Identification of Proteassemblin, a Mammalian Homologue of the Yeast Protein, Ump1p, That Is Required for Normal Proteasome Assembly

We have identified a mammalian homologue of yeast Ump1p by searching for similar proteins in human and mouse expressed sequence tag (EST) databases. Ump1p is an accessory protein that is required for normal proteasome assembly in yeast (1). A mammalian homologue, which we refer to as “proteassemblin,” is a constituent of proteasome assembly intermediates (preproteasomes), but not fully assembled 20S proteasomes, as is Ump1p in yeast. We also provide evidence that proteassemblin is a constituent of pre-immunoproteasomes that contain the precursor of the interferon-γ-inducible subunit LMP2. By analogy with Ump1p, we hypothesize that proteassemblin is required for normal mammalian proteasome assembly.