Regulation by external K+ in a maize inward Shaker channel targets transport activity in the high concentration range
Résumé
An inward Shaker Kþ channel identified in Zea mays (maize), ZmK2.1, displays strong regulation by external Kþ when expressed in Xenopus laevis (African clawed frog) oocytes or COS cells. ZmK2.1 is specifically activated by Kþ with an apparent Km close to 15 mM independent of the membrane hyperpolarization level. In the absence of Kþ, ZmK2.1 appears to enter a nonconducting state. Thus, whatever the membrane potential, this maize channel cannot mediate Kþ influx in the submillimolar concentration range, unlike its relatives in Arabidopsis thaliana. Its expression is restricted to the shoots, the strongest signal (RT-PCR) being associated with vascular/bundle sheath strands. Based on sequence and gene structure, the closest relatives of ZmK2.1 in Arabidopsis are Kþ Arabidopsis Transporter 1 (KAT1) (expressed in guard cells) and KAT2 (expressed in guard cells and leaf phloem). Patch-clamp analyses of guard cell protoplasts reveal a higher functional diversity of Kþ channels in maize than in Arabidopsis. Channels endowed with regulation by external Kþ similar to that of ZmK2.1 (channel activity regulated by external Kþ with a Km close to 15 mM, regulation independent of external Ca2þ) constitute a major component of the maize guard cell inward Kþ channel population. The presence of such channels in maize might reflect physiological traits of C4 and/or monocotyledonous plants.