Tricorn protease exists as an icosahedral supermolecule in vivo

Tricorn protease is the core enzyme of a recently discovered modular proteolytic system. We present evidence that tricorn protease exists in vivo in the form of a higher-order assembly, namely as an icosahedral capsid. Its size exceeds that of many virus particles and represents by far the largest known homooligomeric enzyme complex. Each capsid is built from 20 copies of the tricorn hexameric toroid and thus has a molecular weight of 14.6 MDa. Three-dimensional reconstructions of ice-embedded capsids from electron micrographs show that it is hollow and has large void volumes in its wall. We suggest that the tricorn capsid, in addition to its intrinsic proteolytic activity, serves as the organizing center of a multienzyme complex.