Characterisation of the newly identified human Ump1 homologue POMP and analysis of LMP7(beta 5i) incorporation into 20 S proteasomes

Biogenesis of mammalian 20 S proteasomes occurs via precursor complexes con taining a and unprocessed beta subunits. A human homologue of the yeast pro teasome maturation factor Ump1 was identified in 2D gels of 16 S precursor preparations and designated as POMP (proteasome maturation protein). We sho w that POMP is detected only in precursor fractions and not in fractions co ntaining mature 20 S proteasome. Northern blot experiments revealed that ex pression of POMP is induced after treatment with interferon gamma. To analy se the role of the beta 5 propeptide for proper maturation and incorporatio n of the beta 5 subunit into the complex, human T2 cells, which highly expr ess derivatives of the beta 5i subunit (LMP7), were studied. In contrast to yeast, the presence of the beta 5 propeptide is not essential for incorpor ation of LMP7 into the proteasome complex. Mutated LMP7 subunits either car rying the prosequence of beta 2i (LMP2) or containing a mutation in the act ive threonine site are incorporated like wild-type LMP7, while a LMP7 deriv ative 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 accumula tion, an increased amount of the POMP protein can be detected in these cell s. (C) 2000 Academic Press.