Tortoise, a novel mitochondrial protein, is required for directional responses of Dictyostelium in chemotactic gradients

We have identified a novel gene, Tortoise (TorA), that is required for the efficient chemotaxis of Dicryostelium discoideum cells. Cells lacking TorA sense chemoattractant gradients as indicated by the presence of periodic wa ves of cell shape changes and the localized translocation of cytosolic PH d omains to the membrane. However, they are unable to migrate directionally u p spatial gradients of cAMP. Cells lacking Mek1 display a similar phenotype . Overexpression of Mek1 in torA(-) partially restores chemotaxis, whereas overexpression of TorA in mek1(-) does not rescue the chemotactic phenotype . Regardless of the genetic background, TorA overexpressing cells stop grow ing when separated from a substrate. Surprisingly, TorA-green fluorescent p rotein (GFP) is clustered near one end of mitochondria. Deletion analysis o f the TorA protein reveals distinct regions for chemotactic function, mitoc hondrial localization, and the formation of clusters. TorA is associated wi th a round structure within the mitochondrion that shows enhanced staining with the mitochondrial dye Mitotracker. Cells overexpressing TorA contain m any more of these structures than do wild-type cells. These TorA-containing structures resist extraction with Triton X-100, which dissolves the mitoch ondria. The characterization of TorA demonstrates an unexpected link betwee n mitochondrial function, the chemotactic response, and the capacity to gro w in suspension.