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The 26S proteasome is specialized for regulated protein degradation. It is formed by a regulatory particle (RP) that recognizes ubiquitinated substrates and caps a hollow cylindrical core particle (CP) where substrates are proteolyzed. Structural heterogeneity caused by dynamics makes it challenging to observe ubiquitin chains at the RP by cryogenic electron microscopy (cryo-EM). Here, we present a cryo-EM-based protocol we applied to study the human 26S proteasome with ubiquitin chains by using non-cleavable M1-linked hexaubiquitin (M1-Ub 6 ) unanchored to a substrate. For complete details on the use and execution of this protocol, please refer to Chen et al. (2020)., Competing Interests: The authors declare no competing interests.

The 20S proteasome is a core particle of the eukaryotic proteasome responsible for proteolysis and is composed of layered α and β hetero-heptameric rings. The α7 subunit, which is one of components of the α ring, is known to self-assemble into a double-ringed homo-tetradecamer composed of two layers of the α7 heptameric ring. The α7 tetradecamer is known to disassemble upon the addition of α6 subunit, producing a 1:7 hetero-octameric α6-α7 complex. However, the detailed disassembly mechanism remains unclear. Here, we applied high-speed atomic force microscopy (HS-AFM) to dissect the disassembly process of the α7 double ring caused by interaction with the α6. HS-AFM movies clearly demonstrated two different modes of interaction in which the α6 monomer initially cracks at the interface between the stacked two α7 single rings and the subsequent intercalation of the α6 monomer in the open pore of the α7 single ring blocks the re-association of the single rings into the double ring. This result provides a mechanistic insight about the disassembly process of non-native homo-oligomers formed by proteasome components which is crucial for the initial process for assembly of 20S proteasome.

In eukaryotic cells, the ubiquitin-proteasome system (UPS) is responsible for the regulated degradation of intracellular proteins. The 26S holocomplex comprises the core particle (CP), where proteolysis takes place, and one or two regulatory particles (RPs). The base of the RP is formed by a heterohexameric AAA + ATPase module, which unfolds and translocates substrates into the CP. Applying single-particle cryo-electron microscopy (cryo-EM) and image classification to samples in the presence of different nucleotides and nucleotide analogs, we were able to observe four distinct conformational states (s1 to s4). The resolution of the four conformers allowed for the construction of atomic models of the AAA + ATPase module as it progresses through the functional cycle. In a hitherto unobserved state (s4), the gate controlling access to the CP is open. The structures described in this study allow us to put forward a model for the 26S functional cycle driven by ATP hydrolysis., Competing Interests: The authors declare no conflict of interest.

Accurate modeling of image formation in cryo-electron microscopy is an important requirement for quantitative image interpretation and optimization of the data acquisition strategy. Here we present a forward model that accounts for the specimen's scattering properties, microscope optics, and detector response. The specimen interaction potential is calculated with the isolated atom superposition approximation (IASA) and extended with the influences of solvent's dielectric and ionic properties as well as the molecular electrostatic distribution. We account for an effective charge redistribution via the Poisson-Boltzmann approach and find that the IASA-based potential forms the dominant part of the interaction potential, as the contribution of the redistribution is less than 10%. The electron wave is propagated through the specimen by a multislice approach and the influence of the optics is included via the contrast transfer function. We incorporate the detective quantum efficiency of the camera due to the difference between signal and noise transfer characteristics, instead of using only the modulation transfer function. The full model was validated against experimental images of 20S proteasome, hemoglobin, and GroEL. The simulations adequately predict the effects of phase contrast, changes due to the integrated electron flux, thickness, inelastic scattering, detective quantum efficiency and acceleration voltage. We suggest that beam-induced specimen movements are relevant in the experiments whereas the influence of the solvent amorphousness can be neglected. All simulation parameters are based on physical principles and, when necessary, experimentally determined., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The 26S proteasome is the end point of the ubiquitin-proteasome pathway and degrades ubiquitylated substrates. It is composed of the 20S core particle (CP), where degradation occurs, and the 19S regulatory particle (RP), which ensures substrate specificity of degradation. Whereas the CP is resolved to atomic resolution, the architecture of the RP is largely unknown. We provide a comprehensive analysis of the current structural knowledge on the RP, including structures of the RP subunits, physical protein-protein interactions, and cryoelectron microscopy data. These data allowed us to compute an atomic model for the CP-AAA-ATPase subcomplex. In addition to this atomic model, further subunits can be mapped approximately, which lets us hypothesize on the substrate path during its degradation.

Cryoelectron microscopy (cryo-EM) is an increasingly popular method to elucidate the structures of macromolecular complexes. However, in many applications the resolution of cryo-EM densities is limited to the low or intermediate resolution regime, that is, (10Å)(-1) or worse. Therefore, unambiguous molecular interpretation of cryo-EM densities requires efficient use of additional information, such as atomic structures of related subunits and protein-protein interaction data. Here, we describe how information from different sources can be combined to determine the approximate molecular architecture of complexes. Molecular dynamics based flexible fitting protocols allow subsequent refinement of the atomistic models., (Copyright © 2010 Elsevier Inc. All rights reserved.)

Proteasome-dependent proteolysis is essential for a number of key cellular processes and requires a sophisticated biogenesis pathway to function. Here, we have arrested the assembly process in its dynamic progression at the short-lived 16S state. Structural analysis of the 16S proteasome precursor intermediates by electron microscopy, and single particle analysis reveals major conformational changes in the structure of the beta-ring in comparison with one-half of the 20S proteasome. The individual beta-subunits in the 16S precursor complex rotate with respect to their positions in the x-ray crystallographic structure of the fully assembled 20S. This rearrangement results in a movement of the catalytic residue threonine-1 from the protected location in 16S precursor complexes to a more exposed position in the 20S structure. Thereby, our findings provide a molecular explanation for the structural rearrangements necessary for the dimerization of two 16S precursor complexes and the subsequent final maturation to active 20S proteasomes.