Ubiquitylation on canonical and non-canonical sites targets the transcription factor neurogenin for ubiquitin-mediated proteolysis.

Polyubiquitylation targets multiple proteins for degradation by the proteasome. Typically, the first ubiquitin is linked to lysine residues in the substrate for degradation via an isopeptide bond, although rarely ubiquitin linkage to the N-terminal residue has also been observed. We have recently shown that Neurogenin (NGN), a basic helix-loop-helix transcription factor that plays a central role in regulating neuronal differentiation, is degraded by ubiquitin-mediated proteolysis. We have taken a biochemical and mutagenesis approach to investigate sites of ubiquitylation of NGN, initially using extracts of eggs from the frog Xenopus laevis as a source of ubiquitylation and degradation components. NGN can be targeted for destruction by ubiquitylation via lysines or the N terminus. However, we see that a modified NGN, where canonical lysine ubiquitylation and N-terminally linked ubiquitylation are prevented, is nevertheless ubiquitylated and degraded by the proteasome. We show that polyubiquitin chains covalently attach to non-canonical cysteine residues in NGN, and these non-canonical linkages alone are capable of targeting NGN protein for destruction. Importantly, canonical and non-canonical ubiquitylation occurs simultaneously in the native protein and may differ in importance for driving degradation in interphase and mitosis. We conclude that native NGN is ubiquitylated on multiple canonical and non-canonical sites by cellular ubiquitin ligases, and all types of linkage can contribute to protein turnover.

Regulation of neurogenin stability by ubiquitin-mediated proteolysis.

NGN (neurogenin), a proneural bHLH (basic helix-loop-helix) transcription factor, plays a central role in promoting neuronal specification and differentiation in many regions of the central nervous system. NGN activity has been shown extensively to be controlled at the transcriptional level. However, in addition, recent findings have indicated that the levels of NGN protein may also be regulated. In the present study, we have demonstrated that NGN protein stability was regulated in both Xenopus embryos and P19 embryonal carcinoma cells, a mammalian neuronal model system. In both systems, NGN was a highly unstable protein that was polyubiquitinated for destruction by the proteasome. NGN binds to DNA in complex with its heterodimeric E-protein partners E12 or E47. We observed that NGN was stabilized by the presence of E12/E47. Moreover, NGN was phosphorylated, and mutation of a single threonine residue substantially reduced E12-mediated stabilization of NGN. Thus E-protein partner binding and phosphorylation events act together to stabilize NGN, promoting its accumulation when it can be active.