Lactobacillus reuteri metabolizes two similar three-carbon molecules,
1,2-propanediol and glycerol, within closed polyhedral subcellular
bacterial organelles called bacterial microcompartments (metabolosomes).
The outer shell of the propanediol-utilization (Pdu) metabolosome is
composed of hundreds of mainly hexagonal protein complexes made from six
types of protein subunits that share similar domain structures. The
structure of the bacterial microcompartment protein PduB has a tandem
structural repeat within the subunit and assembles into a trimer with
pseudo-hexagonal symmetry. This trimeric structure forms sheets in the
crystal lattice and is able to fit within a polymeric sheet of the major
shell component PduA to assemble a facet of the polyhedron. There are
three pores within the trimer and these are formed between the tandem
repeats within the subunits. The structure shows that each of these
pores contains three glycerol molecules that interact with conserved
residues, strongly suggesting that these subunit pores channel glycerol
substrate into the metabolosome. In addition to the observation of
glycerol occupying the subunit channels, the presence of glycerol on the
molecular threefold symmetry axis suggests a role in locking closed the
central region.