Quality Control and Substrate‐Dependent Downregulation of the Nutrient Transporter Fur4

Abstract

Upon exposure to stress conditions, unfolded cell-surface nutrient transporters are rapidly internalized and degraded via the multivesicular body (MVB) pathway. Similarly, high concentrations of nutrients result in the downregulation of the corresponding transporters. Our studies using the yeast transporter Fur4 revealed that substrate-induced downregulation and quality control utilize a common mechanism. This mechanism is based on a conformation-sensing domain, termed LID (Loop Interaction Domain), that regulates site-specific ubiquitination (also known as degron). Conformational alterations in the transporter induced by unfolding or substrate binding are transmitted to the LID, rendering the degron accessible for ubiquitination by Rsp5. As a consequence, the transporter is rapidly degraded. We propose that the LID-degron system is a conserved, chaperone-independent mechanism responsible for conformation-induced downregulation of many cell-surface transporters under physiological and pathological conditions.

Substrate-Induced Ubiquitylation and Endocytosis of Yeast Amino Acid Permeases

“…This is reminiscent of what happens with the uric acid and xanthine permease (UapA) of A. nidulans, which undergoes ubiquitylation and endocytosis in response to Am but also to transport of its substrates (24). Substrate transport-induced endocytosis has also been demonstrated in the case of Fur4, the yeast uracil permease (23,25). The situation of Gap1 is more complex, since amino acids provided to the cells simultaneously stimulate two downregulation pathways, the TORC1/Npr1 pathway responding to internal amino acids and the pathway elicited by Gap1-mediated transport (Fig.…”

“…That transport catalysis might induce the Ub-dependent endocytosis of specific yAAPs is conceivable, as such a mechanism has been described for at least two fungal permeases, the uracil permease Fur4 (23) and the uric acid/xanthine permease UapA of Aspergillus nidulans (24). Recent work suggests that uracil-induced ubiquitylation of Fur4 is due to a conformational change causing disruption of an interaction between the membrane-proximal N-terminal tail and internal loops of the permease, thereby rendering N-terminal lysines more accessible to ubiquitylation by Rsp5 (25).…”

Ubiquitin-Dependent Sorting in Endocytosis

“…One interesting possibility is that properly folded proteins are induced to form a similar unfolded "degron"-like motif as a trigger for their ubiquitination and down-regulation in response to particular stimuli. For instance, transporter activity itself induces conformational changes that expose arrestin/Rsp5-binding sites and these might mimic small disordered protein segments thought to be exposed upon damaging (Cain and Kaiser 2011;Merhi and Andre 2012;Keener and Babst 2013). Once ubiquitinated, these proteins follow an endosomal sorting complex required for transport (ESCRT)-dependent trafficking route into multivesicular bodies and onto lysosomes for degradation.…”

Rsp5/Nedd4 clears cells of heat-damaged proteins

“…Conformational changes in Fur4, a high affinity uracil importer, triggers ubiquitylation by Rsp5 and transport to lysosomes 13 . The amino terminal part of Fur4 encompasses sequence elements sensing the conformational status of the protein.…”

“…A topologically related methionine transporter Mup1, which lacks such a PY--like stretch, also undergoes Rsp5--dependent internalization. Yet unlike Fur4, the efficient down--regulation of Mup1 relies on Rsp5 adaptor proteins of the Art family 13,14 .…”