Proteolysis, the last step in the life of a protein, is essential for the maintenance of cellular homeostasis. Proteases remove damaged or denatured proteins, recycle their amino acids, and are important for cell regulation. But how do cells tame thepotentially destructive activities of these enzymes to ensure that they act only where needed? One solution--used by archaebacteria, euhacteria, and eukaryotes--has been to compartmentalize proteolytic enzymes within so-called molecular organelles, ring-shaped protein assemblies in which the proteolytically active center is sequestered from the rest of the cytosol. The paradigm is the proteasome, a cylindrical multimeric protein complex with a central proteolytic chamber, to which access is carefully controlled (see figure, part B). This ancient organizational principle has been extremely successful. Indeed, most protein degradation in the cytosol of archaebacteria and of eukaryotic cells is thought to be proteasome-mediated. But now on page 1385 of this issue, Tamura et al. report the surprising discovery of a new proteasome-like particle, a large tricorn complex (TRI) abundant in the thcrmophilic archaebactcrium Thermoplasma acidophllum (see figure, part A). A comparison of TRI and the proteasome reveals both interesting parallels and differences.
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