Abstract

1,1′-Dimethyl-4,4′-bipyridinium dichloride (methyl viologen; paraquat), an herbicide that causes depletion of NADPH and generates excessive reactive oxygen species (ROS) in vivo, has been used to screen for ROS-sensitiveDrosophila mutants. One mutant so isolated, namedquiver¹(qvr¹), has a leg-shaking phenotype. Mutants of the Shaker (Sh),Hyperkinetic (Hk), and ether a go-go (eag) genes, which encode different K⁺ channel subunits that regulate the A-type K⁺ current (I_A) in different ways, exhibit leg shaking under ether anesthesia and have heightened metabolic rates and shortened life spans. We found thatSh, Hk, and eag mutant flies were all hypersensitive to paraquat. Double-mutant combinations among the three channel mutations andqvr¹ had drastically enhanced sensitivity to paraquat. Synaptic transmission at the larval neuromuscular junction was increased in theqvr¹ mutant to the level ofSh mutants. Similar to eag Sh double mutants, double mutants of eag andqvr¹ showed striking enhancement in synaptic transmission and a wings-down phenotype, the hallmarks of extreme hyperexcitability. Voltage-clamp experiments demonstrated that the qvr¹ mutation specifically disrupted the Sh-dependent I_Acurrent without altering the other currents [I_K, Ca²⁺-activated fast (I_CF) and slow (I_CS) currents, andI_Ca] in larval muscles. Several deficiency strains of the qvr locus failed to complementqvr¹ and confirmed that ether-induced leg shaking, reduced I_A current, and paraquat hypersensitivity map to the same locus. Our results suggest that the qvr gene may encode a novel K⁺ channel-related polypeptide and indicate a strong link between a voltage-activated K⁺ current and vulnerability to ROS.