Wild-type NM23-H1, but not its S120 mutants, suppresses desensitization of muscarinic potassium current

NM23 (NDP kinase) modulates the gating of muscarinic K+ channels by agonists through a mechanism distinct from GTP regeneration. To better define the function of NM23 in this pathway and to identify sites in NM23 that are important for its role in muscarinic K+ channel function, we utilized MDA-MB-435 human breast carcinoma cells that express low levels of NM23-H1. M2 muscarinic receptors and GIRK1/GIRK4 channel subunits were co-expressed in cells stably transfected with vector only (control), wild-type NM23-H1, or several NM23-H1 mutants. Lysates from all cell lines tested exhibit comparable nucleoside diphosphate (NDP) kinase activity. Whole cell patch clamp recordings revealed a substantial reduction of the acute desensitization of muscarinic K+ currents in cells overexpressing NM23-H1. The mutants NM23-H1P96S and NM23-H1S44A resembled wild-type NM23-H1 in their ability to reduce desensitization. In contrast, mutants NM23-H1S120G and NM23-H1S120A completely abolished the effect of NM23-H1 on desensitization of muscarinic K+ currents. Furthermore, NM23-H1S120G potentiated acute desensitization, indicating that this mutant retains the ability to interact with the muscarinic pathway, but has properties antithetical to those of the wild-type protein. We conclude that NM23 acts as a suppressor of the processes leading to the desensitization of muscarinic K+ currents, and that Ser-120 is essential for its actions.