Biology subjects

Jensen, L. E.

Publications and source records attributed to Jensen, L. E..

3 recordsLinked to original sources

bioRxiv2023

A Flat Protein Complex Shapes Rough ER Membrane Sheets

Rough ER sheets are a fundamental domain of the ER and the gateway into the secretory pathway. While reticulon proteins stabilize high-curvature ER tubules, it is unclear if other proteins scaffold the flat membranes of rough ER sheets. Through a proteomics screen using ER sheet localized RNA-binding proteins as bait, we identify the Sigma-1 receptor (SigmaR1) as an ER sheet shaping factor. High-resolution live cell imaging and electron tomography assign SigmaR1 as an ER sheet-localized factor whose levels determine the amount of rough ER sheets in cells. Structure-guided mutagenesis and in vitro reconstitution on giant unilamellar vesicles further support a mechanism whereby SigmaR1 oligomers use their extended arrays of amphipathic helices to bind and flatten the lumenal leaflet of ER membranes. Our results demonstrate an unexpected way for proteins to sense and propagate flat membrane sheets.

cell biology↗

bioRxiv2022

Membrane curvature sensing and stabilization by the autophagic LC3 lipidation machinery

How the highly curved phagophore membrane is stabilized during autophagy initiation is a major open question in autophagosome biogenesis. Here, we use in vitro reconstitution on membrane nanotubes and molecular dynamics simulations to investigate how core autophagy proteins in the LC3 lipidation cascade interact with curved membranes, providing insight into possible roles in regulating membrane shape during autophagosome biogenesis. ATG12-5-16L1 was up to 100-fold enriched on highly curved nanotubes relative to flat membranes. At high surface density, ATG12-5-16L1 binding increased the curvature of the nanotubes. While WIPI2 binding directs membrane recruitment, the amphipathic helix 2 of ATG16L1 is responsible for curvature sensitivity. Molecular dynamics simulations revealed that helix 2 of ATG16L1 inserts shallowly into the membrane, explaining its curvature-sensitive binding to the membrane. These observations show how the binding of the ATG12-5-16L1 complex to the early phagophore rim could stabilize membrane curvature and facilitate autophagosome growth.

biophysics↗

bioRxiv2019

Organization and Regulation of Chromatin by Liquid-Liquid Phase Separation

Genomic DNA is highly compacted in the nucleus of eukaryotic cells as a nucleoprotein assembly called chromatin1. The basic unit of chromatin is the nucleosome, where [~]146 base pair increments of the genome are wrapped and compacted around the core histone proteins2,3. Further genomic organization and compaction occur through higher order assembly of nucleosomes4. This organization regulates many nuclear processes, and is controlled in part by histone post-transtranslational modifications and chromatin-binding proteins. Mechanisms that regulate the assembly and compaction of the genome remain unclear5,6. Here we show that in the presence of physiologic concentrations of mono- and divalent salts, histone tail-driven interactions drive liquid-liquid phase separation (LLPS) of nucleosome arrays, resulting in substantial condensation. Phase separation of nucleosomal arrays is inhibited by histone acetylation, whereas histone H1 promotes phase separation, further compaction, and decreased dynamics within droplets, mirroring the relationship between these modulators and the accessibility of the genome in cells7-10. These results indicate that under physiologically relevant conditions, LLPS is an intrinsic behavior of the chromatin polymer, and suggest a model in which the condensed phase reflects a genomic \"ground state\" that can produce chromatin organization and compaction in vivo. The dynamic nature of this state could enable known modulators of chromatin structure, such as post-translational modifications and chromatin binding proteins, to act upon it and consequently control nuclear processes such as transcription and DNA repair. Our data suggest an important role for LLPS of chromatin in the organization of the eukaryotic genome.

biophysics↗