Long Polyamines Act as Cofactors in PIP2 Activation of Inward Rectifier Potassium (Kir2.1) Channels

Abstract

Phosphatidylinosital-4,5-bisphosphate (PIP2) acts as an essential factor regulating the activity of all Kir channels. In most Kir members, the dependence on PIP2 is modulated by other factors, such as protein kinases (in Kir1), Gβγ (in Kir3), and the sulfonylurea receptor (in Kir6). So far, however, no regulator has been identified in Kir2 channels. Here we show that polyamines, which cause inward rectification by selectively blocking outward current, also regulate the interaction of PIP2 with Kir2.1 channels to maintain channel availability. Using spermine and diamines as polyamine analogs, we demonstrate that both spontaneous and PIP2 antibody–induced rundown of Kir2.1 channels in excised inside-out patches was markedly slowed by long polyamines; in contrast, polyamines with shorter chain length were ineffective. In K188Q mutant channels, which have a low PIP2 affinity, application PIP2 (10 μM) was unable to activate channel activity in the absence of polyamines, but markedly activated channels in the presence of long diamines. Using neomycin as a measure of PIP2 affinity, we found that long polyamines were capable of strengthening either the wild type or K188Q channels' interaction with PIP2. The negatively charged D172 residue inside the transmembrane pore region was critical for the shift of channel–PIP2 binding affinity by long polyamines. Sustained pore block by polyamines was neither sufficient nor necessary for this effect. We conclude that long polyamines serve a dual role as both blockers and coactivators (with PIP2) of Kir2.1 channels.

Tamoxifen Inhibits Inward Rectifier K+ 2.x Family of Inward Rectifier Channels by Interfering with Phosphatidylinositol 4,5-Bisphosphate-Channel Interactions

“…Inhibition of Kir2.3 currents was significantly reduced by a single point mutation of (I213L), which enhances channel interaction with membrane PIP 2 . More recently, it was found that long polyamines were capable of strengthening the PIP 2 -channel interaction (Xie et al, 2005). In the present work, pretreatment with spermine (100 M) decreased the inhibitory effect of tamoxifen on Kir2.1, perhaps by strengthening the channel interaction with PIP 2 (Xie et al, 2005).…”

“…More recently, it was found that long polyamines were capable of strengthening the PIP 2 -channel interaction (Xie et al, 2005). In the present work, pretreatment with spermine (100 M) decreased the inhibitory effect of tamoxifen on Kir2.1, perhaps by strengthening the channel interaction with PIP 2 (Xie et al, 2005). In support of this hypothesis, pretreatment or continuous application of PIP 2 markedly decreased the inhibition induced by tamoxifen, 4-hydroxy-tamoxifen (data not shown), and raloxifene (data not shown).…”

Tamoxifen Inhibits Cardiac ATP-Sensitive and Acetylcholine-Activated K+ Currents in Part by Interfering With Phosphatidylinositol 4,5-Bisphosphate–Channel Interaction

“…Inhibition of Kir6.2 currents by tamoxifen was significantly reduced by a single point mutation (C166S). The mutation C166S has been proposed to alter the orientation of the channel's cytoplasmic region in such a way to enhance the R176/ R177-PIP 2 interaction, resulting in bursts of openings (18,29). Additional and direct support of the hypothesis that tamoxifen may be interfering with PIP 2 -channel interaction was that continuous application of PIP 2 markedly decreased the inhibition induced by tamoxifen.…”

Polyamines Interact with Hydroxyl Radicals in Activating Ca2+ and K+ Transport across the Root Epidermal Plasma Membranes

“…To the best of our knowledge, this possibility has not been considered in the literature so far. However, it was reported that PAs may stabilize the binding of other cofactors essential for the ion channel activity, such as PIP 2 for Kir and plant Shaker K + channels (Liu et al, 2005a;Xie et al, 2005). Thus, the precise biophysical mechanisms of this synergism between PAs and ROS interaction warrant a separate investigation.…”