How Phosphorylation by PINK1 Remodels the Ubiquitin System: A Perspective from Structure and Dynamics.

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Ubiquitin is an important signaling protein in cells. It functions by covalent attachment to substrate proteins and by noncovalent interactions with target proteins. Ubiquitins are also concatenated, and the resulting polyubiquitins recognize target proteins multivalently with enhanced specificity. The function of ubiquitin is enabled by the conformational dynamics of ubiquitin and polyubiquitins, which spans over 12 orders of magnitude in a time scale. Recently, it was found that ubiquitin can be phosphorylated by PINK1 at residues S65 and T66. Only sparsely populated for the unmodified ubiquitin, a C-terminally retracted conformation is stabilized for phosphorylated ubiquitin and is further enriched at an increasing pH. The modulation of tertiary structure further impacts the quaternary arrangements of ubiquitin subunits in polyubiquitins. Additionally, ubiquitin phosphorylation inhibits the activities of many enzymes responsible for attaching and removing polyubiquitins, thus remodeling the composition and length of polyubiquitins. The phosphorylation-remolded polyubiquitins can then recognize different target proteins. As PINK1 and ubiquitin phosphorylation levels are up-regulated under certain pathophysiological conditions, the remodeled ubiquitin system may be involved in the divergence of cell fate.