Mechanism of autocatalytic activation during proteasome assembly

Many large molecular machines are too elaborate to assemble spontaneously and are built through ordered pathways orchestrated by dedicated chaperones. During assembly of the core particle (CP) of the proteasome, where protein degradation occurs, its six active sites are simultaneously activated via cleavage of N-terminal propeptides. Such activation is autocatalytic and coupled to fusion of two half-CP intermediates, which protects cells by preventing activation until enclosure of the active sites within the CP interior. Here we uncover key mechanistic aspects of autocatalytic activation, which proceeds through alignment of the β5 and β2 catalytic triad residues, respectively, with these triads being misaligned before fusion. This mechanism contrasts with most other zymogens, in which catalytic centers are preformed. Our data also clarify the mechanism by which individual subunits can be added in a precise, temporally ordered manner. This work informs two decades-old mysteries in the proteasome field, with broader implications for protease biology and multisubunit complex assembly.

Rational design of proteasome inhibitors based on the structure of the endogenous inhibitor PI31/Fub1

Proteasome inhibitors are widely used anticancer drugs. The three clinically approved agents are modified small peptides that preferentially target one of the proteasome's three active sites (β5) at physiologic concentrations. In addition to these drugs, there is also an endogenous proteasome inhibitor, PI31/Fub1, that enters the proteasome's interior to simultaneously yet specifically inhibit all three active sites. Here, we have used PI31's evolutionarily optimized inhibitory mechanisms to develop a suite of potent and specific β2 inhibitors. The lead compound strongly inhibited growth of multiple myeloma cells as a standalone agent, indicating the compound's cell permeability and establishing β2 as a potential therapeutic target in multiple myeloma. The lead compound also showed strong synergy with the existing β5 inhibitor bortezomib; such combination therapies might help with existing challenges of resistance and severe side effects. These results represent an effective method for rational structure-guided development of proteasome inhibitors.

Structure of the preholoproteasome reveals late steps in proteasome core particle biogenesis

Assembly of the proteasome's core particle (CP), a barrel-shaped chamber of four stacked rings, requires five chaperones and five subunit propeptides. Fusion of two half-CP precursors yields a complete structure but remains immature until active site maturation. Here, using Saccharomyces cerevisiae, we report a high-resolution cryogenic electron microscopy structure of preholoproteasome, a post-fusion assembly intermediate. Our data reveal how CP midline-spanning interactions induce local changes in structure, facilitating maturation. Unexpectedly, we find that cleavage may not be sufficient for propeptide release, as residual interactions with chaperones such as Ump1 hold them in place. We evaluated previous models proposing that dynamic conformational changes in chaperones drive CP fusion and autocatalytic activation by comparing preholoproteasome to pre-fusion intermediates. Instead, the data suggest a scaffolding role for the chaperones Ump1 and Pba1/Pba2. Our data clarify key aspects of CP assembly, suggest that undiscovered mechanisms exist to explain CP fusion/activation, and have relevance for diseases of defective CP biogenesis.

Predictors of nutritional and inflammation risk in hemodialysis patients

BACKGROUND

Malnutrition and chronic inflammation are prevalent complications in hemodialysis (HD) patients. Different nutritional assessment tools are used to identify patients at risk. A composite and comprehensive malnutrition inflammation score (MIS) has been correlated with morbidity and mortality, and appears to be a robust and quantitative tool.

OBJECTIVES

Determine malnutrition risk profile in a sample of portuguese HD patients; determine the association of clinical and laboratory factors with MIS, and the impact of each parameter on MIS.

METHODS AND RESULTS

We performed, between September 15th of 2015 and January 31st of 2016, a cross sectional analysis of 2975 patients, representing 25% of portuguese HD patients. 59% were men (66.7 ± 14.8 years); 31% diabetic; 79% and 21% performed, respectively, high-flux HD and HDF. A MIS >5 was considered to indicate higher risk and was present in 1489 patients (50%). Amongst all parameters, comorbilities/dialysis vintage, transferrin, functional capacity, changes in body weight and decreased fat stores showed the higher impact, while albumin had one of the lowest impact on the nutritional risk.

MULTIVARIABLE ANALYSIS

Higher age (>75 years, OR 1.71, p < 0.001), diabetes (OR 1.25, p = 0.026), lower P levels (OR 1.57,p = 0.001), higher Ca levels (OR 1.51, p < 0.001), higher ERI (OR 1.05, p < 0.001), higher Kt/V (OR 2.14, p < 0.001) and higher CRP (OR 1.01, p < 0.001) were independently associated with a higher risk of MIS>5; higher nPNA (OR 0.29, p < 0.001) and higher Pcreat (OR 0.88, p < 0.001) were associated with a risk reduction of MIS>5 (95% CI).

CONCLUSIONS

Routine clinical and analytic parameters were found to be associated with MIS range that might indicate higher risk, and may represent a simple alert sign for the need of further assessments.