Substrate-mediated remodeling of methionine transport by multiple ubiquitin-dependent mechanisms in yeast cells.

Plasma membrane transport of single amino-acid methionine in yeast is shown to be mediated by at least seven different permeases whose activities are transcriptionaly and post-transcriptionaly regulated by different ubiquitin-dependent mechanisms. Upon high extracellular methionine exposure, three methionine-permease genes are repressed while four others are induced. SCFMet30, SCFGrr1 and Rsp5 ubiquitin ligases are the key actors of the ubiquitin-dependent remodeling of methionine transport. In addition to regulating the activity of Met4, the SCFMet30 ubiquitin ligase is shown to convey an intracellular signal to a membrane initiated signaling pathway by controlling the nuclear concentration of the Stp1 transcription factor. By coupling intra- and extracellular metabolite sensing, SCFMet30 thus allows yeast cells to accurately adjust the intermediary sulfur metabolism to the growth conditions. The multiple ubiquitin-dependent mechanisms that function in methionine transport regulation further exemplify the pervasive role of ubiquitin in the adaptation of single-cell organisms to environmental modifications.