1.S.3. The Bacterial Microcompartment Shell/Pore-forming Protein-3 (BMC-SP3) Family Cyanobacterial CO₂ fixation is promoted by encapsulating and co-localizing the CO₂-fixing enzymes within a protein shell, the carboxysome. A key feature of the carboxysome is its ability to control selectively the flux of metabolites in and out of the shell. The β-carboxysome shell protein CcmP forms a double layer of pseudohexamers with a relatively large central pore (~13 Å diameter), which may allow passage of larger metabolites such as the substrate for CO₂ fixation, ribulose 1,5-bisphosphate, through the shell. Larsson et al. 2017 described two crystal structures, at 1.45 Å and 1.65 Å resolution, of CcmP from Synechococcus elongatus PCC7942 (SeCcmP). The central pore of CcmP is open or closed at its ends, depending on the conformation of two conserved residues, Glu69 and Arg70. The presence of glycerol resulted in a pore that is open at one end and closed at the opposite end. When glycerol was omitted, both ends of the barrel became closed. A binding pocket at the interior of the barrel features residual density with distinct differences in size and shape depending on the conformation, open or closed, of the central pore of SeCcmP, suggestive of a metabolite-driven mechanism for the gating of the pore (Larsson et al. 2017).
References:
Bonacci, W., P.K. Teng, B. Afonso, H. Niederholtmeyer, P. Grob, P.A. Silver, and D.F. Savage. (2012). Modularity of a carbon-fixing protein organelle. Proc. Natl. Acad. Sci. USA 109: 478-483.
Cai, F., M. Sutter, J.C. Cameron, D.N. Stanley, J.N. Kinney, and C.A. Kerfeld. (2013). The structure of CcmP, a tandem bacterial microcompartment domain protein from the β-carboxysome, forms a subcompartment within a microcompartment. J. Biol. Chem. 288: 16055-16063.
Larsson, A.M., D. Hasse, K. Valegård, and I. Andersson. (2017). Crystal structures of β-carboxysome shell protein CcmP: ligand binding correlates with the closed or open central pore. J Exp Bot 68: 3857-3867.
Examples:
TC#	Name	Organismal Type	Example
1.S.3.1.1	Carboxysome shell protein CcmP; (Carbon dioxide concentrating mechanism protein) of 213 aas and as many as 4 peaks of moderate hydrophobicity, in a 2 + 2 TMS arrangement, that could be TMSs. The crystal structure is known (see Larsson et al. 2017 and the family description for details). The structure of CcmP, a tandem bacterial microcompartment domain protein from a β-carboxysome, forms a subcompartment within a microcompartment (Cai et al. 2013). Carboxysomes, encoded by 10 genes, can be heterologously produced in E. coli. Expression of carboxysomes in E. coli resulted in the production of icosahedral complexes similar to those from the native host. In vivo, the complexes were capable of both assembling with carboxysomal proteins and fixing CO₂. Characterization of purified synthetic carboxysomes indicated that they were well formed in structure, contained the expected molecular components, and were capable of fixing CO₂ in vitro. In addition, the postulated pore-forming protein CsoS1D, may modulate function (Bonacci et al. 2012).		*CcmP of Synechococcus elongatus* PCC 7942**

1.S.3.1.2	Microcompartment protein, CcmP, of 212 aas.		CcmP of Spirulina subsalsa

1.S.3.1.3	CsoS1D of 213 aas and 4 moderately hydrophobic peaks in a 2 + 2 TMS arrangement. It is a pore-forming protein in a cyanobacterial carboxysome, and it modulates the function of this enclosed protein machinary (Bonacci et al. 2012).		CsoS1D of Microcystis aeruginosa