Cooperative Transport Mechanism Of Human Monocarboxylate Transporter 2

Monocarboxylates play important roles in metabolism in living cells.They participate in the energy metabolism and biosynthesis.The concentrations of monocarboxylates should be strictly controlled through monocarboxylate transporters by cells.In glycolytically active cells,proton-linked monocarboxylate transporters(MCTs)can efficiently transport monocarboxylates to maintain the proper metabolism for cells.However,in glycolytically inactive cells,the transport activity of MCTs should be reduced or even inactive to maintain a physiological monocarboxylate concentration.To demonstrate how MCTs maintain the monocarboxylate homeostasis,we studied and analyzed the transport activity of human monocarboxylate transporter 2(MCT2).Based on our functional study,the transport activity of MCT2 dependents on the concentration of intracellular substrate concentration.This property means MCT2 may transport substrate cooperatively.We solved the structure of human MCT2 at 3.8 ? with cryo-electron microscopy.By analyzing the cryo-EM structure,we found that human MCT2 has an unusual dimerization.The N-and C-terminal domains of MCT2 subunits are involved in dimerization.This unusual form of dimerization may explain why MCT2 transports substrate cooperatively.Structure-based mutagenesis in combination with pyruvate transport assay provides insights into the key region responsible for the cooperative transport of MCT2.With molecular docking and functional study,we also determined the key residues responsible for substrate recognition and proton coupling of MCT2.Based on the structure comparison between MCT2 and SfMCT,we finally proposed a model of cooperative transport in human MCT2.