Polyamines and potassium channels: A 25-year romance

Abstract

Potassium channels that exhibit the property of inward rectification (Kir channels) are present in most cells. Cloning of the first Kir channel genes 25 years ago led to recognition that inward rectification is a consequence of voltage-dependent block by cytoplasmic polyamines, which are also ubiquitously present in animal cells. Upon cellular depolarization, these polycationic metabolites enter the Kir channel pore from the intracellular side, blocking the movement of K⁺ ions through the channel. As a consequence, high K⁺ conductance at rest can provide very stable negative resting potentials, but polyamine-mediated blockade at depolarized potentials ensures, for instance, the long plateau phase of the cardiac action potential, an essential feature for a stable cardiac rhythm. Despite much investigation of the polyamine block, where exactly polyamines get to within the Kir channel pore and how the steep voltage dependence arises remain unclear. This Minireview will summarize current understanding of the relevance and molecular mechanisms of polyamine block and offer some ideas to try to help resolve the fundamental issue of the voltage dependence of polyamine block.

Keywords: ion channel; polyamine; potassium channel; protein structure; spermidine.

Figure 1.

A, key features of inward rectification. Under normal physiological conditions (high [K⁺] inside and low [K⁺] outside), electrodiffusive conductance through a K⁺-selective pore would result in large outward currents at positive potentials (top, dashed line). Inward rectification refers to decreasing currents at more positive voltages, with either a weak voltage dependence (top, yellow line) or strong voltage dependence (red line). The voltage dependence is assessed as the current relative to the unblocked or nonrectifying current (relative conductance, Grel) as a function of voltage (bottom). B, structure of Kir channels. Two opposing (a and c) subunits of Kir2.2, resolved at 2.8 Å (75) (Protein Data Bank code 5KUM) are visible, with the other two (b and d) subunits hidden, to show the ion-permeable pore of the channel. The pore is lined by distinct selectivity filter (SF), transmembrane inner cavity, and cytoplasmic pore regions. Residues that control polyamine-induced rectification in Kir2.1 are indicated (red), as well as residues that induce strong rectification when an acidic substitution is introduced in Kir6.2 (yellow). All residues are numbered according to Kir2.1 sequence. C, alignment of strong (Kir2.1 and Kir4.1) and weak (Kir1.1 and Kir6.2) Kir channel members. Representative members of Kir channel subfamilies are aligned between residues 100 and 320 (in Kir2.1). Note conservation of negative residues at the rectification controller in strong inward rectifier Kir2.1 and Kir4.1 and variable conservation of other key pore-lining negative residues.

Figure 2.

A, characteristics of polyamine-dependent rectification (redrawn from Ref. 38). Left, Grel versus voltage plots for Kir2.3 currents in the presence of 250 μm spermine⁴⁺ (SPM), spermidine³⁺ (SPD), or putrescine²⁺ (PUT). Note that both potency and steepness of the voltage dependence increase as the polyamine size and charge increase. Right, Grel versus voltage plots for Kir2.3 currents in the presence of increasing spermine concentrations (25, 50, 100, 150, and 250 μm). The Grel-V_m curves shift in parallel. Note that there is both a shallow and steep component to the voltage dependence for spermidine and spermine. B, left, “long pore plugging” model of polyamine-dependent rectification. Shallow rectification is assumed to arise from movement of the polyamine into the cytoplasmic cavity (pale site), up to the entrance to the inner cavity, potentially carrying some polyamine charge into the electric field. Steep rectification is assumed to arise from movement of the polyamine deep into the pore, between the rectification controller and the selectivity filter (dark site), with substantial polymine charge moving across the field and displacing K⁺ ions from the filter in the process. Center, “extended pore-filing” model of polyamine-dependent rectification. Shallow rectification is assumed to arise from movement of the polyamine into the cytoplasmic cavity (pale site), in the process “pushing” on a “chain” of K⁺ ions that extends through the selectivity filter. Steep rectification is assumed to arise from forward movement of the polyamine toward the rectification controller (dark site), moving up to four or five K⁺ ions through the selectivity filter in the process. Right, “cavity-trapping” model for polyamine-dependent rectification. We propose a new hybrid model that may reconcile objections to earlier models. Shallow rectification is again assumed to arise from movement of the polyamine into the cytoplasmic cavity (pale site), potentially displacing K⁺ ions into the inner cavity. The charge associated with block, determining how steep rectification will be, is determined by the net charge on the wall of the inner cavity. With four negative charges at the rectification controller, there will be four K⁺ counterions present. Steep rectification is then assumed to arise from movement of the polyamine into the inner cavity, through a narrow single filing region at the entrance, in the process displacing these K⁺ ions into and through the selectivity filter.