Studies of the role of ubiquitination in the interaction of ubiquilin with the loop and carboxyl terminal regions of presenilin-2.
Ubiquilin was originally identified as a presenilin-interacting protein. We previously reported that ubiquilin interacts with both the loop and carboxyl terminus of presenilin proteins and that the ubiquitin-associated (UBA) domain of ubiquilin, which binds poly ubiquitin chains, is important for mediating this interaction. In the present study, we examined whether ubiquitination of presenilin-2 (PS2) is required for interaction with ubiquilin-1 by mutating lysine residues that may be targets for ubiquitination in the presenilin loop and carboxyl terminus regions. Mutation of two lysine residues in the PS2-loop region suggested that ubiquitination is not required for interaction with ubiquilin-1 and may, in fact, even negatively regulate the interaction. Similarly, we found that ubiquitination of the PS2 carboxyl terminus (PS2-C-terminus) is not required for interaction with ubiquilin-1, although our results suggest that it could play some role. Instead, we found that the mutation of either one of the two lysine residues in the carboxyl terminus of PS2 or the proline residues in the highly conserved PALP motif in this region results in destabilization of the mutant PS2 polypeptides because of increased degradation by the proteasome. Furthermore, by GST-pull-down assays we found that the mutant polypeptides were unable to bind ubiquilin, suggesting that loss of ubiquilin interaction leads to destabilization of presenilin polypeptides. Paradoxically, however, knockdown of ubiquilin expression by RNA interference did not alter the rate of turnover of PS2 proteins in cells. Instead, we found that PS2 synthesis was reduced, and PS2 fragment production was increased, suggesting that ubiquilin expression modulates biogenesis and endoproteolysis of presenilin proteins.
Ford
Diana L
DL
Medical Biotechnology Center, Program in Neurodegenerative Diseases, University of Maryland Biotechnology Institute, 725 West Lombard Street, Baltimore, Maryland 21201, UAS.
Monteiro
Mervyn J
MJ
D048868:Adaptor Proteins, Signal Transducing; D000071183:Autophagy-Related Proteins; D001665:Binding Sites; D002352:Carrier Proteins / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000378:metabolism*; D002460:Cell Line; D006801:Humans; D047468:Immunoprecipitation; D008239:Lysine / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D053766:Presenilin-2 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011392:Proline / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D011485:Protein Binding; D034741:RNA, Small Interfering / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic; D020798:Two-Hybrid System Techniques
D048868:Adaptor Proteins, Signal Transducing; D000071183:Autophagy-Related Proteins; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D053766:Presenilin-2; D034741:RNA, Small Interfering; D029701:Saccharomyces cerevisiae Proteins; C107792:UBQLN1 protein, human; D011392:Proline; D046988:Proteasome Endopeptidase Complex; D008239:Lysine