Raimund Dutzler

Volume-regulated anion channels, which are activated in response to hypotonicity, are heteromers composed of LRRC8 family members. They consist of the obligatory subunit LRRC8A and at least another paralog that determines their substrate preference. Here we investigate heteromeric assemblies composed of LRRC8A and D subunits, the latter of which confer permeability to amino acids, osmolytes and anti-cancer drugs. Using patch-clamp electrophysiology, we show that LRRC8A/D channels are reversibly activated by cell swelling and that their response is modulated by sybodies targeting the A subunits, which either potentiate or repress channel activity. Structures of the LRRC8A/D channel in complex with the inhibitory sybody Sb1 define the channel stoichiometry of four A and two D subunits, with Sb1 stabilizing the A subunits in a similar closed channel conformation as found in homomeric LRRC8A assemblies. In LRRC8A/D heteromers, the two D subunits weaken the arrangement of A subunits and thus enhance the activation properties of the channel. This channel conformation is strongly perturbed upon binding of the potentiating sybody Sb4, which disrupts the packing of the cytoplasmic domains linking their mobility to pore opening.

Volume-regulated anion channels (VRACs), formed by heteromers of LRRC8 subunits, mediate the transport of anions and organic osmolytes in numerous physiological processes. Although their gating mechanism remains poorly understood, conformational rearrangements of the cytosolic leucine-rich repeat domains (LRRDs) have been implicated as a key step towards activation. Here, we identify the intracellular subdomain (ISD) of the obligatory LRRC8A as a key element coupling LRRD motion to pore opening. Substitution of leucine 402 with tryptophan (L402W) within a hydrophobic pocket of the ISD rendered LRRC8A homomers constitutively open, independent of osmotic stimulation or pharmacological modulation. Mutation of the nearby tryptophan 168 to leucine (W168L) also enhanced channel activity whereas the combination of both mutations (W168L-L402W) partly attenuated the hyperactive phenotype of L402W. FRET measurements revealed increased LRRD flexibility of L402W, which was reduced in W168L-L402W or by coexpression with an inhibiting sybody. The cryo-EM structure of L402W showed pronounced ISD rearrangements and outward tilting of the LRRDs, while the double mutant W168L-L402W contains features explaining its intermediate phenotype. Together, these data identify the ISD as a regulatory hub that transmits conformational changes from the LRRDs to the transmembrane gate of VRAC.

Membrane contact sites are hubs for interorganellar lipid transport within eukaryotic cells. As a principal tether bridging the endoplasmic reticulum (ER) and the plasma membrane in Saccharomyces cerevisiae, the protein IST2 has a major role during lipid transport between both compartments. Here, we show a comprehensive investigation elucidating the structural and mechanistic properties of IST2 and its interaction with the soluble lipid transfer protein OSH6. The ER-embedded transmembrane domain of IST2 is homologous to the TMEM16 family and acts as a constitutively active lipid scramblase. The extended C terminus binds to the plasma membrane and the phosphatidylserine–phosphatidylinositol 4-phosphate exchanger OSH6. Through cellular growth assays and biochemical and structural studies, we characterized the interaction between both proteins and show that OSH6 remains associated with IST2 during lipid shuttling between membranes. These results highlight the role of the IST2–OSH6 complex in lipid trafficking and offer initial insights into the relevance of scramblases for carrier-like lipid transport mechanisms.

The Tweety homologues (TTYHs) constitute a family of eukaryotic membrane proteins that, on the basis of structural features, were recently proposed to contribute to lipid transfer between soluble carriers and cellular membranes¹. However, in the absence of supporting data, this function was hypothetical. Here through pull-down of endogenous proteins, we identify APOE as the interaction partner of human TTYH2. Subcellular fractionation and immunocytochemistry assays showed that both proteins colocalize in endosomal compartments. Characterization of the specific interaction between APOE and TTYH2 through binding assays and structural studies enabled us to identify an epitope in an extended domain of TTYH2 that faces the endosomal lumen. Structures of complexes with APOE-containing lipoprotein particles revealed a binding mode that places lipids in a suitable position to facilitate their diffusion into the membrane. Moreover, in vitro studies revealed that lipid transfer is accelerated by TTYH2. Collectively, our findings indicate that TTYH2 has a role in the unloading of APOE-containing lipoproteins after they are endocytosed. These results define a new protein class that facilitates the extraction of lipids from and their insertion into cellular membranes. Although ubiquitous, this process could be of particular relevance in the brain, where APOE is involved in the transfer of lipids between astrocytes and neurons.

Membrane contact sites (MCS) are hubs for inter-organellar lipid transport within eukaryotic cells. As one of the principal tethers bridging the ER and the plasma membrane in Saccharomyces cerevisiae , the protein Ist2 plays a major role during lipid transport between both compartments. Here, we present a comprehensive investigation elucidating structural and mechanistic properties of Ist2 and its interaction with the soluble lipid transport protein Osh6. The ER-embedded transmembrane domain of Ist2 is homologous to the TMEM16 family and acts as constitutively active lipid scramblase. The extended C-terminus binds to the plasma membrane and the PS/PI ₄ P exchanger Osh6. Through cellular growth assays, biochemical and structural studies, we have elucidated the interaction between both proteins and provide evidence that Osh6 remains associated with Ist2 during lipid shuttling between membranes. These results highlight the role of the Ist2/Osh6 complex in lipid trafficking and offer new insights into the relevance of scramblases at MCSs.

Iron and manganese are essential nutrients whose transport across membranes is catalyzed by members of the SLC11 family. In humans, this protein family contains two paralogs, the ubiquitously expressed DMT1, which is involved in the uptake and distribution of Fe²⁺ and Mn²⁺, and NRAMP1, which participates in the resistance against infections and nutrient recycling. Despite previous studies contributing to our mechanistic understanding of the family, the structures of human SLC11 proteins and their relationship to functional properties have remained elusive. Here we describe the cryo-electron microscopy structures of DMT1 and NRAMP1 and relate them to their functional properties. We show that both proteins catalyze selective metal ion transport coupled to the symport of H⁺, but additionally also mediate uncoupled H⁺ flux. Their structures, while sharing general properties with known prokaryotic homologs, display distinct features that lead to stronger transition metal ion selectivity.

Volume-regulated anion channels (VRACs) are multimeric proteins composed of different paralogs of the LRRC8 family. They are activated in response to hypotonic swelling, but little is known about their specific functions. We studied two human individuals with the same congenital syndrome affecting blood vessels, brain, eyes, and bones. The LRRC8C gene harbored de novo variants in both patients, located in a region of the gene encoding the boundary between the pore and a cytoplasmic domain, which is depleted of sequence variations in control subjects. When studied by cryo-EM, both LRRC8C mutant proteins assembled as their wild-type counterparts, but showed increased flexibility, suggesting a destabilization of subunit interactions. When co-expressed with the obligatory LRRC8A subunit, the mutants exhibited enhanced activation, resulting in channel activity even at isotonic conditions in which wild-type channels are closed. We conclude that structural perturbations of LRRC8C impair channel gating and constitute the mechanistic basis of the dominant gain-of-function effect of these pathogenic variants. The pleiotropic phenotype of this novel clinical entity associated with monoallelic LRRC8C variants indicates the fundamental roles of VRACs in different tissues and organs.

The CALHM proteins constitute a family of large pore channels that contains six closely related paralogs in humans. Two family members, CALHM1 and 3, have been associated with the release of ATP during taste sensation. Both proteins form heteromeric channels that activate at positive potential and decreased extracellular Ca²⁺ concentration. Although the structures of several family members displayed large oligomeric organizations of different size, their function has in most cases remained elusive. Our previous study has identified the paralogs CALHM2, 4 and, 6 to be highly expressed in the placenta and defined their structural properties as membrane proteins exhibiting features of large pore channels with unknown activation properties (Drożdżyk et al., 2020). Here, we investigated whether these placental paralogs would form heteromers and characterized heteromeric complexes consisting of CALHM2 and CALHM4 subunits using specific binders as fiducial markers. Both proteins assemble with different stoichiometries with the largest population containing CALHM2 as the predominant component. In these oligomers, the subunits segregate and reside in their preferred conformation found in homomeric channels. Our study has thus revealed the properties that govern the formation of CALHM heteromers in a process of potential relevance in a cellular context.

The CALHM proteins constitute a family of large pore channels that contains six closely related paralogs in human. Two family members, CALHM1 and 3, have been associated with the release of ATP during taste sensation. Both proteins form heteromeric channels that activate at positive potential and decreased extracellular Ca2+ concentration. Although the structures of several family members displayed large oligomeric organizations of different size, their function has in most cases remained elusive. Our previous study has identified the paralogs CALHM2, 4 and 6 to be highly expressed in the placenta and defined their structural properties as membrane proteins exhibiting features of large pore channels with unknown activation properties (Drozdzyk et al., 2020). Here we investigated whether these placental paralogs would form heteromers and characterized heteromeric complexes consisting of CALHM2 and CALHM4 subunits using specific binders as fiducial markers. Both proteins assemble with different stoichiometries with the largest population containing CALHM2 as predominant component. In these oligomers, the subunits segregate and reside in their preferred conformation found in homomeric channels. Our study has thus revealed the properties that govern the formation of CALHM heteromers in a process of potential relevance in a cellular context.

The CALHM proteins constitute a family of large pore channels that contains six closely related paralogs in human. Two family members, CALHM1 and 3, have been associated with the release of ATP during taste sensation. Both proteins form heteromeric channels that activate at positive potential and decreased extracellular Ca ²⁺ concentration. Although the structures of several family members displayed large oligomeric organizations of different size, their function has in most cases remained elusive. Our previous study has identified the paralogs CALHM2, 4 and 6 to be highly expressed in the placenta and defined their structural properties as membrane proteins exhibiting features of large pore channels with unknown activation properties (Drozdzyk et al., 2020). Here we investigated whether these placental paralogs would form heteromers and characterized heteromeric complexes consisting of CALHM2 and CALHM4 subunits using specific binders as fiducial markers. Both proteins assemble with different stoichiometries with the largest population containing CALHM2 as predominant component. In these oligomers, the subunits segregate and reside in their preferred conformation found in homomeric channels. Our study has thus revealed the properties that govern the formation of CALHM heteromers in a process of potential relevance in a cellular context.