The GTP-binding regulatory proteins of neuroblastoma × glioma, NG108-15, and glioma, C6, cells. Immunochemical evidence of a pertussis toxin substrate that is neither N_i nor N_o

Amounts of the guanine nucleotide binding regulatory proteins which are also pertussis toxin substrates (such as N_i and N_o) were measured in rat glioma, C6BU-1, cells and in neuroblastoma × glioma, NG108-15, hybrid cells. Measurements were performed both by quantitating pertussis toxin catalyzed ADP-ribosylation and by quantitative immunoblotting with affinity purified antibodies specific for N_i or N_o. The amounts of pertussis toxin substrate in C6 and NG108-15 cells are 7.5 and 0.6 pmol mg membrane protein, respectively. These levels are minimum values and higher estimates of the total amounts of N proteins in the two cells are obtained by quantitative immunoblot analysis of the β-subunit common to all N proteins. Immunoblots with specific antibodies show that NG108-15 cells contain 3.8 pmol mg of N_o and detectable but small ( < 0.1 pmol mg) amounts of N_i. In contrast, C6 cell membranes contain no detectable N_o and only 0.14 pmol mg N_i. Thus, C6 cells contain large amounts of a pertussis toxin substrate which is neither N_i nor N_o.