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Published online 5 February 2001. doi:10.1083/jcb.152.3.621
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© The Rockefeller University Press, 0021-9525/2001//621 $5.00
The Journal of Cell Biology, Volume 152, Number 3, , 2001 621-632

Original Article

Tortoise, a Novel Mitochondrial Protein, Is Required for Directional Responses of Dictyostelium in Chemotactic Gradients



Saskia van Esa, Deborah Wesselsb, David R. Sollb, Jane Borleisa, and Peter N. Devreotesa

a Department of Cell Biology and Anatomy, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
b Department of Biological Sciences, University of Iowa, Iowa City, Iowa 52242
Department of Cell Biology and Anatomy, Johns Hopkins University School of Medicine, 725 North Wolfe St., Baltimore, MD 21205.(410) 955-5759(410) 955-4699

pnd{at}welch.jhu.edu

We have identified a novel gene, Tortoise (TorA), that is required for the efficient chemotaxis of Dictyostelium discoideum cells. Cells lacking TorA sense chemoattractant gradients as indicated by the presence of periodic waves of cell shape changes and the localized translocation of cytosolic PH domains to the membrane. However, they are unable to migrate directionally up 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 growing when separated from a substrate. Surprisingly, TorA–green fluorescent protein (GFP) is clustered near one end of mitochondria. Deletion analysis of the TorA protein reveals distinct regions for chemotactic function, mitochondrial localization, and the formation of clusters. TorA is associated with a round structure within the mitochondrion that shows enhanced staining with the mitochondrial dye Mitotracker. Cells overexpressing TorA contain many more of these structures than do wild-type cells. These TorA-containing structures resist extraction with Triton X-100, which dissolves the mitochondria. The characterization of TorA demonstrates an unexpected link between mitochondrial function, the chemotactic response, and the capacity to grow in suspension.

Key Words: TorA • Mek1 • chemotaxis • mitochondria • Dictyostelium

© 2001 The Rockefeller University Press

The online version of this article contains supplemental material.

Abbreviations used in this paper: cGMP, guanosine 3', 5' cyclic monophosphate; Crac, cytosolic regulator of adenylyl cyclase; DIAS, dynamic image analyzing system; GFP, green fluorescent protein; REMI, restriction enzyme–mediated integration; TorA, Tortoise.


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