Subunit positional effects revealed by novel heteromeric inwardly rectifying K⁺ channels

K(ir) 4.1 is an inward rectifier potassium channel subunit isolated from rat brain which forms homomeric channels when expressed in Xenopus oocytes; K(ir) 5.1 is a structurally related subunit which does not. Co-injection of mRNAs encoding K(ir) 4.1 and K(ir) 5.1 resulted in potassium currents that (i) were much larger than those seen from expression of K(ir) 4.1 alone, (ii) increased rather than decreased during several seconds at strongly negative potentials and (iii) had an underlying unitary conductance of 43 pS rather than the 12 pS seen with K(ir) 4.1 alone. In contrast, the properties of K(ir) 1.1, 2.1, 2.3, 3.1, 3.2 or 3.4 were not altered by coexpression with K(ir) 5.1. Expression of a concatenated cDNA encoding two or four linked subunits produced currents with the properties of co-expressed K(ir) 4.1 and K(ir) 5.1 when the subunits were connected 4-5 or 4-5-4-5, but not when they were connected 4-4-5-5. The results indicate that K(ir) 5.1 associates specifically with K(ir) 4.1 to form heteromeric channels, and suggest that they do so normally in the subunit order 4-5-4-5. Further, the relative order of subunits within the channel contributes to their functional properties.