Choline Transport Activity Regulates Phosphatidylcholine Synthesis through Choline Transporter Hnm1 Stability

Background: Yeast cells grown in choline exhibit a reduction in the level of the choline transporter Hnm1. Results: Hnm1 transporter activity triggers its endocytosis by a mechanism dependent on the ubiquitin ligase Rsp5 and the casein kinases Yck1/2. Conclusion: Choline transport activity affects the stability of the Hnm1 choline transporter. Significance: Regulation of choline transporter activity constitutes an important regulatory node for phosphatidylcholine homeostasis. Choline is a precursor for the synthesis of phosphatidylcholine through the CDP-choline pathway. Saccharomyces cerevisiae expresses a single high affinity choline transporter at the plasma membrane, encoded by the HNM1 gene. We show that exposing cells to increasing levels of choline results in two different regulatory mechanisms impacting Hnm1 activity. Initial exposure to choline results in a rapid decrease in Hnm1-mediated transport at the level of transporter activity, whereas chronic exposure results in Hnm1 degradation through an endocytic mechanism that depends on the ubiquitin ligase Rsp5 and the casein kinase 1 redundant pair Yck1/Yck2. We present details of how the choline transporter is a major regulator of phosphatidylcholine synthesis.