Time flies… Our very first publication reporting a potential role of ubiquitin in downregulation of plasma membrane transporters was released in 1995, so 20 years ago. Claudine Hein, a PhD student of the lab, constructed a myc-tagged version of the Gap1 permease, allowing her to show that the inhibition of Gap1 activity observed after addition of ammonium coincides with degradation of the protein. This result surprised us as the inhibitory effect of ammonium on Gap1 was thought to be reversible. This regulation of Gap1 by ammonium had been described in 1983 by Marcelle Grenson who also isolated a mutant strain, npi1 (nitrogen permease inactivator 1), in which this regulation is defective (Grenson, 1983). Jean-Yves Springael, another PhD student of the lab, used this mutant to clone the NPI1 gene and realized that it corresponds to the RSP5 gene just shown by Johan Huibregtse and coll. to code for a new type of ubiquitin ligase, but whose molecular function remained unclear (Huibregtse et al. 1995). Finally, the paper reports the data of a collaboration with the group of Rosine Haguenauer-Tsapis (J. Monod Institute, Paris) showing that Npi1/Rsp5 is also required for stress-induced degradation of the uracil permease, a process known to involve prior endocytosis of the protein. Altogether, these observations indicated that the yeast Npi1/Rsp5 ubiquitin ligase plays a central role in endocytosis of several transporters. Furthermore, Npi1/Rsp5 turned out to be conserved in mammals, suggesting that ubiquitin could serve as a signal for protein endocytosis in more complex species as well (what was later confirmed). This article, published in 1995 in the journal Molecular Microbiology, remains one of our best cited papers (306 citations in August 2015). Time flies …
