Abstract
Guard cells adjust their volume by changing their ion content due to intense fluxes that, for K(+), are believed to flow through inward or outward Shaker channels. Because Shaker channels can be homo- or heterotetramers and Arabidopsis guard cells express at least five genes encoding inward Shaker subunits, including the two major ones, KAT1 and KAT2, the molecular identity of inward Shaker channels operating therein is not yet completely elucidated. Here, we first addressed the properties of KAT1-KAT2 heteromers by expressing KAT1-KAT2 tandems in Xenopus oocytes. Then, computer analyses of the data suggested that coexpression of free KAT1 and KAT2 subunits resulted mainly in heteromeric channels made of two subunits of each type due to some preferential association of KAT1-KAT2 heterodimers at the first step of channel assembly. This was further supported by the analysis of KAT2 effect on KAT1 targeting in tobacco cells. Finally, patch-clamp recordings of native inward channels in wild-type and mutant genotypes strongly suggested that this preferential heteromerization occurs in planta and that Arabidopsis guard cell inward Shaker channels are mainly heteromers of KAT1 and KAT2 subunits.
Highlights
Brane belonging to the Shaker family [1, 2]
We investigated the features of KAT1 and KAT2 homomers and of heteromeric KAT1-KAT2 channels of defined stoichiometry and compared them with those of channels formed by unbiased assembly and with those of native inward-rectifying Kϩ channels in Arabidopsis guard cells
In heterologous expression systems as well as in vivo, assembly of heteromeric KAT1-KAT2 channels is favored above that of homomeric KAT1 and KAT2 channels, so the native inward conductance in Arabidopsis guard cells relies mainly on channels made of two KAT1 and two KAT2 subunits
Summary
Brane belonging to the Shaker family [1, 2]. These potassium channels were found to play pleiotropic roles in guard cells, improving stomatal reactivity to external or internal signals (light, CO2 availability, or evaporative demand) and enhancing the ability of the plant to adapt to fluctuating and/or stressing natural environments [1]. Yeast twohybrid interaction tests and coexpression experiments in Xenopus oocytes have already demonstrated that KAT1 and KAT2 subunits have the potential to interact and to form heteromeric channels [19]. It is not known whether they show any preference for homomeric or heteromeric assembly as has been reported for AtKC1 and AKT1 [27] and for KAT2 and AKT2 [20]. In heterologous expression systems as well as in vivo, assembly of heteromeric KAT1-KAT2 channels is favored above that of homomeric KAT1 and KAT2 channels, so the native inward conductance in Arabidopsis guard cells relies mainly on channels made of two KAT1 and two KAT2 subunits
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