In addition, the ATPase subunits supply energy for unfolding target proteins, so that they can be translocated into the interior cavities of 20S proteasomes, where the active sites are located.

The lid subcomplex consists of multiple non-ATPase subunits (Rpn3, Rpn5 to -9, Rpn11, Rpn12, and Rpn15).

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Polyubiquitinated proteins are also recognized directly by the 19S proteasome subunit Rpn10 (3).

Rpn10 is composed of one N-terminal von Willebrand factor A (VWA) domain and one or two C-terminal ubiquitin-interacting motifs (UIM).

Rpn10 is a subunit of the 26S proteasome that recognizes polyubiquitinated proteins.

The importance of Rpn10 in ubiquitin-mediated proteolysis is debatable, since a deficiency of Rpn10 causes different phenotypes in different organisms.

To date, the role of mammalian Rpn10 has not been examined genetically.

Moreover, vertebrates have five splice variants of Rpn10 whose expressions are developmentally regulated, but their biological significance is not understood.

Rpn10 was the first protein recognized to bind to polyubiquitin chains through UIM domains.

Surprisingly, genetic studies with yeast showed that deletion of the gene resulted in little loss of viability, in contrast to most other proteasome genes, which are essential for life (6, 43).

There are three UBL-UBA proteins in budding yeast called Rad23, Ddi1, and Dsk2 (2, 8, 34).

Furthermore, several UBL-UBA proteins are also found in mammals, and some of them act in a manner similar to that of their yeast counterparts (12, 21).

Finally, a yeast strain that has mutations in the UIM domain of Rpn10 showed a clear deficiency in ubiquitin chain recognition, and this Δ mutation, indicating that the UIM domain of Rpn10 and UBL-UBA proteins work redundantly (4).