AtTPK4, an Arabidopsis tandem-pore K ⁺ channel, poised to control the pollen membrane voltage in a pH- and Ca ²⁺ -dependent manner

Abstract

The Arabidopsis tandem-pore K ؉ (TPK) channels displaying four transmembrane domains and two pore regions share structural homologies with their animal counterparts of the KCNK family. In contrast to the Shaker-like Arabidopsis channels (six transmembrane domains͞one pore region), the functional properties and the biological role of plant TPK channels have not been elucidated yet. Here, we show that AtTPK4 (KCO4) localizes to the plasma membrane and is predominantly expressed in pollen. AtTPK4 (KCO4) resembles the electrical properties of a voltage-independent K ؉ channel after expression in Xenopus oocytes and yeast. Hyperpolarizing as well as depolarizing membrane voltages elicited instantaneous K ؉ currents, which were blocked by extracellular calcium and cytoplasmic protons. Functional complementation assays using a K ؉ transport-deficient yeast confirmed the biophysical and pharmacological properties of the AtTPK4 channel. The features of AtTPK4 point toward a role in potassium homeostasis and membrane voltage control of the growing pollen tube. Thus, AtTPK4 represents a member of plant tandem-pore-K ؉ channels, resembling the characteristics of its animal counterparts as well as plant-specific features with respect to modulation of channel activity by acidosis and calcium.

Ca2+-Dependent Protein Kinase11 and 24 Modulate the Activity of the Inward Rectifying K+ Channels inArabidopsisPollen Tubes

“…The other was defined as instantaneously activated, voltage-independent conductance that is mediated by TPK4 in Arabidopsis . K + in currents mediated by TPK4-like channels contributed to 20 to 30% of the total K + in currents across the pollen grain and pollen tube plasma membrane in previous reports (Mouline et al, 2002;Becker et al, 2004), but they were not more than 10% of the whole K + in currents in our experiments ( Figure 6A). The difference could result from differences in the experimental methods, such as those used for protoplast isolation or the electrophysiological recording mode.…”

“…The Downstream Target of the Two CDPKs K + in currents across the plasma membrane of pollen tube consist of two components (Obermeyer and Blatt, 1995;Griessner and Obermeyer, 2003;Becker et al, 2004). One component was defined as the hyperpolarization-activated, time-, and voltagedependent inward rectifier.…”

The role of ion fluxes in polarized cell growth and morphogenesis: the pollen tube as an experimental paradigm

“…The functional properties of AKT5 are unknown. Other members from the family may also play a role (Mouline et al, 2002;Becker et al, 2004). More recently, a K + channel, AtTPK4, from the KCO/TPK family was shown to be responsible for a marginal proportion of the tube K + conductance .…”

“…K + conductance (Obermayer and Blatt, 1995;Fan et al, 1999;Fan et al, 2001;Fan et al, 2003;Griessner and Obermeyer, 2003;Becker et al, 2004) and K + channels (Mouline et al, 2002) are regulated by Em in pollen. Ca 2+ channels have been identified by patch-clamp in the pollen grain.…”

“…The transcription of several K + -channels has been identified in pollen using RT-PCR (Mouline et al, 2002;Becker et al, 2004) or micro-arrays analysis (Becker et al, 2003;Honys and Twell, 2004;Pina et al, 2005). Using pharmacological studies coupled to genetic and electrophysiological approaches, a K + channel from the Shaker family (SPIK) has been shown to play a role in this high affinity K + uptake in Arabidopsis pollen (Mouline et al, 2002), specially on hyper-polarization conditions.…”