title: Minimal length requirement for proteasomal degradation of ubiquitin-dependent substrates.
abstract: An erroneous transcriptional process, known as molecular misreading, gives rise to an alternative transcript of the ubiquitin B (UBB) gene. This transcript encodes the protein UBB(+1), which comprises a ubiquitin moiety and a 19-aa C-terminal extension. UBB(+1) is found in affected neurons in neurodegenerative diseases and behaves as an atypical ubiquitin fusion degradation (UFD) proteasome substrate that is poorly degraded and impedes the ubiquitin/proteasome system. Here, we show that the limited length of UBB(+1) is responsible for its inefficient degradation and inhibitory activity. Designed UFD substrates with an equally short 19-aa or a 20-aa C-terminal extension were also poorly degraded and had a general inhibitory activity on the ubiquitin/proteasome system in two unrelated cell lines. Extending the polypeptide to 25 aa sufficed to convert the protein into an efficiently degraded proteasome substrate that lacked inhibitory activity. A similar length dependency was found for degradation of two UFD substrates in Saccharomyces cerevisiae, which suggests that the mechanisms underlying this length constraint are highly conserved. Extending UBB(+1) also converted this protein into an efficient substrate of the proteasome. These observations provide an explanation for the accumulation of UBB(+1) in neurodegenerative disorders and offers new insights into the physical constraints determining proteasomal degradation.
authors.affiliation: Department of Cell and Molecular Biology, The Medical Nobel Institute, Karolinska Institutet, Von Eulers väg 3, S-17177 Stockholm, Sweden.
authors.forename: Lisette G G C
authors.initials: LG
authors.lastname: Verhoef
authors.forename: Christian
authors.initials: C
authors.lastname: Heinen
authors.forename: Alexandra
authors.initials: A
authors.lastname: Selivanova
authors.forename: Els F
authors.initials: EF
authors.lastname: Halff
authors.forename: Florian A
authors.initials: FA
authors.lastname: Salomons
authors.forename: Nico P
authors.initials: NP
authors.lastname: Dantuma
mesh_terms: D000595:Amino Acid Sequence; D045744:Cell Line, Tumor; D006801:Humans; D008969:Molecular Sequence Data; D009154:Mutation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity; D014158:Transcription, Genetic; D025801:Ubiquitin / Q000235:genetics* / Q000378:metabolism*
chemical_list: D025801:Ubiquitin; D046988:Proteasome Endopeptidase Complex