LAUSR

&NA; Recent reports have identified defensins as a new type of potassium channel inhibitors; differential binding mechanisms of human &bgr;‐defensins hBD1 and hBD2 point to complex interactions between human &bgr;‐defensins and potassium channels. We investigated the inhibitory effects of human defensins hBD3 and hBD4 on potassium channels. The data indicate that hBD3 is a voltage‐gated channel subfamily A member 3 (Kv1.3) inhibitor with an IC50 value of 187.6 ± 25.7 nM; 1 &mgr;M hBD4 inhibited 34.0 ± 0.2% of Kv1.3 channel currents. Moreover, 1 &mgr;M hBD3 inhibited 50.6 ± 3.6% of Kv1.2 channel currents and had smaller effects on Kv1.1, SKCa3, and IKCa channel currents; these effects differed from the Kv1.3 channel‐specific inhibitors hBD1 and hBD2. Similar to the pharmacological profiles of hBD1 and hBD2, hBD4 had lower inhibitory effects on Kv1.1, Kv1.2, SKCa3, and IKCa channels. Subsequent mutagenesis and channel activation experiments confirmed that hBD3 binds in a manner similar to that of hBD1, interacting with the outer pore region of the Kv1.3 channel without affecting Kv1.3 channel activation. Thus, the data indicate that the human &bgr;‐defensin family is a novel group of potassium channel inhibitors with diverse types of human &bgr;‐defensin‐potassium channel interactions. HighlightshBD3 and hBD4 are endogenous Kv1.3 channel blockers with differential affinities.hBD3 is the first endogenous Kv1.2 channel blocker.hBD3 and hBD4 show less inhibitory effects on many other human potassium channels.hBD3 adopts the hBD1‐like pore‐blocking mode to interact with Kv1.3 channel.