The membrane electric field regulates the PIP2-binding site to gate the KCNQ1 channel.

Voltage-dependent ion channels underlie the propagation of action potentials and other forms of electrical activity in cells. In these proteins, voltage sensor domains (VSDs) regulate opening and closing of the pore through the displacement of their positive-charged S4 helix in response to the membrane voltage. The movement of S4 at hyperpolarizing membrane voltages in some channels is thought to directly clamp the pore shut through the S4-S5 linker helix. The KCNQ1 channel (also known as Kv7.1), which is important for heart rhythm, is regulated not only by membrane voltage but also by the signaling lipid phosphatidylinositol 4,5-bisphosphate (PIP2). KCNQ1 requires PIP2 to open and to couple the movement of S4 in the VSD to the pore. To understand the mechanism of this voltage regulation, we use cryogenic electron microscopy to visualize the movement of S4 in the human KCNQ1 channel in lipid membrane vesicles with a voltage difference across the membrane, i.e., an applied electric field in the membrane. Hyperpolarizing voltages displace S4 in such a manner as to sterically occlude the PIP2-binding site. Thus, in KCNQ1, the voltage sensor acts primarily as a regulator of PIP2 binding. The voltage sensors' influence on the channel's gate is indirect through the reaction sequence: voltage sensor movement → alter PIP2 ligand affinity → alter pore opening.

Voltage-sensor movements in the Eag Kv channel under an applied electric field.

Voltage-dependent ion channels regulate the opening of their pores by sensing the membrane voltage. This process underlies the propagation of action potentials and other forms of electrical activity in cells. The voltage dependence of these channels is governed by the transmembrane displacement of the positive charged S4 helix within their voltage-sensor domains. We use cryo-electron microscopy to visualize this movement in the mammalian Eag voltage-dependent potassium channel in lipid membrane vesicles with a voltage difference across the membrane. Multiple structural configurations show that the applied electric field displaces S4 toward the cytoplasm by two helical turns, resulting in an extended interfacial helix near the inner membrane leaflet. The position of S4 in this down conformation is sterically incompatible with an open pore, thus explaining how movement of the voltage sensor at hyperpolarizing membrane voltages locks the pore shut in this kind of voltage-dependent K+ (Kv) channel. The structures solved in lipid bilayer vesicles detail the intricate interplay between Kv channels and membranes, from showing how arginines are stabilized deep within the membrane and near phospholipid headgroups, to demonstrating how the channel reshapes the inner leaflet of the membrane itself.

Plastid phylogenomic insights into the evolution of Caryophyllales.

The Caryophyllales includes 40 families and 12,500 species, representing a large and diverse clade of angiosperms. Collectively, members of the clade grow on all continents and in all terrestrial biomes and often occupy extreme habitats (e.g., xeric, salty). The order is characterized by many taxa with unusual adaptations including carnivory, halophytism, and multiple origins of C4 photosynthesis. However, deep phylogenetic relationships within the order have long been problematic due to putative rapid divergence. To resolve the deep-level relationships of Caryophyllales, we performed phylogenomic analyses of all 40 families of Caryophyllales. We time-calibrated the molecular phylogeny of this clade, and evaluated putative correlations among plastid structural changes and rates of molecular substitution. We recovered a well-resolved and well-supported phylogeny of the Caryophyllales that was largely congruent with previous estimates of this order. Our results provide improved support for the phylogenetic position of several key families within this clade. The crown age of Caryophyllales was estimated at ca. 114.4 million years ago (Ma), with periods of rapid divergence in the mid-Cretaceous. A strong, positive correlation between nucleotide substitution rate and plastid structural changes was detected. Our study highlights the importance of broad taxon sampling in phylogenomic inference and provides a firm basis for future investigations of molecular, morphological, and ecophysiological evolution in Caryophyllales.

High-Sensitivity Detection of Nanometer 1H-19F Distances for Protein Structure Determination by 1H-Detected Fast MAS NMR.

Protein structure determination by solid-state NMR requires the measurement of many interatomic distances through dipole-dipole couplings. To obtain multiple long-range distance restraints rapidly and with high sensitivity, here we demonstrate a new 1H-detected fast magic-angle-spinning NMR technique that yields many long distances in a two-dimensional (2D)-resolved fashion. The distances are measured up to ∼15 Å, with an accuracy of better than 10%, between 1H and 19F, two nuclear spins that have the highest gyromagnetic ratios. Exogenous fluorines are sparsely introduced into the aromatic residues of the protein, which is perdeuterated and back-exchanged to give amide protons. This 1H-19F distance experiment, termed 2D heteronuclear single-quantum coherence rotational-echo double-resonance (HSQC-REDOR), is demonstrated on the singly fluorinated model protein, GB1. We extracted 33 distances between 5-19F-Trp43 and backbone amide protons, using 2D spectral series that were measured in less than 3 days. Combining these 1H-19F distance restraints with 13C-19F distances and chemical shifts, we calculated a GB1 structure with a backbone root-mean-square deviation of 1.73 Å from the high-resolution structure. This 1H-detected 1H-19F distance technique promises to provide a highly efficient tool for constraining the three-dimensional structures of proteins and protein-ligand complexes, with not only precise and fast measurements but also access to truly long-range distances.