Heterotetramer formation and charybdotoxin sensitivity of two K+ channels cloned from smooth muscle

Delayed rectifier K+ channels are involved in the electrical activity of all excitable cells. The relationship between native K+ currents recorded from these cells and cloned K+ channel cDNAs has been difficult to ascertain partly because of contradictions in pharmacological characteristics between native and expressed currents. Through the study of the charybdotoxin (CTX) pharmacology of two cloned smooth muscle delayed rectifier K+ channels (cKv 1.2 and cKv1.5) expressed in oocytes, evidence for heterotetramer formation was obtained. We have shown that the presence of even a single CTX-insensitive subunit renders the heterotetrameric channel insensitive to CTX. The two K+ channel clones differ in an amino acid at the mouth of the pore region, which may be in a position to block the access of CTX to its binding site and hence determine CTX sensitivity of the heterotetrameric channel. These results may explain discrepancies reported between native and cloned smooth muscle K+ channels.