Arabidopsis membrane-anchored ubiquitin-fold (MUB) proteins localize a specific subset of ubiquitin-conjugating (E2) enzymes to the plasma membrane.

The covalent attachment of ubiquitin (Ub) to various intracellular proteins plays important roles in altering the function, localization, processing, and degradation of the modified target. A minimal ubiquitylation pathway uses a three-enzyme cascade (E1, E2, and E3) to activate Ub and select target proteins for modification. Although diverse E3 families provide much of the target specificity, several factors have emerged recently that coordinate the subcellular localization of the ubiquitylation machinery. Here, we show that the family of membrane-anchored ubiquitin-fold (MUB) proteins recruits and docks specific E2s to the plasma membrane. Protein interaction screens with Arabidopsis MUBs revealed that interacting E2s are limited to a well defined subgroup that is phylogenetically related to human UbcH5 and yeast Ubc4/5 families. MUBs appear to interact noncovalently with an E2 surface opposite the active site that forms a covalent linkage with Ub. Bimolecular fluorescence complementation demonstrated that MUBs bind simultaneously to the plasma membrane via a prenyl tail and to the E2 in planta. These findings suggest that MUBs contribute subcellular specificity to ubiquitylation by docking the conjugation machinery to the plasma membrane.