Biology subjects

Haapanen, O.

Publications and source records attributed to Haapanen, O..

6 recordsLinked to original sources

bioRxiv2026

Structural Rewiring of IL-7R Dimerization by an Oncogenic Transmembrane Mutation Can Be Reversed by Rational Design

Mutations within the transmembrane domains (TMDs) of single-pass transmembrane receptors often cause aberrant, ligand-independent receptor signaling associated with diverse malignancies, but their mechanism of action remain largely unknown. These TMD mutations are generally not targetable as they are buried in membrane. Here, we determined the mechanism of a gain-of-function (GOF) TMD mutation of interleukin-7 receptor (IL-7R) associated with T-cell acute lymphoblastic leukemia, and addressed the possibility of directly targeting the TMD mutation by using rationally designed transmembrane helices to restore order to uncontrolled signaling. We find that the GOF mutation of IL-7R severely shifts the TMD homodimerization interface, causing the receptor to homodimerize in a geometry that activates downstream signaling independent of ligand. Designed transmembrane helices that interfere with the new interface, delivered with mRNA technology, selectively block ligand-independent but not ligand-dependent signaling. Our study provides a conceptual framework for understanding and repairing disease-causing TMD mutations of single-pass cytokine receptors.

biophysics↗

bioRxiv2024

Conformational dynamics of a histidine molecular switch in a cation/proton antiporter

Multisubunit Mrp type sodium proton (Na+/H+) antiporters are indispensable for the growth of alkali and salt tolerant bacteria and archaea under challenging conditions. Several subunits of the membrane protein complex are closely related to the membrane bound subunits of mitochondrial respiratory complex I, a key enzyme in aerobic energy metabolism. The molecular mechanism of ion translocation by complex I and Mrp antiporters has remained largely unknown and is the subject of intense debate. Here, we combine the power of site-directed mutagenesis and large-scale atomistic molecular dynamics simulations to elucidate the molecular basis of proton-conducting function of the MrpA subunit of the antiporter. We show that point mutations directly affect the transport activity by perturbing the conformational dynamics of a key histidine residue that acts as a molecular switch. Importantly, we find that charge and protonation state variations drive hydrogen bonding rearrangements and hydration changes, which result in coupled sidechain and backbone level conformational changes of the histidine and neighboring residues. We propose the histidine switch as a unique functional element central to proton transport function in Mrp antiporters and respiratory complex I. Significance statementEnergy conversion in biology is catalyzed by several membrane-bound enzyme complexes that drive transmembrane charge translocation. A key question is how these charges are moved against a membrane gradient in an efficient manner, i.e., what kind of gating and fail-safe mechanisms are employed by the proteins to ensure charge transfer directionality. By studying cation/proton Mrp antiporter, which shares homology with the mitochondrial complex I, we describe a conserved histidine residue acting as a molecular switch crucial for gating transfer of protons. Our work emphasizes the functional significance of conserved and conformationally mobile motifs in proteins. The results will benefit the mechanistic understanding of naturally existing proteins and design of artificial enzymes.

biophysics↗

bioRxiv2022

Structural basis of mitochondrial membrane bending by I-II-III2-IV2 supercomplex

Mitochondrial energy conversion requires an intricate architecture of the inner mitochondrial membrane1. Here we show that in ciliates, the membrane curvature is provided by a supercomplex containing all four respiratory chain components. We report cryo-electron microscopy and cryo-tomography structures of the supercomplex that comprises 150 different proteins and 311 bound lipids, forming a stable 5.8-megadalton assembly. Due to subunit acquisition and extension, complex I associates with a complex IV dimer, generating a wedge-shaped gap that serves as a binding site for complex II. Together with a tilted complex III dimer association, it results in a curved membrane region. Using molecular dynamics simulations, we demonstrate that the divergent supercomplex actively contributes to the membrane curvature induction and cristae tubulation. Our findings explain how the architecture of the native I-II-III2-IV2 supercomplex reflects the functional specialization of bioenergetics by shaping the membrane.

biochemistry↗

bioRxiv2022

Tunnel dynamics of quinone derivatives and its coupling to protein conformational rearrangements in respiratory complex I

Respiratory complex I in mitochondria and bacteria catalyzes the transfer of electrons from NADH to quinone (Q). The free energy available from the reaction is used to pump protons and to establish a membrane proton electrochemical gradient, which drives ATP synthesis. Even though several high-resolution structures of complex I have been resolved, how Q reduction is linked with proton pumping, remains unknown. Here, microsecond long molecular dynamics (MD) simulations were performed on Yarrowia lipolytica complex I structures where Q molecules have been resolved in the ~30 [A] long Q tunnel. MD simulations of several different redox/protonation states of Q reveal the coupling between the Q dynamics and the restructuring of conserved loops and ion pairs. Oxidized quinone stabilizes towards the N2 FeS cluster, a binding mode not previously described in Yarrowia lipolytica complex I structures. On the other hand, reduced (and protonated) species tend to diffuse towards the Q binding sites closer to the tunnel entrance. Mechanistic and physiological relevance of these results are discussed.

biophysics↗

bioRxiv2022

Ion transfer mechanisms in Mrp-type antiporters from high resolution cryoEM and molecular dynamics simulations

Multiple resistance and pH adaptation (Mrp) cation/proton antiporters are essential for growth of a variety of halophilic and alkaliphilic bacteria under stress conditions. Mrp-type antiporters are closely related to the membrane domain of respiratory complex I. We determined the structure of the Mrp antiporter from Bacillus pseudofirmus by electron cryo-microscopy at 2.2 [A] resolution. The structure resolves more than 99% of the sidechains of the seven membrane subunits MrpA to MrpG plus 360 water molecules, including [~]70 in putative ion translocation pathways. Molecular dynamics simulations based on the high-resolution structure revealed details of the antiport mechanism. We find that switching the position of a histidine residue between three hydrated pathways in the MrpA subunit is critical for proton transfer that drives gated transmembrane sodium translocation. Several lines of evidence indicate that the same histidine-switch mechanism operates in respiratory complex I.

biochemistry↗

bioRxiv2021

High-resolution structure and dynamics of mitochondrial complex I - insights into the proton pumping mechanism

Mitochondrial NADH:ubiquinone oxidoreductase (complex I) is a 1 MDa membrane protein complex with a central role in energy metabolism. Redox-driven proton translocation by complex I contributes substantially to the proton motive force that drives ATP synthase. Several structures of complex I from bacteria and mitochondria have been determined but its catalytic mechanism has remained controversial. We here present the cryo-EM structure of complex I from Yarrowia lipolytica at 2.1 [A] resolution, which reveals the positions of more than 1600 protein-bound water molecules, of which [~]100 are located in putative proton translocation pathways. Another structure of the same complex under steady-state activity conditions at 3.4 [A] resolution indicates conformational transitions that we associate with proton injection into the central hydrophilic axis. By combining high-resolution structural data with site-directed mutagenesis and large-scale molecular dynamics simulations, we define details of the proton translocation pathways, and offer new insights into the redox-coupled proton pumping mechanism of complex I.

biochemistry↗