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Eukaryotic Cell, October 2004, p. 1176-1184, Vol. 3, No. 5
1535-9778/04/$08.00+0     DOI: 10.1128/EC.3.5.1176-1184.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

A Cell Number Counting Factor Regulates Akt/Protein Kinase B To Regulate Dictyostelium discoideum Group Size

Tong Gao,1 David Knecht,2 Lei Tang,3 R. Diane Hatton,1 and Richard H. Gomer1,3*

Howard Hughes Medical Institute,1 Department of Biochemistry and Cell Biology, Rice University, Houston, Texas,3 Department of Molecular and Cellular Biology, University of Connecticut, Storrs, Connecticut2

Received 10 June 2004/ Accepted 23 July 2004

Little is known about how individual cells can organize themselves to form structures of a given size. During development, Dictyostelium discoideum aggregates in dendritic streams and forms groups of ~20,000 cells. D. discoideum regulates group size by secreting and simultaneously sensing a multiprotein complex called counting factor (CF). If there are too many cells in a stream, the associated high concentration of CF will decrease cell-cell adhesion and increase cell motility, causing aggregation streams to break up. The pulses of cyclic AMP (cAMP) that mediate aggregation cause a transient translocation of Akt/protein kinase B (Akt/PKB) to the leading edge of the plasma membrane and a concomitant activation of the kinase activity, which in turn stimulates motility. We found that countin– cells (which lack bioactive CF) and wild-type cells starved in the presence of anticountin antibodies (which block CF activity) showed a decreased level of cAMP-stimulated Akt/PKB membrane translocation and kinase activity compared to parental wild-type cells. Recombinant countin has the bioactivity of CF, and a 1-min treatment of cells with recombinant countin potentiated Akt/PKB translocation to membranes and Akt/PKB activity. Western blotting of total cell lysates indicated that countin does not affect the total level of Akt/PKB. Fluorescence microscopy of cells expressing an Akt/PKB pleckstrin homology domain-green fluorescent protein (PH-GFP) fusion protein indicated that recombinant countin and anti-countin antibodies do not obviously alter the distribution of Akt/PKB PH-GFP when it translocates to the membrane. Our data indicate that CF increases motility by potentiating the cAMP-stimulated activation and translocation of Akt/PKB.

* Corresponding author. Mailing address: Department of Biochemistry and Cell Biology, MS-140, Rice University, 6100 S. Main St., Houston, TX 77005-1892. Phone: (713) 348-4872. Fax: (713) 348-5154. E-mail: richard{at}bioc.rice.edu.

Eukaryotic Cell, October 2004, p. 1176-1184, Vol. 3, No. 5
1535-9778/04/$08.00+0     DOI: 10.1128/EC.3.5.1176-1184.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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