The BK channel (also known as Slo1) is almost ubiquitously expressed in the body with many important physiological functions, such as regulating neurotransmitter release by acting at presynaptic sites of neurons. Mutations of the channel may cause diverse diseases. Physiological functions of Slo1 depend to great degrees on its expression level in the cell membrane and interactions with regulatory proteins. Genetic screen for mutants that suppress a sluggish phenotype caused by a hyperactive Slo1 in C. elegans led to the identification of two proteins required for Slo1 physiological functions in vivo, including a melatonin receptor and an ubiquitin E3 ligase. Electrophysiological and behavioral analyses indicate that Slo1 mediates melatonin’s sleep-promoting effect in worms, and that Slo1’s physiological roles in regulating neurotransmitter release and sleep depend on melatonin secretion and activation of the melatonin receptor. In a heterologous expression, human Slo1 is activated by melatonin through the MT1 but not MT2 melatonin receptor. However, it remains to be determined where Slo1 acts in the nervous system to regulate sleep in worms, and whether mammalian Slo1 in native neurons may be also activated by melatonin through a specific melatonin receptor. Mass spectrometry analyses identified a protein greatly increased in mutants of the E3 ligase compared with wild type. Mutations of the gene encoding this protein led to increased Slo1 function, suggesting that it is a novel inhibitory regulator of Slo1, and that the E3 ligase regulates Slo1 by facilitating degradation of this putative inhibitory regulator. Further studies are needed to define a molecular pathway through which the E3 ligase regulates Slo1. This project is to investigate 1) how the E3 ligase regulates Slo1 through the inhibitory regulator and other proteins; 2) where and how Slo1 acts in the nervous system to regulate sleep in C. elegans; and 3) why MT1 but not MT2 may allow Slo1 activation by melatonin in the heterologous expression system, and whether melatonin can also regulate Slo1 in mouse brain through MT1 but not MT2. We will answer these questions using a combination of electrophysiological, genetic, cellular, and molecular biological approaches. Results of the proposed studies are expected to produce important new knowledge about how Slo1 interacts with other proteins to regulate cellular excitability, neurotransmitter release, and behavior.