Adenoviruses infect a wide range of vertebrates including humans. Their icosahedral capsids are composed of three major proteins: the trimeric hexon forms the facets and the penton, a noncovalent complex of the pentameric penton base and trimeric fibre proteins, is located at the 12 capsid vertices. Several proteins (IIIa, VI, VIII and IX) stabilise the capsid. We have obtained a 10 Å resolution map of the human adenovirus 5 by image analysis from cryo-electron micrographs (cryoEMs). This map, in combination with the X-ray structures of the penton base and hexon, was used to build a quasi-atomic model of the arrangement of the two major capsid components and to analyse the hexon-hexon and hexon-penton interactions. The secondary proteins, notably VIII, were located by comparing cryoEM maps of native and pIX deletion mutant virions. Minor proteins IX and IIIa are located on the outside of the capsid, whereas protein VIII is organised with a T ¼ 2 lattice on the inner face of the capsid. The capsid organisation is compared with the known X-ray structure of bacteriophage PRD1.

Cellular proteolysis involves large oligomeric peptidases that play key roles in the regulation of many cellular processes. The cobalt-activated peptidase TET1 from the hyperthermophilic Archaea Pyrococcus horikoshii (PhTET1) was found to assemble as a 12-subunit tetrahedron and as a 24-subunit octahedral particle. Both quaternary structures were solved by combining x-ray crystallography and cryoelectron microscopy data. The internal organization of the PhTET1 particles reveals highly self-compartmentalized systems made of networks of access channels extended by vast catalytic chambers. The two edifices display aminopeptidase activity, and their organizations indicate substrate navigation mechanisms different from those described in other large peptidase complexes. Compared with the tetrahedron, the octahedron forms a more expanded hollow structure, representing a new type of giant peptidase complex. PhTET1 assembles into two different quaternary structures because of quasi-equivalent contacts that previously have only been identified in viral capsids.Cytosolic proteolysis is a key cellular process. In addition to its role in amino acid metabolism, it is responsible for protein quality control and the adjustment of the turnover of regulatory molecules (1, 2). Initial proteolysis is carried out by nonspecific endoproteases (1). Through oligomerization, these enzymes form barrel-like cellular subcompartments to protect cytoplasmic proteins from unwanted cleavage; the active sites are situated in proteolytic chambers, only accessible to unfolded polypeptides. In Eukarya and Archaea, the 20 S proteasome is the core of this proteolytic system (3, 4). The proteins to be degraded are unfolded and translocated into the proteolytic complex by energy-dependent regulatory particles (5). Polypeptide degradation results in multiple endoproteolytic cleavage, yielding short peptides (6 -12 residues in length) (6). The final breakdown of these fragments is probably achieved by a collection of ATP-independent exopeptidases, which have been proposed to be essential components of the protein degradation system (7,8).Structural studies on the Tricorn protease complex, an archaeal carboxypeptidase (peptidyl-di/tripeptidase), have shown that some exopeptidases can also form multimeric ring structures, with the active sites sequestered within an internal chamber (9). This feature of compartmentalization was also recognized in the three-dimensional structures of three ATP-independent intracellular aminopeptidases: Gal6, leucine aminopeptidase, and the D-aminopeptidase DppA (10 -12). In eukaryotic cells, the tripeptidyl-peptidase II also forms a giant, well organized toroidal oligomeric structure (13). All these exopeptidases could function as scavengers of the oligopeptides generated by the ATP-dependent endoproteases (14).TET is another type of a large hollow peptidase complex. It was characterized initially in the halophilic Archaea Haloarcula marismortui (15). The complex has a tetrahedral shape and is a broad substrate specificity ...