Three-dimensional cryoelectron microscopic reconstruction of the 2.25-MDa homomultimeric phosphoenolpyruvate synthase from Staphylothermus marinus.

The phosphoenolpyruvate synthase (EC 2.7.9.2) of the hyperthermophilic archaeon Staphylothermus marinus forms a 2.25 MDa homomultimeric complex of 24 subunits. Here, computational analysis of low-dose cryoelectron micrographs was used to ascertain that the major rotational symmetry axes were 2-fold, 3-fold, and 4-fold. These symmetry considerations were used to perform a three-dimensional reconstruction to a spatial resolution of 4 nm. This assembly has an octahedral architecture with a solvent accessible interior and with ill-defined yet seemingly flexible appendages on the periphery. This macromolecular assembly is unusually large in mass compared with most other known globular proteins--especially other identified phosphoenolpyruvate synthases which are usually dimeric--and its elaborate quaternary arrangement might represent an adaptation to its extreme environment.