Biology subjects

Ngassam, V. N.

Publications and source records attributed to Ngassam, V. N..

2 recordsLinked to original sources

bioRxiv2025

Apolipoprotein interaction induces shape remodeling and lipid phase separation in giant unilamellar vesicles

Apolipoprotein A-I (ApoA-I) - a 243-residue amphipathic protein containing an N-terminal globular domain and a primarily helical C-terminal lipid binding domain - is a principal protein component of high-density lipoprotein (HDL) or "good" cholesterol, which is an essential component of lipid homeostasis in humans. Synthesized in the liver and intestine and excreted in the blood, ApoA-I undergoes complex, cooperative, and dynamic self-assembly with membrane lipids, producing unlipi-dated (or weakly lipidated), nascent discoidal, and mature HDL states. In vitro studies demonstrate that the reconstitution of purified protein and lipids restores this cooperative self-assembly. However, the kinetic pathways by which these mesoscopic, proteolipidic assemblies form remain incompletely understood. Here, we monitor the dynamics of ApoA-I-membrane interactions through real-time monitoring of morphological changes, which ensue when ApoA-I is incubated with minimal giant unilamellar vesicles (GUVs) composed of single phospholipids or phase-separating phospholipid-cholesterol mixtures. Our fluorescence microscopy measurements reveal that the interaction initiates a gross, morphological remodeling of the parent vesicle proceeding through discrete stages involving membrane poration, solute leakage, vesiculation, and lipid-lipid phase separation. Our atomic force microscopy measurements confirm that the outcome includes discoidal nanoparticles. This qualitative phenomenology is robust and fully reproducible for different protein mutants and alleles (WT APOA-1, {Delta}49ApoA-I, ApoE-3, and ApoE-4) and other lipid mixtures (including mixtures containing phosphoserine lipids). Our molecular simulations recapitulate the essential shape changes and further reveal the composition dependence of the interactions. Together, these findings outline key steps in protein-lipid interactions that facilitate the assembly of mesoscopic reconstituted lipoproteins and nanodiscs.

biophysics↗

bioRxiv2020

Recurrent Dynamics of Rupture Transitions of Single Giant Vesicles at Solid Surfaces

Single giant vesicles (GVs) rupture spontaneously from their salt-laden suspension onto solid surfaces. At hydrophilic surfaces, they rupture via a recurrent burst-heal dynamics: during burst, single pores nucleate at the contact boundary of the adhering vesicles facilitating asymmetric spreading and producing a "heart" shaped membrane patch. During the healing phase, the competing pore closure produces a daughter vesicle. At hydrophobic surfaces, by contrast, the GVs rupture via a distinctly different, yet recurrent, bouncing ball rhythm: Rendered tense by the substrate interactions, GVs porate and spread monomolecular layer on the hydrophobic surface in a symmetric manner. Here too, the competition from pore closure produces a daughter vesicle, which re-engages with the substrate. In both cases, the pattern of burst-reseal events repeats multiple times splashing and spreading the vesicular fragments as bilayer patches at the solid surface in a pulsatory manner. These remarkable recurrent dynamics arise not because of the elastic properties of the solid surface but because the competition between membrane spreading and pore healing, prompted by the surface-energy dependent adhesion, determine the course of the topological transition. STATEMENT OF SIGNIFICANCEGiant lipid vesicles adhering to a solid surface experience strong mechanical stresses. The contacting membrane segment loses thermal fluctuations and accumulates mechanical tension, the equilibration of which can give rise to global shape changes, lipid phase separation, and traction forces. Beyond a threshold tension, vesicles porate, unravel, and spread. Here, we find that a competition from pore-healing can make rupture iterative, rather than a single all-or-nothing event. During burst, single pores expand, spreading a lipid bilayer on the hydrophilic surface and a monolayer on the hydrophobic one. During heal, pore-healing can produce daughter vesicles. This burst-reseal event reiterates "splashing" portions of single vesicles at the solid surface and "bouncing" the remainder as a secondary vesicle in multiple steps.

biophysics↗