Cholera toxin inhibits chemotaxis by a cAMP-independent mechanism

Cholera toxin inhibits chemotaxis of the RAW264 mouse macrophage cell line. The degree of inhibition by cholera toxin increases upon incubation with the cells, suggesting that the entry of the toxin is required for inhibition of chemotaxis. In the absence of guanine nucleotides, cholera toxin catalyzes the [32P]ADP-ribosylation of RAW264 cell membrane proteins of Mr 41,000, Mr 45,000, and a doublet of Mr 48,000-50,000. GTP increases the labeling of the Mr 45,000 protein and the Mr 48,000-50,000 doublet, and it decreases the labeling of the Mr 41,000 protein. Experiments with cholera toxin treatment of intact cells indicate that the Mr 45,000 protein is the major membrane protein ADP-ribosylated by the toxin in vivo. Cholera toxin increases cAMP levels in RAW264 cells, but increased cAMP levels do not correlate with inhibition of chemotaxis, because isoproterenol and forskolin, which also increase cAMP levels, have no effect on chemotaxis.