The large-conductance, Ca2+- and voltage-activated potassium channel (BK) alpha subunit is modulated by one of four types of beta subunits, each imparting unique electrophysiological properties. BK beta4 is expressed in brain. It slows both activation and deactivation, with only small shifts in V50, and confers resistance to block by charybdotoxin and iberiotoxin. In mice, deletion of beta4 causes temporal lobe epilepsy. We explored the contacts between alpha and beta4 subunits by determining the extent of endogenous disulfide bond formation between cysteines substituted in the extracellular flanks of the two beta4 transmembrane (TM) helices, TM1 and TM2, and in the extracellular flanks of each of the seven alphaTM helices, S0-S6. We found that the extracellular ends of beta4 TM2 and alpha S0 are close and that beta4 TM1 is close to both S1 and S2. At their extracellular ends, TM1 and TM2 are not close to S3, S4, S5 or S6. Beta4 TM1 and TM2 are like pincers on either side of the alpha voltage-sensor domain, S0-S4. Crosslinking of beta4 TM2 to S0 further slowed activation and deactivation kinetics, with either no effect on V50, or causing a small hyperpolarizing shift. Thus, crosslinking enhances the predominant effect of beta4 on the transition rates between the activated and deactivated states, with little effect on the free energy differences between these states. Supported by NS054946.