1.S.4.  The Bacterial Microcompartment Shell/Pore-forming Protein-4 (BMC-SP4) Family

In the glycyl-radical propanediol (Grp) MCP, the BMC-domain shell proteins [bacterial microcompartment (in reference to the shell protein domain)] assemble to form a polyhedral barrier that encapsulates the enzymatic contents of the MCP. The Grp MCP contains a number of shell proteins with unusual sequence features. GrpU is one such shell protein whose amino acid sequence is particularly divergent from other members of the BMC-domain superfamily of proteins that effectively defines all MCPs. Expression, purification, and subsequent characterization of the protein showed that it binds an iron-sulfur cluster.  X-ray crystal structures of two GrpU orthologs provided structural insight into the homohexameric BMC-domain shell proteins of the Grp system. The X-ray structures show that the metal cluster resides in the central pore of the BMC shell protein at a position of broken 6-fold symmetry. The result is a structurally polymorphic iron-sulfur cluster binding site that appears to be unique among metalloproteins studied to date (Thompson et al. 2014).