Expression of an outward-rectifying potassium channel from maize mRNA and complementary RNA in Xenopus oocytes

Injection of Xenopus oocytes with poly(A)+ mRNA isolated from different plants (maize, cucumber, and squash) results in the appearance of a voltage- and time-dependent, potassium-selective, outward current that is similar to the outward-rectifying potassium current recorded in many higher plant cells. Maize shoots were found to be especially enriched in mRNA encoding such activity. A cDNA library of maize shoot mRNA was constructed in the vector lambda ZAPII and was used to synthesize RNA complementary to the cDNA (cRNA). Injection of the cRNA gave rise to an outward-rectifying potassium current with properties similar to the currents obtained by poly(A)+ mRNA injection. These results demonstrate that higher plant mRNA can be properly translated into a product that produces a voltage-regulated potassium channel in the plasma membrane of Xenopus oocytes. Thus, Xenopus oocytes can be used as a heterologous expression system for the functional identification and isolation of plant ion channel genes as well as for the study of structure-function relationship of plant ion channels.