A minor shell protein of the choline degradation-specific bacterial microcompartment (BMC). Proteins such as this one with circularly permuted BMC domains may play a key role in conferring heterogeneity and flexibility in this BMC. Amine and polyamine metabolism; choline degradation. Has been crystallized in 5 structures (all are mutated, 3 have an N-terminal His-tag), most are homohexameric with a central pore. In two the homohexamer lies flat with a beta-barrel on the flat face created by the protruding N termini of the six chains (PubMed:32885887). In 2 others the hexamer is not flat but has a six-fold screw axis; the screw pitch is 33.8 or 41.9 Angstroms depending on the structure (PubMed:32885887). Interacts with the BMC major shell protein (By similarity). One side of the hexamer is concave and lined by hydrophobic residues, the other side has a slightly protruding, 6-stranded beta-barrel. Belongs to the EutS/PduU family.