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).