Cap formation by various ligands on lymphocytes shows the same dependence on high cellular ATP levels.

The effects of inhibitors of mitochondrial ATP synthesis and the calcium ionophore, A23187, on the capping of surface immunoglobulin, concanavalin A receptors and theta antigen on mouse spleen or thymus cells have been examined. (i) For all of these capping ligands and inhibitors, the cellular ATP level must be above 80% of the normal level in resting lymphocytes for 90% of maximal cap formation to occur. Below 50% of the normal ATP level, less than 10% of maximal capping occurs. There is, therefore, a common dependence for all three capping systems on the cellular ATP level, irrespective of the metabolic inhibitor used. (ii) Inhibition of cap formation by A23187 follows the same profile for ATP dependence as the mitochondrial inhibitors, but in contrast to those inhibitors, A23187 requires extracellular calcium to decrease the ATP level and inhibit capping. Other agents can affect cap formation without reducing the ATP level. For example, concanavalin A inhibits its own cap formation and cytochalasin B reduces the rate of cap formation at concentrations which do not alter the cellular ATP level. (iii) From these and other data we conclude that there are cellular functions essential for cap formation, other than the maintenance of ionic gradients, that require a high concentration of cellular ATP. The possibility that high levels of ATP are required for the function of the cytoskeleton in lymphocytes is discussed.