1.S.5. The Bacterial Microcompartment Shell/Pore-forming Protein5 (Pfam00936; BMC-SP5) Family The shell proteins are composed of at least two domain classes: those with the bacterial microcompartment (BMC; Pfam00936) domain, which oligomerize to form (pseudo)hexamers (this famiy), and those with the CcmL/EutN (Pfam03319) domain which form pentamers in carboxysomes. These two shell protein types are proposed to be the basis for the carboxysome's icosahedral geometry. The shell proteins are also thought to allow the flux of metabolites across the shell through the presence of the small pore formed by their hexameric/pentameric symmetry axes. Kenney et al. described bioinformatic and structural analyses that highlighted the important primary, tertiary, and quaternary structural features of these conserved shell subunits. In the future, further understanding of these molecular building blocks may provide the basis for enhancing CO₂ fixation in other organisms or creating novel biological nanostructures (Kinney et al. 2011).
References:
Brinsmade, S.R., T. Paldon, and J.C. Escalante-Semerena. (2005). Minimal functions and physiological conditions required for growth of salmonella enterica on ethanolamine in the absence of the metabolosome. J. Bacteriol. 187: 8039-8046.
Cameron, J.C., S.C. Wilson, S.L. Bernstein, and C.A. Kerfeld. (2013). Biogenesis of a bacterial organelle: the carboxysome assembly pathway. Cell 155: 1131-1140.
Kinney, J.N., S.D. Axen, and C.A. Kerfeld. (2011). Comparative analysis of carboxysome shell proteins. Photosynth Res 109: 21-32.
Kofoid, E., C. Rappleye, I. Stojiljkovic, and J. Roth. (1999). The 17-gene ethanolamine (eut) operon of Salmonella typhimurium encodes five homologues of carboxysome shell proteins. J. Bacteriol. 181: 5317-5329.
Examples:
TC#	Name	Organismal Type	Example
1.S.5.1.1	CcmL shell protein of cyanobacterial carboxysomes, The protein is of 99 aas with two very moderately hydrophobic regions, the larger N-terminal region, and the shorter C-terminal region. It probably forms vertices in the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. It has been modeled to induce curvature upon insertion into an otherwise flat hexagonal molecular layer of CcmK subunits (Cameron et al. 2013).		CcmL of Synechococcus elongatus (ATCC33912); also called Anacystis nodulans (R2)

1.S.5.1.2	EutN (CchB) shell protein of the ethanolamine utilization microcompartiment. It is of 99 aas with a broad region of hydrophobicity throughout the N-terminal regioni, and a shorter but more hydrophobic region at the C-terminus. It has been described by (Kinney et al. 2011). The ethanolamine (EA) catabolic bacterial microcompartment (BMC) probably concentrates low levels of ethanolamine catabolic enzymes, concentrates volatile reaction intermediates, keeps the level of toxic acetaldehyde low, generates enough acetyl-CoA to support cell growth, and maintains a pool of free coenzyme A (CoA) and NAD (Brinsmade et al. 2005; Kofoid et al. 1999).		EutN of Salmonella enterica (subtype Typhimurium)