Members of the Arabidopsis AtTPK/KCO family form homomeric vacuolar channels in planta

Abstract

SummaryThe Arabidopsis thaliana K + channel family of AtTPK/KCO proteins consists of six members including a 'singlepore' (K ir -type) and five 'tandem-pore' channels. AtTPK4 is currently the only ion channel of this family for which a function has been demonstrated in planta. The protein is located at the plasma membrane forming a voltage-independent K + channel that is blocked by extracellular calcium ions. In contrast, AtTPK1 is a tonoplast-localized protein, that establishes a K + -selective, voltage-independent ion channel activated by cytosolic calcium when expressed in a heterologous system, i.e. yeast. Here, we provide evidence that other AtTPK/KCO channel subunits, i.e. AtTPK2, AtTPK3, AtTPK5 and AtKCO3, are also targeted to the vacuolar membrane, opening the possibility that they interact at the target membrane to form heteromeric ion channels. However, when testing the cellular expression patterns of AtTPK/KCO genes we observed distinct expression domains that overlap in only a few tissues of the Arabidopsis plant, making it unlikely that different channel subunits interact to form heteromeric channels. This conclusion was substantiated by in planta expression of combinations of selected tonoplast AtTPK/KCO proteins. Fluorescence resonance energy transfer assays indicate that protein interaction occurs between identical channel subunits (most efficiently between AtTPK1 or AtKCO3) but not between different channel subunits. The finding could be confirmed by bimolecular fluorescence complementation assays. We conclude that tonoplast-located AtTPK/KCO subunits form homomeric ion channels in vivo.

Common Functions or Only Phylogenetically Related? The Large Family of PLAC8 Motif-Containing/PCR Genes

“…These results indicate that AtPCR1 and AtPCR2 act as heavy metal exporters. Several cases have been described where homo-oligomers of membrane proteins containing only two membrane-spanning domains form functional potassium channels (Bryan and Aguilar-Bryan, 1999;Gazzarrini et al, 2009;Voelker et al, 2006). Furthermore, the bacterial magnesium transporter, CorA, has also only two hydrophobic α-helices (Payandesh and Pai, 2006).…”

Comparative Transcriptomics of Arabidopsis Sperm Cells

“…Interestingly, in mammals a glycosyl hydrolase (hyaluronidase) protein, PH-20, located at both the plasma and acrosomal membranes in spermatozoa, is involved in adhesion to the zona pellucida (Lathrop et al, 1990). Another gene of note is KCO2|At5g46370, encoding a member of the Arabidopsis K + channel family AtTPK(KCO) (Voelker et al, 2006). Within the same family, AtTPK4(KCO4) is predominantly expressed in pollen tubes and is spontaneously activated and modulated by external Ca 2+ and cytosolic pH (Becker et al, 2004).…”

The Small GTPase ROP10 of Medicago truncatula Is Required for Both Tip Growth of Root Hairs and Nod Factor-Induced Root Hair Deformation

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