3). CcmL and CsoS4A have been structurally characterized (Tanaka et al. 2008); both form pentamers and have a pronounced concave/convex sidedness similar to the hexamers. In contrast to the hexameric shell proteins, the electrostatic potential of these proteins is predominantly

positive (Fig. 6). The structures of CcmL and CsoS4A can be superimposed with an RMSD of 0.74 Å over 58 C-α atoms. The largest difference between the primary structures of these two proteins is in the region corresponding to an 8–10 amino acid loop on the concave face of the pentamer that seems to influence the charge of the concave face. A similar difference is seen between the paralogs CsoS4A and CsoS4B. In this region CsoS4B has more positively MK5108 datasheet charged residues than CsoS4A. The pores Based on the current models of mTOR inhibitor carboxysome function and structure, pores in the shell protein hexamers provide conduits for the flux of metabolites; bicarbonate ions and RuBP diffuse in and 3PGA to diffuses out, while preventing the BTSA1 order leakage of CO2 from the interior (Dou et al. 2008). The shell also prevents oxygen from diffusing in, reducing unwanted photorespiration by RuBisCO (Marcus et al. 1992). As the shell localizes CA and RuBisCO together, the overall rate of CO2 fixation by RuBisCO is enhanced; effectively, the carboxysome provides a focal point for the carbon concentrating mechanism (CCM) (Fig. 2). A key characteristic of carboxysome shell proteins is a narrow (~4–7 Å

diameter; Kerfeld et al. 2005) central pore that is formed at the 5- and 6-fold axis of symmetry by a loop in the hexamers and pentamers, respectively. Residues forming this loop tend to be conserved

among paralogs; for example, these residues are K-I-G-S and R-(A/V)-G-S in CcmK2 and CcmK4, respectively (Table 1). Such differences in residues flanking the pore likely Protein kinase N1 influence the flux of metabolites into or out of the carboxysome by influencing the size and charge of the pore. All of the pores of structurally characterized carboxysome shell proteins are positively charged at the narrowest point (Fig. 9); presumably this provides a favorable attractive force for negatively charged metabolites such as bicarbonate. At the same time, a charged pore would not attract molecules lacking a dipole moment, such as CO2 and oxygen (Fig. 9). Table 1 List of structurally characterized BMC-domain proteins from the carboxysome and their dimensions Pfam00936 protein Carboxysome type Hexamer diameterb (Å) Hexamer edge lengthc (Å) Pore residues Pore diameter (Å) CsoS1A [2G13] α 72 36 FVGG 4 CsoS1C [3H8Y] α 72 36 FVGG 4 CcmK1 [3BN4] β 75 37 KIGS 4.8 (5.5) CcmK2 [2A1B] β 75 35 KIGS 5.5 (7) CcmK4 [2A10] β 75 37 RAGS 4 CsoS1Da [3F56] α 72 36 ERAF 12.5 (14) PDB IDs of the listed structures are in brackets. aCsoS1D is a tandem BMC-domain protein; values for the dimensions of the pseudohexamer are reported. b Hexamer diameter was measured from one vertex to its opposite vertex.