Minimal length requirement for proteasomal degradation of ubiquitin-dependent substrates

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.-Verhoef, L. G. G. C., Heinen, C., Selivanova, A., Halff, E. F., Salomons, F. A., Dantuma, N. P. Minimal length requirement for proteasomal degradation of ubiquitin-dependent substrates. FASEB J. 23, 123-133 (2009)