Regulation of the human voltage-gated proton channel by membrane sterols

Structural basis of lipid-dependent allosteric gating mechanisms for PC1-PC2 ion channel

The polycystin complex, consisting of one polycystin-1 (PC1) and three polycystin-2 (PC2), forms a cation channel localized to the primary cilium and is critically involved in autosomal dominant polycystic kidney disease (ADPKD). This study reveals an allosteric gating mechanism of the PC1-PC2 channel modulated by specific membrane lipids. In typical membrane environments, phosphatidylglycerol (PG) and phosphatidic acid (PA) bind to the channel central pore, maintaining it in a closed state. Dissociation of these lipids transitions the channel to a pre-open state. The cilia-enriched oxysterol 7β,27-dihydroxycholesterol (7β,27-DHC) stabilizes the channel in a more open but still non-conductive conformation through an allosteric mechanism. Lipid-mediated regulation is coupled to large conformational rearrangements of the TOP and voltage-sensor-like domains (VSDs) of the third PC2 subunit, which eventually leads to pore opening. This lipid-dependent modulation is also observed in a gain-of-function channel. These findings reveal a distinct gating mechanism for the asymmetric 1:3 PC1-PC2 complex. Autosomal dominant polycystic kidney disease involves dysfunction of the PC1–PC2 ion channel. Here the authors reveal lipid-dependent allosteric gating and define multiple structural states showing how specific membrane lipids control channel opening.

Identification of a transient receptor potential channel that is regulated by phospholipid asymmetry

Phospholipid asymmetry is a hallmark of mammalian cell membranes and reflects the selective distribution of distinct phospholipid species between the two leaflets of the lipid bilayer. Although this asymmetry is tightly maintained, the membrane proteins whose functions depend on it remain largely unknown. To perturb phospholipid asymmetry experimentally, we expressed a constitutively active phospholipid scramblase and thereby identified transient receptor potential melastatin 8 (TRPM8) as an ion channel regulated by this membrane property. Activation of TRPM8 by both l-menthol and innocuous cold was markedly suppressed following disruption of phospholipid asymmetry. Likewise, selective depletion of phosphatidylserine (PS), a phospholipid enriched in the cytoplasmic leaflet, using a cytosolically targeted PS decarboxylase attenuated TRPM8 activation, indicating that cytoplasmic PS is required for proper TRPM8 function. Mechanistically, our findings suggest that cytoplasmic PS supports efficient TRPM8 activation by maintaining the biochemical state of the channel. Together, these findings identify TRPM8 as a phospholipid asymmetry-dependent ion channel and establish an experimental strategy for systematically identifying membrane proteins regulated by phospholipid asymmetry. This work provides a foundation for future studies investigating the biological significance of this fundamental membrane property.

Conditional Transmembrane Peptides as Allosteric Modulators: Thermodynamic and Functional Perspectives in Membrane Protein Regulation.

Membrane proteins participate in most cellular processes from sensing the cellular environment to regulating gene expression. The fundamental unit in eukaryotic membrane proteins is the transmembrane (TM) helix, which in addition to anchoring the protein to the lipid bilayer, often plays a central functional role. We review here advances in our understanding of the factors underpinning the thermodynamic stability of TM helices in cellular membranes. Interestingly, a class of TM sequences exists that has the ability to, when isolated as a peptide, be stable both in solution and in the membrane. This Review covers such conditional TM peptides, including their discovery in the form of the pHLIP peptide, the factors that determine their membrane insertion, and the basis for their use in cancer theranostics. We also discuss how this knowledge has been the conceptual basis for the design of sequences that partition into lipid bilayers and target membrane proteins. These peptides act as both positive and negative allosteric regulators of their membrane targets. Such TM allosteric conditional peptides (TMACs) additionally constitute tools that can advance our understanding of the activity and regulation of membrane proteins.

Molecular mechanisms of Kir channel gating

Abstract Inward-rectifying K+ (Kir) channels are ubiquitously present in variety of cells and play an important role in maintaining resting membrane potential and supporting K+ homeostasis. They are an important family of K+ channels that connects cellular metabolism to membrane excitability, and exhibit complex lipid–protein interaction landscape. Dysfunction of Kir channels is, therefore, associated with multi-factorial diseases and are important drug targets. Recent high-resolution structural dynamics and functional studies of several Kir channels have significantly advanced our understanding of the mechanisms of voltage-dependent pore block and channel gating regulation mediated by lipids, and other modulators. In the present minireview, based on recent knowledge derived from prokaryotic and eukaryotic Kir channels, we highlight the binding sites of the channel for various ligands/modulators, and also provide an emerging model focusing on the structural rearrangements associated with the transition of the channel from the closed/deactivated to open/activated conformation during lipid-dependent gating, which should be broadly applicable to all Kir channels.

Lipid modulation in a model K+ channel occurs through alterations at both, the inner and the outer channel gates.

This paper reports on a plausible mechanism of modulation by anionic phospholipids of KcsA, a model prokaryotic potassium channel. A first conclusion is that the association of anionic phospholipids with KcsA occurs at discrete regions located near both the extracellular and intracellular channel gates. This same behaviour is consistently observed in both detergent micelles and lipid bilayer-based experimental systems, indicating that the interaction does not depend on bulk bilayer characteristics. In the outer gate (i.e., the selectivity filter) anionic lipid binding to the so called non-annular lipid binding sites, drives the selectivity filter into a non-inactivating, conductive conformation, with a fairly high affinity to bind K+. Likewise, at the inner gate, the anionic lipid binding site seems to include cationic amino acid residues at the so called M0 helix. Here, interaction with anionic lipids facilitates the untangle of the C-terminal α-helical bundle at more neutral pH values, thus, facilitating inner gate opening. Both of these phenomena seem to occur independently and should favour the so called Conductive/Open state in the proposed gating cycle of the channel, which in the absence of anionic lipids is just a short-lived, transient state. Consequently, it is expected that the interactions of anionic lipid with KcsA should result in a great increase in ion channel activity. Indeed, this is the case, as reported in functional studies from different laboratories. Such an ability of specific lipids to regulate channel function points out to an interest in future developments of lipid-like drugs to control channelopathies.

Physiological Regulation and Pharmacology of Two-Pore Calcium Channels of the Endolysosomal System