1.S.1.1.1
The PduA shell protein of 99 aas which forms a hexameric array with a pore in the array for diffusion of 1,2-propanediol but not propionaldehyde (Park et al. 2017).  A serine that protrudes into the pore at the point of construction to form a hydrogen bond with propionaldehyde prevent it's free diffusion. Substrates enter these microcompartments by passing through the central pores in hexameric assemblies of shell proteins. Limiting the escape of toxic metabolic intermediates created inside the microcompartments confers a selective advantage for the host organism. The pore of the PduA hexamer has a lower energy barrier for passage of the propanediol substrate compared to the toxic propionaldehyde generated within the microcompartment (Park et al. 2017).  PduA forms a selectively permeable pore tailored for the influx of 1,2-propanediol while restricting the efflux of propionaldehyde, a toxic intermediate of 1,2-propanediol catabolism (Chowdhury et al. 2015). The hexamer-hexamer interactions seen in PduA crystals persist in the cytoplasmic structures and reveal the profound influence of the two key amino acids, Lys-26 and Arg-79, on tiling, not only in the crystal lattice but also in the bacterial cytoplasm (Pang et al. 2014). There are several shell proteins, PduA and PduT being two of them, and their crystal structures have been determined (Crowley et al. 2010).  PduA forms a symmetric homohexamer whose central pore appears tailored for facilitating transport of the 1,2-propanediol substrate. PduT is a novel, tandem domain shell protein that assembles as a pseudohexameric homotrimer. Its structure reveals an unexpected site for binding an [Fe-S] cluster at the center of the PduT pore (Crowley et al. 2010). The location of a metal redox cofactor in the pore of a shell protein suggests a novel mechanism for either transferring redox equivalents across the shell or for regenerating luminal [Fe-S] clusters.It is a minor shell protein of the Salmonella enterica microcompartment (BMC) dedicated to 1,2-propanediol (1,2-PD) degradation. The isolated BMC shell component protein ratio for J:A:B':B:K:T:U is approximately 15:10:7:6:1:1:2 (Crowley et al. 2010). PduJ is 89% identical to PduA; PduB is protein 1.S.2.1.2; PduU is protein 1.S.2.3.1 in TCDB.