Characterisation of the newly identified human Ump1 homologue POMP and analysis of LMP7(β5i) incorporation into 20 S proteasomes

Biogenesis of mammalian 20 S proteasomes occurs via precursor complexes containing α and unprocessed β subunits. A human homologue of the yeast proteasome maturation factor Ump1 was identified in 2D gels of 16 S precursor preparations and designated as POMP (proteasome maturation protein). We show that POMP is detected only in precursor fractions and not in fractions containing mature 20 S proteasome. Northern blot experiments revealed that expression of POMP is induced after treatment with interferon γ. To analyse the role of the β5 propeptide for proper maturation and incorporation of the β5 subunit into the complex, human T2 cells, which highly express derivatives of the β5i subunit (LMP7), were studied. In contrast to yeast, the presence of the β5 propeptide is not essential for incorporation of LMP7 into the proteasome complex. Mutated LMP7 subunits either carrying the prosequence of β2i (LMP2) or containing a mutation in the active threonine site are incorporated like wild-type LMP7, while a LMP7 derivative lacking the prosequence completely is incorporated to a lesser extent. Although the absence of the prosequence does not affect incorporation of LMP7, its deletion leads to delayed proteasome maturation and thereby to an accumulation of precursor complexes. As a result of the precursor accumulation, an increased amount of the POMP protein can be detected in these cells.