Biology subjects

Maschmann, Z. A.

Publications and source records attributed to Maschmann, Z. A..

3 recordsLinked to original sources

bioRxiv2026

Signaling mechanism of the transmembrane energy receptor Aer

The E. coli aerotaxis receptor Aer is a bacterial chemoreceptor that senses intracellular redox changes via an N terminal PAS domain bound to a flavin adenine dinucleotide (FAD) cofactor. Distinct from canonical methyl-accepting chemotaxis proteins (MCPs) such as Tar/Tsr, Aer lacks a periplasmic ligand-binding domain and adaptive methylation, transmitting conformational signals laterally from the PAS domain to the HAMP domain and the methylation helix-like cap (MHL cap) of the kinase control domain (KCD). To elucidate the Aer signalling mechanism, we determined cryo electron microscopy (cryo EM) structures of full length Aer in oxidized flavin quinone (kinase on) and reduced semiquinone (kinase off) states. Structural comparison reveals redox linked rearrangements of the FAD binding pocket, reorientation of PAS-HAMP interactions, and strikingly altered MHL cap stability. PAS-MHL-cap contact in the oxidized state compresses the receptor and stabilized proximal KCD helices, whereas reduction disrupts these contacts, increasing KCD flexibility. To probe distal effects on KCD architecture, we performed nanodisc reconstitution and pulse dipolar ESR spectroscopy on spin labelled positions along the four helix bundle. Distance distributions indicate redox dependent changes in helix separation, particularly at the C terminal MHL2 region, consistent with PAS driven loosening of KCD packing in kinase off states. These data support a model in which FAD redox chemistry reorganizes flavin pocket residues that in turn subtly alter PAS conformation to influence PAS-HAMP and PAS-MHL-cap packing and hence KCD conformational stability. The findings reveal an Aer specific signaling axis distinct from periplasmic ligand binding MCPs that has adapted MCP architecture for lateral PAS input and cytoplasmic redox sensing.

biochemistry↗

bioRxiv2025

Opsins are Phospholipid Scramblases in All Domains of Life

Opsins are highly abundant retinal proteins in the membranes of photoheterotrophic bacteria. However, some microbial genomes encode an opsin but lack the gene for the final enzyme in retinal synthesis. To account for this paradox, we hypothesized that bacterial opsins play a role in membrane structure and/or biogenesis independent from their potential for light-driven signaling or proton pumping. After purifying actinorhodopsin from a cell-free expression system and from E. coli membranes upon overexpression, we demonstrated both in vitro and in silico that actinorhodopsin from Nanopelagicus ca. is a phospholipid scramblase, serving in its pentameric state as a retinal-independent phospholipid diffusion channel. Phospholipid headgroups move along a transbilayer path between actinorhodopsin protomers, to equilibrate lipid content in the inner and outer leaflets. Two profound activities, membrane biosynthesis and capture of light energy, are thus facilitated by one ancient bacterial polypeptide. Light-dependent activity and light-independent phospholipid scrambling are shared functions of eukaryotic, archaeal, and bacterial rhodopsins. ImportanceCells are surrounded by membranes which concentrate metabolites and protect cellular contents. Most biomembranes are phospholipid bilayers, in which the phospholipids of each leaflet orient their greasy tails inward and polar groups outward. Bilayer biogenesis depends on phospholipids synthesized on the cytofacial side of the membrane reorienting to the extracellular membrane leaflet. This reorientation requires proteins, termed scramblases, and it was shown that rhodopsins -- 7-helix photoactive membrane proteins bound to the cofactor retinal -- from organisms as widely divergent as mammals and archaea possess scramblase activity. Now we conclusively demonstrate using purified proteins in laboratory membranes as well as computational approaches, that bacterial rhodopsins are also phospholipid scramblases. This work is important because it highlights a surprising commonality among bacteria, archaea and eukaryotes and because it shows that rhodopsins - ancient proteins found in the last universal common ancestor - manifest two seemingly unrelated biochemical functions in one protein.

biophysics↗

bioRxiv2023

Signal Transduction and Kinase Control in Nanodisc Reconstituted E. coli Aerotaxis Receptor

WITHDRAWAL STATEMENTThe authors have withdrawn this manuscript because its being revised. The authors wish to withdraw this manuscript for two reasons. Firstly, the relevance and validity of some of the mass spectrometry data in the manuscript are being questioned by the authors. Secondly, some of the authors had not adequately reviewed the manuscript before it was uploaded to bioRxiv. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author.

biophysics↗