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SummaryIn this work we unmask a novel downregulation mechanism of the uric acid/xanthine transporter UapA, the prototype member of the ubiquitous Nucleobase-Ascorbate Transporter family, directly related to its function. In the presence of substrates, UapA is endocytosed, sorted into the multivesicular body pathway and degraded in vacuoles. Substrateinduced endocytosis, unlike ammonium-induced turnover, is absolutely dependent on UapA activity and several lines of evidence showed that the signal for increased endocytosis is the actual translocation of substrates through the UapA protein. The use of several UapA functional mutants with altered kinetics and specificity has further shown that transportdependent UapA endocytosis occurs through a mechanism, which senses subtle conformational changes associated with the transport cycle. We also show that distinct mechanisms of UapA endocytosis necessitate ubiquitination of a single Lys residue (K572) by HulA Rsp5. Finally, we demonstrate that in the presence of substrates, non-functional UapA versions can be endocytosed in trans if expressed in the simultaneous presence of active UapA versions, even if the latter cannot be endocytosed themselves.

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Processing bodies (PBs) and stress granules (SGs) are related, cytoplasmic RNA-protein complexes that contribute to post-transcriptional gene regulation in all eukaryotic cells. Both structures contain translationally repressed mRNAs and several proteins involved in silencing, stabilization or degradation of mRNAs, especially under environmental stress. Here, we monitored the dynamic formation of PBs and SGs, in somatic cells of adult worms, using fluorescently tagged protein markers of each complex. Both complexes were accumulated in response to various stress conditions, but distinct modes of SG formation were induced, depending on the insult. We also observed an age-dependent accumulation of PBs but not of SGs. We further showed that direct alterations in PB-related genes can influence aging and normal stress responses, beyond their developmental role. In addition, disruption of SG-related genes had diverse effects on development, fertility, lifespan and stress resistance of worms. Our work therefore underlines the important roles of mRNA metabolism factors in several vital cellular processes and provides insight into their diverse functions in a multicellular organism.