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Eukaryotic Cell, January 2005, p. 72-81, Vol. 4, No. 1
1535-9778/05/$08.00+0     doi:10.1128/EC.4.1.72-81.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Exposure of Cells to a Cell Number-Counting Factor Decreases the Activity of Glucose-6-Phosphatase To Decrease Intracellular Glucose Levels in Dictyostelium discoideum{dagger}

Wonhee Jang and Richard H. Gomer*

Howard Hughes Medical Institute and Department of Biochemistry and Cell Biology, Rice University, Houston, Texas

Received 4 August 2004/ Accepted 11 October 2004

The development of Dictyostelium discoideum is a model for tissue size regulation, as these cells form groups of {approx}2 x 104 cells. The group size is regulated in part by a negative feedback pathway mediated by a secreted multipolypeptide complex called counting factor (CF). CF signal transduction involves decreasing intracellular CF glucose levels. A component of CF, countin, has the bioactivity of the entire CF complex, and an 8-min exposure of cells to recombinant countin decreases intracellular glucose levels. To understand how CF regulates intracellular glucose, we examined the effect of CF on enzymes involved in glucose metabolism. Exposure of cells to CF has little effect on amylase or glycogen phosphorylase, enzymes involved in glucose production from glycogen. Glucokinase activity (the first specific step of glycolysis) is inhibited by high levels of CF but is not affected by an 8-min exposure to countin. The second enzyme specific for glycolysis, phosphofructokinase, is not regulated by CF. There are two corresponding enzymes in the gluconeogenesis pathway, fructose-1,6-bisphosphatase and glucose-6-phosphatase. The first is not regulated by CF or countin, whereas glucose-6-phosphatase is regulated by both CF and an 8-min exposure to countin. The countin-induced changes in the Km and Vmax of glucose-6-phosphatase cause a decrease in glucose production that can account for the countin-induced decrease in intracellular glucose levels. It thus appears that part of the CF signal transduction pathway involves inhibiting the activity of glucose-6-phosphatase, decreasing intracellular glucose levels and affecting the levels of other metabolites, to regulate group size.

* Corresponding author. Mailing address: Howard Hughes Medical Institute and 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.

{dagger} This paper is dedicated to the memory of Fred Rudolph.

Eukaryotic Cell, January 2005, p. 72-81, Vol. 4, No. 1
1535-9778/05/$08.00+0     doi:10.1128/EC.4.1.72-81.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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