Secondary anionic phospholipid binding site and gating mechanism in Kir2.1 inward rectifier channels

Phosphoinositide control of membrane protein function: a frontier led by studies on ion channels

Anionic phospholipids are critical constituents of the inner leaflet of the plasma membrane, ensuring appropriate membrane topology of transmembrane proteins. Additionally, in eukaryotes, the negatively charged phosphoinositides serve as key signals not only through their hydrolysis products but also through direct control of transmembrane protein function. Direct phosphoinositide control of the activity of ion channels and transporters has been the most convincing case of the critical importance of phospholipid-protein interactions in the functional control of membrane proteins. Furthermore, second messengers, such as [Ca(2+)]i, or posttranslational modifications, such as phosphorylation, can directly or allosterically fine-tune phospholipid-protein interactions and modulate activity. Recent advances in structure determination of membrane proteins have allowed investigators to obtain complexes of ion channels with phosphoinositides and to use computational and experimental approaches to probe the dynamic mechanisms by which lipid-protein interactions control active and inactive protein states.

Phosphoinositide regulation of inward rectifier potassium (Kir) channels

Inward rectifier potassium (Kir) channels are integral membrane proteins charged with a key role in establishing the resting membrane potential of excitable cells through selective control of the permeation of K⁺ ions across cell membranes. In conjunction with secondary anionic phospholipids, members of this family are directly regulated by phosphoinositides (PIPs) in the absence of other proteins or downstream signaling pathways. Different Kir isoforms display distinct specificities for the activating PIPs but all eukaryotic Kir channels are activated by PI(4,5)P₂. On the other hand, the bacterial KirBac1.1 channel is inhibited by PIPs. Recent crystal structures of eukaryotic Kir channels in apo and lipid bound forms reveal one specific binding site per subunit, formed at the interface of N- and C-terminal domains, just beyond the transmembrane segments and clearly involving some of the key residues previously identified as controlling PI(4,5)P₂ sensitivity. Computational, biochemical, and biophysical approaches have attempted to address the energetic determinants of PIP binding and selectivity among Kir channel isoforms, as well as the conformational changes that trigger channel gating. Here we review our current understanding of the molecular determinants of PIP regulation of Kir channel activity, including in context with other lipid modulators, and provide further discussion on the key questions that remain to be answered.