Crystal structures of GfcC, a group 4 capsule operon protein from Escherichia coli, and YraM, an outer membrane lipoprotein from Haemophilus influenzae

GfcC is a small (26 kDa) periplasmic protein encoded within the seven gene gfc operon comprised of gfcABCDE, etp and etk of E. coli. This operon in enteropathogenic strains is essential for the expression of a group 4 polysaccharide capsule whose oligosaccharide repeat units are identical to those in lipopolysaccharide . The GfcC structure shows striking similarities with the outer membrane lipoprotein Wza from group 1 polysaccharide export. Both have two beta-grasp domains for each monomer and an amphipathic C-terminal helical region. GfcC and its homologs also have a 40-residue long helical hairpin inserted into the first beta-grasp domain not found in Wza. The C-terminal helices of Wza form a novel octameric alpha-helical barrel in the outer membrane. This barrel most likely exports the polysaccharide from the periplasm where it is synthesized. Although GfcC has a similar helix, it is shorter and folded back onto a beta-grasp domain and hence GfcC exists as a soluble monomer in solution. Since the group 4 operon also has a wza homolog, gfcE, the function of GfcC is not clear. The following gene encodes GfcD, predicted to be a large beta-barrel outer membrane lipoprotein. Homologs of gfcC and gfcD are fused in several species, notably in Burkholderia sp., suggesting an interaction in vivo between the two encoded proteins.;YraM, a 575-residue outer membrane lipoprotein essential for growth in H. influenzae, has two distinct domains, an N-terminal domain with tetratricopeptide repeats and a C-terminal periplasmic binding protein-like domain. The C-domain has conserved residues clustered in the cleft where other binding proteins bind small molecule ligands. The N-domain resembles other proteins that bind protein or peptidoglycan ligands. The two domains are arranged adjacent connected by a 6-residue linker. Normal mode analysis (specifically, Elastic network modeling) suggests flexibility in the overall conformation between the two domains with the linker acting as a hinge. The function of YraM is unknown but it represents a novel fusion of domains with two distinct binding capabilities---one a small molecule ligand (to the C-domain) and the other possibly a protein (to the N-domain), both binding partners are yet to be identified.