Voltage-gated K⁺ (K_v) channels are essentially involved in the excitability of the nervous system and the heart. Their opening is required for action potential repolarization, maintenance of the resting membrane potential, spike frequency adaptation and secretion of signalling molecules. It is well known that members of the K_v1 channels may form heteromeric channels. To investigate the exact composition and the functional role of heteromeric K_v channels, molecules recognizing specific subunit combinations with high affinity would be very helpful.

Here we present data for conotoxin kM-RIIIJ, which blocks homomeric K_v1.2 channels expressed in HEK293 cells with an IC₅₀ of about 250 nM. All other homomeric K_v1 channels are blocked with IC₅₀ higher than 1 mM. Heteromeric K_v1 channels containing K_v1.2 subunits were constructed. Surprisingly, kM-RIIIJ differentiates between different dimeric concatemers. Heteromeric channels composed of K_v1.2 and either K_v1.1 or K_v1.6 subunits had a significantly higher kM-RIIIJ sensitivity compared to homomeric K_v1.2 channels (IC₅₀ of 18 and 6 nM for K_v1.1/1.2 and K_v1.6/1.2 concatemers, respectively). To evaluate the kM-RIIIJ sensitivity of K_v1 channels with known composition we constructed concatemers of four subunits. Constructs containing three K_v1.2 and either one K_v1.1 or one K_v1.6 subunit had an even higher kM-RIIIJ sensitivity than channels with a 2:2 stoichiometry (K_v1.1/1.2/1.2/1.2: IC₅₀ 1.2 nM; K_v1.6/1.2/1.2/1.2: IC₅₀ 0.5 nM).

In conclusion, the mammalian target of the conotoxin kM-RIIIJ are heteromeric K_v1 channels containing three subunits of K_v1.2. Therefore, kM-RIIIJ is a promising tool to investigate the function of heteromeric K_v1 channels with a specific composition.