Specific regions of heteromeric subunits involved in enhancement of G protein-gated K ⁺ channel activity

Heterologous coexpression of recombinant, G protein-gated, inwardly rectifying K ⁺ (GIRK) channel subunits has yielded large currents, severalfold greater than those obtained from expression of the individual subunits. Such current enhancement has been obtained from coexpression of the inactive GIRK1 subunit with the low activity GIRK2-5 subunits in Xenopus oocytes. Using deletion and chimeric constructs, we now report the identification of a C-terminal region unique to GIRK1 and a larger central region of GIRK4 highly homologous to GIRK1, both of which are critical for production of large currents. Chimeras containing these two regions produced homomeric channels, exhibiting currents severalfold greater than those from either wild-type subunit alone. G protein regulation of such chimeric channel currents resembled that of wild-type currents. Green fluorescent protein- tagged channels showed that the amount of chimeric channel expressed on the oocyte cell surface was similar to its wild-type counterpart, suggesting that the enhanced activity was not due to differences in relative levels of expression but rather to the coexistence of the chimeric regions. Single- channel recordings of the active chimeras exhibited patterns of activities with open-time kinetics and conductance characteristics representative of those of GIRK4, indicating that the presence of the GIRK1 C-terminal region caused an increase in the frequency of channel openings without affecting their duration.