A high-resolution single-particle cryo-EM hydrated structure of Streptococcus pyogenes enolase offers insights into its function as a plasminogen receptor

Cellular plasminogen (Pg) receptors (PgR) are utilized to recruit Pg, stimulate its activation to the serine protease, plasmin (Pm), and sterically protect the generated Pm from inactivation by natural host inhibitors. The net result is that cells contain a stable proteolytic surface used for biological mechanisms involved in cell migration. One such PgR is the moonlighting enzyme, enolase, some of which leaves the cytoplasm and resides at the cell surface to potentially function as a PgR. Since microbes employ conscription of host Pg by PgRs as one virulence mechanism, we explored the structural basis of the ability of Streptococcus pyogenes enolase (Sen) to function in this regard. Employing single-particle cryo-electron microscopy (cryo-EM), recombinant Sen from S. pyogenes was modeled at 2.6 Å as a stable symmetrical homooctamer displaying point group 422 (D4) symmetry, with a monomeric subunit molecular weight of ~49 kDa. Subunit-subunit interactions showed four major and four minor interfaces in the octamer. Binding sites for hPg were previously proposed to include the COOH-terminal K434,435 residues of Sen, but in native Sen these residues are buried within the minor interfaces of the octamer and do not function as a Pg binding epitope. Whereas Sen and hPg do not interact in solution, when Sen is bound to a surface, hPg interacts with Sen independently of K434,434. We propose that the octameric structure of Sen is important to its ability to interact with hPg, but disruption of its overall octameric conformation without dissociation of the octamer exposes neoepitopes for hPg binding.