Yeast nutrient transceptors provide novel insight in the functionality of membrane transporters

Abstract

In the yeast Saccharomyces cerevisiae several nutrient transporters have been identified that possess an additional function as nutrient receptor. These transporters are induced when yeast cells are starved for their substrate, which triggers entry into stationary phase and acquirement of a low protein kinase A (PKA) phenotype. Re-addition of the lacking nutrient triggers exit from stationary phase and sudden activation of the PKA pathway, the latter being mediated by the nutrient transceptors. At the same time, the transceptors are ubiquitinated, endocytosed and sorted to the vacuole for breakdown. Investigation of the signaling function of the transceptors has provided a new read-out and new tools for gaining insight into the functionality of transporters. Identification of amino acid residues that bind co-transported ions in symporters has been challenging because the inactivation of transport by site-directed mutagenesis is not conclusive with respect to the cause of the inactivation. The discovery of nontransported agonists of the signaling function in transceptors has shown that transport is not required for signaling. Inactivation of transport with maintenance of signaling in transceptors supports that a true proton-binding residue was mutagenised. Determining the relationship between transport and induction of endocytosis has also been challenging, since inactivation of transport by mutagenesis easily causes loss of all affinity for the substrate. The use of analogues with different combinations of transport and signaling capacities has revealed that transport, ubiquitination and endocytosis can be uncoupled in several unexpected ways. The results obtained are consistent with transporters undergoing multiple substrate-induced conformational changes, which allow interaction with different accessory proteins to trigger specific downstream events.

A novel approach using C. elegans DNA damage-induced apoptosis to characterize the dynamics of uptake transporters for therapeutic drug discoveries

“…Conversely, when nutrients are plentiful OCT-1 might function to sustain the basal expression of OCT-2. Precedence for this mode of regulation exists in S. cerevisiae , Drosophila melanogaster and Homo sapiens 41. For instance, in S. cerevisiae the Ssy1 sensor, a plasma membrane protein belonging to the amino acid permease family, is endowed with no, or only limited, transport function42.…”

Key Residues and Phosphate Release Routes in the Saccharomyces cerevisiae Pho84 Transceptor

“…Furthermore, Pho84 activates the protein kinase A (PKA) pathway, which senses and signals the uptake of external P i in phosphate-starved cells. This dual functionality of Pho84 has led to its classification as a transceptor (6). A three-dimensional model of Pho84 in the open inward-facing conformation has been created by homology modeling using the glycerol 3-phosphate transporter GlpT as a template (7).…”

Premature termination ofGAT1transcription explains paradoxical negative correlation between nitrogen-responsive mRNA, but constitutive low-level protein production

“…Fine tuning of nitrogen metabolism, allowing yeast cells to make the most of a plentiful nitrogen supply or cope with a very poor one, is achieved at the transcriptional and posttranslational levels. Posttranslational control targets the activity of amino acid permeases by controlling their modification, internalization and vacuolar degradation ( 13,14 ; reviewed in 15,16 ), whereas transcriptional control restrains the production of enzymes and permeases needed to utilize non-preferred, poor nitrogen sources when readily usable, good nitrogen sources are available (for recent reviews, see [17][18][19] ). Four GATAfamily transcription factors are central to this latter control: 2 activators, Gln3 and Gat1/Nil1 and 2 repressors, Dal80/Uga43 and Gzf3/Deh1/Nil2.…”