Components of the Carboxysome

      The best-studied carboxysomes are those of Halothiobacillus neapolitanus. They are composed of nine major proteins: the large (CbbL) and small (CbbS) subunits of RuBisCO, six shell proteins: CsoslA,B, and C, Csos2A and B, Csos3, and one protein of unknown function (Cannon and Shively 1983, Holthuijzen et al. 1986). CsoslA,B, and C are nearly identical in amino acid sequence and are encoded by a highly conserved three gene repeat which appears to be the result of a gene duplication event (English et al. 1994). Csos2 exists in two forms: Csos2A and Csos2B, which differ in their amount of posttranslational glycosylation (Baker et al. 1999). Csos3 whose presence was initially missed on Coomassie-stained gels has also been demonstrated to be a part of the carboxysome shell (Baker et al. 2000). Csosl appears to be the most prevalent shell protein, followed by the two forms of Csos2 while Csos3 appears to be a minor shell protein. The presence of carbonic anhydrase in the carboxysomes of the cyanobacteria, Synechococcus PCC7942, has been reported. This finding however, has been contested by others who have attributed the presence of carbonic anhydrase activity to contamination of the preparations (Shively et al. 1998). Still others think that differences in the carboxysomes of the cyanobacteria and chemoautotrophs could account for this disparity, suggesting a scenario where carbonic anhydrase is present in the carboxysomes of cyanobacteria and absent from those of the thiobacilli ((Badger and Price 2003)). In all cases, however, no other Calvin cycle enyzmes have been reported to be associated with the carboxysome.

Function of the Carboxysome

      Though not entirely understood, it is thought that the carboxysome is actively

involved in concentrating CO2 and may be part of a larger CO2-concentrating mechanism