Milton/Trak 1/2 Protein and the Transport of Mitochondria

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This proposal focuses on the dynamic behavior of mitochondria in cells. In particular, it examines two pathways that control that behavior via different posttranslational modifications of the motor-adaptor protein milton/TRAK1/2. The movement of mitochondria is driven by two microtubule-based motors, Kinesin-1 and Dynein, and these motors are bound to the mitochondrial surface by a motor/adaptor complex consisting of the proteins milton and Miro (also called TRAK1/2 and RhoT1/2). The first regulatory pathway we here investigate involves the enzyme O-GlcNAc Transferase (OGT), an enzyme that catalyzes the addition of the sugar residue N-Acetyl Glucosamine to serine and threonine residues on proteins. We and others have found that this enzyme GlcNAcylates milton. We also find that expression of OGT in neurons arrests the movement of the mitochondria and that this occurs directly through its substrate milton. Because OGT is thought to be a nutrient sensor that is more active when glucose levels are high, we hypothesized that the OGT pathway will stop mitochondria under conditions of high extracellular glucose and indeed we observe that mitochondrial movement is decreased in axons via this pathway when glucose levels are raised. We propose to examine the mechanism by which OGT halts mitochondria and also the significance of OGT and milton GlcNAcylation for the distribution of mitochondria in axons. The second regulatory pathway that is examined in this grant also concerns mitochondrial/cytoskeletal interactions and their regulation by a posttranslational modification, in this case phosphorylation of milton. Through the motors that move mitochondria and also additional likely anchoring proteins, mitochondria normally exist in a close relationship with microtubules. We find that this situation is very different in dividing cells. When a cell enters mitosis, the microtubules are dismantled and reform as the spindle apparatus. During mitosis, mitochondria are excluded from the regions of the cell that contain microtubules. We have found that milton becomes phosphorylated during the cell cycle, probably by cdk1. We will investigate the significance of this phosphorylation for the state of the motor/adaptor complex and its significance for the mitotic redistribution of mitochondria.