Abstract
We report the cloning and characterization of a Drosophila proteasome 11 S REGgamma (PA28) homolog. The 28-kDa protein shows 47% identity to the human REGgamma and strongly enhances the trypsin-like activities of both Drosophila and mammalian 20 S proteasomes. Surprisingly, the Drosophila REG was found to inhibit the proteasome's chymotrypsin-like activity against the fluorogenic peptide succinyl-LLVY-7-amino-4-methylcoumarin. Immunocytological analysis reveals that the Drosophila REG is localized to the nucleus but is distributed throughout the cell when nuclear envelope breakdown occurs during mitosis. Through site-directed mutagenesis studies, we have identified a functional nuclear localization signal present in the homolog-specific insert region. The Drosophila PA28 NLS is similar to the oncogene c-Myc nuclear localization motif. Comparison between uninduced and innate immune induced Drosophila cells suggests that the REGgamma proteasome activator has a role independent of the invertebrate immune system. Our results support the idea that gamma class proteasome activators have an ancient conserved function within metazoans and were present prior to the emergence of the alpha and beta REG classes.
Highlights
The proteasome is the major nonlysosomal protease present in eukaryotic cells and has a central role in regulating protein levels within the cell
The existence of such complexes has led to the proposal by Hendil et al that the REG complex (PA28) may participate in the degradation of intact proteins when associated in a hybrid proteasome
Our results are the first demonstration that nonmammalian REG sequences do code for functionally active REG complexes
Summary
The proteasome is the major nonlysosomal protease present in eukaryotic cells and has a central role in regulating protein levels within the cell. Proteasomes participate in the generation of peptide antigens for the major histocompatibility complex class I system [2]. The 11 S REG complex has recently been proposed to participate in a mixed ternary complex This complex consists of 20 S proteasomes associating both with 19 S regulatory and REG complexes [14]. A novel protein complex was characterized that was capable of enhancing proteasomal activity without the addition of ATP [13, 15] This activator was found to be composed of two subunits termed ␣ and  and was shown to have high similarity to a previously described nuclear protein: Ki autoantigen. The human REG␥ is able to markedly stimulate the trypsinlike activity of the 20 S proteasome in vitro, preferentially
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