A Snf1-related nutrient-responsive kinase antagonizes endocytosis in yeast

Endocytosis is regulated in response to changing environmental conditions to adjust plasma membrane (PM) protein composition for optimal cell growth. Protein networks involved in cargo capture and sorting, membrane sculpting and deformation, and vesicle scission have been well-characterized, but less is known about the networks that sense extracellular cues and relay signals to trigger endocytosis of specific cargo. Hal4 and Hal5 are yeast Snf1-related kinases that were previously reported to regulate nutrient transporter stability by an unknown mechanism. Here we demonstrate that loss of Hal4 and Hal5 activates endocytosis of many different kinds of PM proteins, including Art1-mediated and Art1-independent endocytic events. Acute inhibition of Hal5 in the absence of Hal4 triggers rapid endocytosis, suggesting that Hal kinases function in a nutrient-sensing relay upstream of the endocytic response. Interestingly, Hal5 localizes to the PM, but shifts away from the cell surface in response to stimulation with specific nutrients. We propose that Hal5 functions as a nutrient-responsive regulator of PM protein stability, antagonizing endocytosis and promoting stability of endocytic cargos at the PM in nutrient-limiting conditions.Author summary: Cellular homeostasis, a fundamental requirement for all living organisms, is maintained in part through evolutionarily conserved mechanisms that regulate the abundance and activity of ion and nutrient transporters at the cell surface. These mechanisms often incorporate signaling networks that sense changes in the environment and relay signals to alter protein composition at the plasma membrane, often by inducing endocytosis of specific transporters in order to adjust and optimize transport activities at the cell surface. Here, we investigate two kinases in yeast–Hal4 and Hal5 –that are related to the yeast and human AMP sensing kinases. Loss of both Hal4 and Hal5 was previously reported to result in destabilization of ion and nutrient transporters by an unknown mechanism. Our data indicates that Hal kinases function broadly in the regulation of many different classes of endocytic cargo. Hal5 localizes to the plasma membrane in a manner that is responsive to nutrient availability and acute loss of Hal5 activity triggers rapid internalization of endocytic cargo. By uncovering a role for Hal5 as a nutrient-responsive regulator of endocytosis, this research sheds light on how signaling molecules regulate membrane trafficking events to coordinate adaptive growth responses.