Dimensions of the hexamers were measured using PyMOL
(DeLano 2002), and all pore diameters were measured for this study using HOLE (Smart et al. 1996). Previously published pore diameters are in parenthesis if the difference was >0.5 Å between this analysis and published values Fig. 9 Electrostatic comparison of pores from structurally characterized BMC shell proteins, viewed from the concave side. Pore residues are shown as green sticks. Red denotes negative charge; blue denotes positive this website charge The pores of the pentamers are also narrow with diameters of ~5 and ~3.5 Å for CcmL and CsoS4A, respectively. They are also positively charged, even more so than the hexamers (Fig. 6). At its narrowest point, the pore for CcmL is formed by R-G-S-A-A and CsoS4A’s is formed by G-S-S-A-A (Table 2). Although the pore residues of carboxysome Pfam03319 orthologs are not as well conserved as their hexameric counterparts, sequence comparison reveals some conservation, with a pore motif of X-(G/S)-S-A-A (Fig. 4b). Table 2 List of structurally characterized pentameric Pfam03319 domain-containing proteins from the buy Inhibitor Library carboxysome and their dimensions Pfam03319 protein Carboxysome type Pentamer diametera (Å) Pentamer edge lengthb
(Å) Pore residues Pore diameter (Å) CcmL [2QW7] β 58 36 RGSAA 5 CsoS4A [2RCF] α 57 34 GSSAA 3.5 PDB IDs of the Mannose-binding protein-associated serine protease listed structures are in brackets. a Pentamer diameter was measured from one vertex to its opposite edged. b Pentamer edge length was measured from one vertex to its shared edge vertex. Dimensions of the pentamers were measured using PyMOL (DeLano 2002), and all pore diameters for this study
were measured using HOLE (Smart et al. 1996) Tandem BMC proteins Among the genes encoding components of both the α- and β-carboxysomes are some containing fusions of BMC domains (Fig. 3): CsoS1D in the α-carboxysome and CcmO and a CsoS1D ortholog (slr0169 in Synechocystis sp. PCC6803) in the β-carboxysome. In 2009, the first structure of a tandem BMC protein was determined, CsoS1D of Prochlorococcus marinus MED4 (Klein et al. 2009). This protein was not predicted to contain two BMC domains; the N-terminal domain lacks obvious sequence similarity to any other BMC domain. However, the α-carbon backbones of the two domains superimpose with an RMSD of 1.27 Å over 95 atoms; guided by a structure-based sequence alignment, the domains are 18% identical. CsoS1D forms trimers resulting in pseudohexamers that are similar in dimensions to hexameric shell proteins (Table 1), with pronounced concave and convex sides (Fig. 9). The edges of the pseudohexamers contain the conserved D-X-X-X-K edge motif and CsoS1D could be readily fitted into existing models of the facets of the α-carboxysome shell (Fig. 5) (Klein et al. 2009).