DROSOPHILA PROTEASOME DM25 SUBUNIT SUBSTITUTES THE MOUSE MC3 SUBUNIT IN HYBRID PROTEASOMES - THE N-TERMINAL DOMAIN IS ESSENTIAL FOR SUBUNITINCORPORATION

The proteasome is a multisubunit 20 S proteinase complex involved in u biquitin-dependent and -independent intracellular protein metabolism. Individual subunits of the alpha- and beta-type share extensive sequen ce homology and are encoded as members of two related and evolutionari ly conserved gene families. Due to the lack of viable deletion mutants of essential alpha-type proteasome subunits in higher eukaryotes, an identification and analysis of potentially homologous subunits of diff erent species was so far not possible. It is shown here that the novel Drosophila alpha-type Dm25 subunit can be incorporated into mouse pro teasomes of stably transfected NIH 3T3 cells. The Dm25 subunit is able to substitute the mouse MC3 alpha-type subunit in proteasomes, indica ting a high structural and possibly also functional homology of the tw o subunits. In contrast and pointing at the importance of the slightly hydrophobic N-terminal region stabile expression of a Dm25 subunit, w hich is truncated at its N terminus and lacks PROS box I, results in a subunit which cannot be incorporated into mouse proteasomes. The abil ity to form hybrid proteasomes involving essential nondeletable subuni ts now opens the possibility for structural and also functional analys is of such subunits by mutagenesis in higher eukaryotes.