In:
Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, Vol. 107, No. 6 ( 2010-02-09), p. 2455-2460
Abstract:
Cyanobacterial 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 β-cyanobacteria, including Thermosynechococcus elongatus BP-1, lack the conventional carboxysomal β-CA, ccaA . The N-terminal domain of the carboxysomal protein CcmM is homologous to γ-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 γ-CA reported. The 2.0 Å structure of CcmM209 reveals a trimeric, left-handed β-helix structure that closely resembles Cam, except that residues 198–207 form a third α-helix stabilized by an essential Cys194-Cys200 disulfide bond. Deleting residues 194–209 (CcmM193) results in an inactive protein whose 1.1 Å 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 β-carboxysome appears to be able to maintain an oxidizing interior by precluding the entry of thioredoxin and other endogenous reducing agents.
Type of Medium:
Online Resource
ISSN:
0027-8424
,
1091-6490
DOI:
10.1073/pnas.0910866107
Language:
English
Publisher:
Proceedings of the National Academy of Sciences
Publication Date:
2010
detail.hit.zdb_id:
209104-5
detail.hit.zdb_id:
1461794-8
SSG:
11
SSG:
12
