Lipid Composition but Not Curvature Is the Determinant Factor for the Low Molecular Mobility Observed on the Membrane of Virus-Like Vesicles

Abstract

Human Immunodeficiency Virus type-1 (HIV-1) acquires its lipid membrane from the plasma membrane of the infected cell from which it buds out. Previous studies have shown that the HIV-1 envelope is an environment of very low mobility, with the diffusion of incorporated proteins two orders of magnitude slower than in the plasma membrane. One of the reasons for this difference is thought to be the HIV-1 membrane composition that is characterised by a high degree of rigidity and lipid packing, which has, until now, been difficult to assess experimentally. To further refine the model of the molecular mobility on the HIV-1 surface, we herein investigated the relative importance of membrane composition and curvature in simplified model membrane systems, large unilamellar vesicles (LUVs) of different lipid compositions and sizes (0.1–1 µm), using super-resolution stimulated emission depletion (STED) microscopy-based fluorescence correlation spectroscopy (STED-FCS). Establishing an approach that is also applicable to measurements of molecule dynamics in virus-sized particles, we found, at least for the 0.1–1 µm sized vesicles, that the lipid composition and thus membrane rigidity, but not the curvature, play an important role in the decreased molecular mobility on the vesicles’ surface. This observation suggests that the composition of the envelope rather than the particle geometry contributes to the previously described low mobility of proteins on the HIV-1 surface. Our vesicle-based study thus provides further insight into the dynamic properties of the surface of individual HIV-1 particles, as well as paves the methodological way towards better characterisation of the properties and function of viral lipid envelopes in general.

STED super-resolution imaging of membrane packing and dynamics by exchangeable polarity-sensitive dyes

“…Twochannel FCS acquisitions permit the simultaneous calculation of diffusion coefficients and lipid packing by means of the GP function. By employing this approach, we found that the lipid packing and bilayer dynamics anticorrelate in model and live-cell membranes, suggesting that the slow diffusion coefficients commonly measured in plasma membranes can be ascribed to the high degree of lipid packing of this structure, which arises from its distinct lipid composition 50 , as previously suggested for highly packed viral membranes 52 .…”

HIV-1 Gag specifically restricts PI(4,5)P2 and cholesterol mobility in living cells creating a nanodomain platform for virus assembly

“…How enveloped viruses acquire their lipid membrane shell is of great importance for their biogenesis and their capacity to infect cells. In the case of HIV-1, recently, it has been shown that the envelope lipid composition was a determinant of low molecular mobility of the Env protein ( 3 , 16 ). Several works have been conducted to reveal different envelope viruses having their own unique lipid composition ( 2 , 7 , 9 , 42 ), implying their distinct biological requirements during infection, and suggesting their capacity in sorting specific host lipids into their virus membrane.…”

“…Fortunately, recent advances in super-resolution microscopy techniques have enabled for the characterization of molecular mobility at the nanoscale, i.e., on the scale of individual (<140 nm) HIV-1 particles. For example, experiments using a combination of super-resolution [stimulated emission depletion (STED) microscopy and fluorescence correlation spectroscopy (STED-FCS) ( 13 ) or scanning STED-FCS (sSTED-FCS) ( 14 , 15 )] revealed a low mobility of molecules on the HIV-1 surface ( 3 ) due to the high degree of lipid packing in the virus membrane ( 15 , 16 ).…”

Fluorescence Microscopy of the HIV-1 Envelope

“…An alternative system to lipid extraction is the use of artificial model membranes, i.e., membranes made of simple lipid mixtures that resemble the chemical and biophysical properties of the HIV lipid envelope. Imaging of artificial model systems has been employed to determine lipid dynamics and molecular order of virus-like membranes [14,55,56].…”