1.S.1. The Bacterial Microcompartment Shell/Pore-forming Protein-1 (BMC-SP1) Family 

Bacterial micro/nano-compartments are protein-based 'organelles' or 'machines' that function in the catalysis of specific reactions or reaction pathways, where the essential enzymes are surrounded by a proteineaceious shell, some of the proteins of which contain pores for the diffusion of certain metabolites but not others (Saier 2013). The shell proteins and their pores have been fairly extensively characterized (Cheng et al. 2008). For example, the PduU shell protein from the 1,2-propanediol-metabolizing organelle in Salmonella typhimurium is a hexameric structure of a 99 aas protein containing a well defiined pore (Crowley et al. 2008) while the EutL shell protein of the ethanolamine ammonia lyase microcompartment in E. coli is a much larger protein of 219 aas that forms a trimer with a hexagonally shpaped tile structure (Sagermann et al. 2009) and the EutB shell protein in the ethanol utilizing microcompartment of Chlostridium kluyveri is also trimeric (Heldt et al. 2009).  There appear to be many such substrate-specific organelles in prokaryotes, and many of these have homologous pore-forming shell proteins. 

The crystal structures of several carboxysome shell proteins are known. The shell proteins are composed of two domain classes: those with the bacterial microcompartment (BMC; Pfam00936) domain, which oligomerize to form (pseudo)hexamers, 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. In this review, we describe bioinformatic and structural analyses that highlight 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 CO(2) fixation in other organisms or creating novel biological nanostructures.