, supplies CO(2) to ribulose bisphosphate carboxylase (RuBisCO) in the carboxysome. Active when the disulfide bond (194-200) is oxidized, suggesting the interior of the carboxysome is oxidizing.[2] 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 forms of CcmM (Probable). CcmN interacts with the N-terminus of full length CcmM, and then recruits the shell proteins (CcmK) via CcmN's encapsulation peptide. Shell formation requires both 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 (By similarity).[UniProtKB:Q03513][3] 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 PubMedCyanobacterial RuBisCO is sequestered in large, icosahedral, protein-bounded microcompartments called carboxysomes. Bicarbonate is pumped into the cytosol, diffuses into the carboxysome through small pores in its shell, and is then converted to CO(2) by carbonic anhydrase (CA) prior to fixation. Paradoxically, many beta-cyanobacteria, including Thermosynechococcus elongatus BP-1, lack the conventional carboxysomal beta-CA, ccaA. The N-terminal domain of the carboxysomal protein CcmM is homologous to gamma-CA from Methanosarcina thermophila (Cam) but recombinant CcmM derived from ccaA-containing cyanobacteria show no CA activity. We demonstrate here that either full length CcmM from T. elongatus, or a construct truncated after 209 residues (CcmM209), is active as a CA-the first catalytically active bacterial gamma-CA reported. The 2.0 A structure of CcmM209 reveals a trimeric, left-handed beta-helix structure that closely resembles Cam, except that residues 198-207 form a third alpha-helix stabilized by an essential Cys194-Cys200 disulfide bond. Deleting residues 194-209 (CcmM193) results in an inactive protein whose 1.1 A structure shows disordering of the N- and C-termini, and reorganization of the trimeric interface and active site. Under reducing conditions, CcmM209 is similarly partially disordered and inactive as a CA. CcmM protein in fresh E. coli cell extracts is inactive, implying that the cellular reducing machinery can reduce and inactivate CcmM, while diamide, a thiol oxidizing agent, activates the enzyme. Thus, like membrane-bound eukaryotic cellular compartments, the beta-carboxysome appears to be able to maintain an oxidizing interior by precluding the entry of thioredoxin and other endogenous reducing agents. Structural basis of the oxidative activation of the carboxysomal gamma-carbonic anhydrase, CcmM.,Pena KL, Castel SE, de Araujo C, Espie GS, Kimber MS Proc Natl Acad Sci U S A. 2010 Feb 9;107(6):2455-60. Epub 2010 Jan 25. PMID:20133749[4] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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