title: A protein in crude cytosol regulates glucose-6-phosphatase activity in crude microsomes to regulate group size in Dictyostelium.
abstract: Dictyostelium discoideum form groups of approximately 2 x 10(4) cells. The group size is regulated in part by a negative feedback pathway mediated by a secreted multipolypeptide complex called counting factor (CF). The CF signal transduction pathway involves CF-repressing internal glucose levels by increasing the K(m) of glucose-6-phosphatase. Little is known about how this enzyme is regulated. Glucose-6-phosphatase is associated with microsomes in both Dictyostelium and mammals. We find that the activity of glucose-6-phosphatase in crude microsomes from cells with high, normal, or low CF activity had a negative correlation with the amount of CF present in these cell lines. In crude cytosols (supernatants from ultracentrifugation of cell lysates), the glucose-6-phosphatase activity had a positive correlation with CF accumulation. The crude cytosols were further fractionated into a fraction containing molecules greater than 10 kDa (S>10K) and molecules less than 10 KDa (S<10K). S>10K from wild-type cells strongly repressed the activity of glucose-6-phosphatase in wild-type microsomes, whereas S>10K from countin(-) cells (cells with low CF activity) significantly increased the activity of glucose-6-phosphatase in wild-type microsomes by decreasing K(m). The regulatory activities in the wild-type and countin(-) S>10Ks are heat-labile and protease-sensitive, suggesting that they are proteins. S<10K from both wild-type and countin(-) cells did not significantly change glucose-6-phosphatase activity. Together, the data suggest that, as a part of a pathway modulating multicellular group size, CF regulates one or more proteins greater than 10 KDa in crude cytosol that affect microsome-associated glucose-6-phosphatase activity.
authors.affiliation: Howard Hughes Medical Institute and Department of Biochemistry and Cell Biology, Rice University, Houston, TX 77005, USA.
authors.forename: Wonhee
authors.initials: W
authors.lastname: Jang
authors.forename: Richard H
authors.initials: RH
authors.lastname: Gomer
mesh_terms: D000818:Animals; D003600:Cytosol / Q000201:enzymology / Q000378:metabolism*; D003902:Detergents / Q000494:pharmacology; D004023:Dictyostelium / Q000201:enzymology*; D004305:Dose-Response Relationship, Drug; D005952:Glucose-6-Phosphatase / Q000378:metabolism*; D006358:Hot Temperature; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D008861:Microsomes / Q000201:enzymology* / Q000378:metabolism; D008954:Models, Biological; D015398:Signal Transduction
chemical_list: D003902:Detergents; D005952:Glucose-6-Phosphatase