New insights into the human 26S proteasome function and regulation

The 26S proteasome is a protease complex essential for proteostasis and strict regulation of diverse critical physiological processes, the mechanisms of which are still not fully described. The human 26S proteasome purification was optimized without exogenous nucleotides, to preserve the endogenous nucleotide occupancy and conformation of its AAA-ATPase subunits. This unveiled important effects on the proteasome function and structure resulting from exposure to Ca2+ or Mg2+, with important physiological implications. This sample, with an added model degron designed to mimic the minimum canonical ubiquitin signal for proteasomal recognition, was analysed by high-resolution cryo-EM. Two proteasome conformations were resolved, with only one capable of degron binding. The structural data show that this occurs without major conformation rearrangements and allows to infer into the allosteric communication between ubiquitin degron binding and the peptidase activities. These results revise existing concepts on the 26S proteasome function and regulation, opening important opportunities for further research.