Di-ubiquitin modification of ubiquitin ligase adaptors in membrane protein downregulation

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PROJECT SUMMARY/ABSTRACT Post-translational modification by ubiquitin is an essential mechanism to alter protein function in eukaryotes. Ubiquitin, a 76 amino acid protein, is attached to specific proteins via a cascade of ubiquitin activating enzyme E1, conjugating enzyme E2, and ubiquitin ligase E3. Ubiquitination plays an essential role in a broad aspect of cellular processes, including transcription, DNA repair, signal transduction, autophagy, cell cycle, immune response, and membrane trafficking. Aberration in the ubiquitination system leads to a number of human diseases, such as neurodegenerative diseases and cancers. Within the ubiquitination cascade, E3s primarily dictate the specificity of the ubiquitination system and thus are often the focal points of research and attractive therapeutic targets. The Nedd4 family E3s are an essential family of HECT-type E3s, members of which contain an N-terminal C2 domain followed by 2-4 WW domains and the C-terminal HECT domain. Nedd4 E3s recognize substrates carrying a “PPxY” motif through their WW domains. However, most substrates lack such a motif but engage with the ligase through “PPxY” motif-containing adaptors. We recently discovered that in yeast, the ubiquitin E3 ligase adaptor protein Art1 is primed with di-ubiquitination and the attachment of the di-Ub chain to a specific lysine residue in Art1 is warranted for its full activity. In this proposal, we plan to investigate the physiological function of adaptor di-ubiquitination and to elucidate the molecular mechanisms of the modular ubiquitination platform form with Nedd4 E3 ligase and di-ubiquitinated adaptors. Specifically, we will pursue the following aims: Aim 1: To investigate the mechanism of Nedd4 E3 adaptor di-ubiquitination. Aim 2: To determine the role of di-ubiquitinated adaptor-E3 complexes in substrate ubiquitination. Aim 3: To elucidate the molecular architecture of di-ubiquitinated adaptors with the Nedd4 E3 ligases. Uncovering the physiological role of Need4 E3 adaptors di-ubiquitination will be of critical importance to understand the molecular basis of how E3 adaptors specifically recognize substrate proteins and efficiently present the substrates for ubiquitination by HECT E3 ligases. We expect the successful implementation of this proposal will not only make significant contributions to the understanding of the molecular mechanisms underlying the Nedd4 E3 ligase/adaptor mediated ubiquitination, but also shed light on the mechanism of protein quality control governed by targeted ubiquitination.