The target site and role of NDP kinase in cellular signaling networks

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This study has been done to investigate the functional role of NDPK as a GTP-supplying molecule in cellular signaling system (hormone receptor-G-protein-adenylate cyclase). To this goal, we first utilized dominant negative NDPK-expressing PC12D cells, to see whether the mutant NDPK overexpressed in the cell interferes with the hormonal response. When we compared the PACAP-stimulated cAMP level between PC12D cell clone that was transfected with mutant inactive recombinant NDPK (rNDPKmα or mβ) and control cell clone, the stimulation by PACAP tended to be suppressed in the transfected cell with rNDPKα compared with the control cell. However, this suppression varied from experiment to experiment.The plasma membranes from PC12D cells contain NDPK activity which acts to form GTP from added GDP, leading to AC activation by PACAP plus GDP. Therefore, we then examined whether the NDPKmα mixed with the plasma membranes can affect the endogenous NDPK and, as a result, alter the cAMP formation stimulated by PACAP. The result showed no obvious effect by NDPKmα. Also, the AC activity in membranes from PC12D-rNDPKmα stimulated by PACAP plus GDP did not differ from that from the contol cells.As we demonstrated previously, however, the PACAP-stimulated AC activity of the purified membranes was completely dependent on GTP, and the effect of PACAP plus GDP was supressed by UDP as an inhibitor of NDPK. Therefore, it is clear that PC12D AC system is closely associated with NDPK as demonstrated for that of rat liver membrane. It remains to be studied whether the plasma membrane-associated NDPK is derived from the cytosolic one, or it is a novel membrane specific isoform distinct from the cytosolic NDPKs.