The <scp>A</scp>rabidopsis mitochondrial membrane‐bound ubiquitin protease <scp>UBP</scp>27 contributes to mitochondrial morphogenesis

<jats:title>Summary</jats:title><jats:p>Mitochondria are essential organelles with dynamic morphology and function. Post‐translational modifications (<jats:styled-content style="fixed-case">PTM</jats:styled-content>s), which include protein ubiquitination, are critically involved in animal and yeast mitochondrial dynamics. How <jats:styled-content style="fixed-case">PTM</jats:styled-content>s contribute to plant mitochondrial dynamics is just beginning to be elucidated, and mitochondrial enzymes involved in ubiquitination have not been reported from plants. In this study, we identified an <jats:styled-content style="fixed-case">A</jats:styled-content>rabidopsis mitochondrial localized ubiquitin protease, <jats:styled-content style="fixed-case">UBP</jats:styled-content>27, through a screen that combined bioinformatics and fluorescent fusion protein targeting analysis. We characterized <jats:styled-content style="fixed-case">UBP</jats:styled-content>27 with respect to its membrane topology and enzymatic activities, and analysed the mitochondrial morphological changes in <jats:italic><jats:styled-content style="fixed-case">UBP</jats:styled-content>27</jats:italic><jats:styled-content style="fixed-case">T</jats:styled-content>‐<jats:styled-content style="fixed-case">DNA</jats:styled-content> insertion mutants and overexpression lines. We have shown that <jats:styled-content style="fixed-case">UBP</jats:styled-content>27 is embedded in the mitochondrial outer membrane with an <jats:styled-content style="fixed-case">N</jats:styled-content><jats:sub>in</jats:sub>–<jats:styled-content style="fixed-case">C</jats:styled-content><jats:sub>out</jats:sub> orientation and possesses ubiquitin protease activities <jats:italic>in vitro</jats:italic>. <jats:styled-content style="fixed-case">UBP</jats:styled-content>27 demonstrates similar sub‐cellular localization, domain structure, membrane topology and enzymatic activities with two mitochondrial deubiquitinases, yeast <jats:styled-content style="fixed-case">S</jats:styled-content>c<jats:styled-content style="fixed-case">UBP</jats:styled-content>16 and human <jats:styled-content style="fixed-case">H</jats:styled-content>s<jats:styled-content style="fixed-case">USP</jats:styled-content>30, which indicated that these proteins are functional orthologues in eukaryotes. Although loss‐of‐function mutants of <jats:italic><jats:styled-content style="fixed-case">UBP</jats:styled-content>27</jats:italic> do not show obvious phenotypes in plant growth and mitochondrial morphology, <jats:italic><jats:styled-content style="fixed-case">UBP</jats:styled-content>27</jats:italic> overexpression can change mitochondrial morphology from rod to spherical shape and reduce the mitochondrial association of dynamin‐related protein 3 (<jats:styled-content style="fixed-case">DRP</jats:styled-content>3) proteins, large <jats:styled-content style="fixed-case">GTP</jats:styled-content>ases that serve as the main mitochondrial fission factors. Thus, our study has uncovered a plant ubiquitin protease that plays a role in mitochondrial morphogenesis possibly through modulation of the function of organelle division proteins.</jats:p>