PubMed · 10782086
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R J Ellis. 2000. Chaperone substrates inside the cell.. https://doi.org/10.1016/s0968-0004(00)01576-0
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Structural and kinetic studies of a SUMORanGAP1-Ubc9-Nup358/RanBP2 complex (Reverter and Lima, 2005) provide the first high-resolution view of SUMO recognition by a SUMO binding motif and also reveal a novel mechanism for E3 ubiquitin-like protein ligases, with the Nup358/RanBP2 E3 teaming up with both SUMO and the E2 (Ubc9) to stimulate tagging.
Correct folding is essential to protein function, which has led to the evolution of sophisticated chaperone systems. Protein folding occurs primarily in the cytoplasm and in the endoplasmic reticulum (ER). The differing redox and ionic milieus inside these two compartments, and the different functions and destinations of the client proteins folded therein, have necessitated the existence of distinct chaperone networks. Both networks exploit the exquisite sensitivity of cysteines to redox state, but they respond in opposite directions, reflecting the different conditions in the cytosol (reducing) and in the ER (more oxidizing). Thus, the cytosolic chaperone Hsp33 forms active dimers in response to oxidation, linking the responses to thermal and oxidative stress, and allows the cell to "remember" the experience: Folded proteins are released upon Hsp33 reduction, whereas unfolded substrates are released only in the presence of additional chaperone complexes that are able to refold them. In contrast, the ER oxidoreductase protein disulphide isomerase (PDI) appears to function as a chaperone primarily when reduced. Owing to the reactivity of their thiol groups, cysteines provide molecular switches that can be used to control the folding and to reversibly modify the structure and function of a protein. Cysteine oxidation provides as versatile a system as protein phosphorylation for the modification of specific substrates and the propagation of signaling cascades. Moreover, it offers the important advantage that cysteines can undergo different modifications, thus providing a molecular code that rapidly reports and responds to redox changes in the environment.
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