The pore-forming shell protein, EutL, of 219 aas, of the bacterial ethanolamine-utilizing (ethanolamine ammonialyase )microcompartment (BMC) (Takenoya et al. 2010). The crystal structure has been described (Sagermann et al. 2009). It allows E. coli to use thanolamine as the sole source for carbon and nitrogen. The crystal structure of this shell protein at 2.2-Å resolution was determined. It is the largest representative of this BMC's shell proteins. In the crystal, EutL forms a trimer that exhibits a hexagonally shaped tile structure. The tiles arrange into a tightly packed 2D array that resembles the proteinaceous membrane of the intact BMC. In contrast to other BMC shell proteins, which have only 1 pore per tile, EutL exhibits 3 pores per tile, thereby significantly increasing the overall porosity of this protein membrane. Each of the individual pores is lined with negatively charged residues and aromatic residues that are proposed to facilitate passive transport of specific solutes. The characteristic shape of the hexagonal tile, which is also found in the microcompartments of carbon-fixating bacteria, may present an inherent and fundamental building unit that may provide a general explanation for the formation of differently sized microcompartments (Sagermann et al. 2009).  Ethanolamine, the substrate of the Eut microcompartment, acts as a negative allosteric regulator of EutL pore opening (Thompson et al. 2015). Specifically, a series of X-ray crystal structures of EutL from Clostridium perfringens, along with equilibrium binding studies, revealed that ethanolamine binds to EutL at a site that exists in the closed-pore conformation and which is incompatible with opening of the large pore for cofactor transport. The allosteric mechanism proposed is consistent with the cofactor requirements of the Eut microcompartment, leading to a new model for EutL function (Thompson et al. 2015).