Membrane Curvature, Trans-Membrane Area Asymmetry, Budding, Fission and Organelle Geometry.

Alexander A Mironov,Anna Mironov,Galina V Beznoussenko,Jure Derganc

doi:10.3390/ijms21207594

Abstract

In biology, the modern scientific fashion is to mostly study proteins. Much less attention is paid to lipids. However, lipids themselves are extremely important for the formation and functioning of cellular membrane organelles. Here, the role of the geometry of the lipid bilayer in regulation of organelle shape is analyzed. It is proposed that during rapid shape transition, the number of lipid heads and their size (i.e., due to the change in lipid head charge) inside lipid leaflets modulates the geometrical properties of organelles, in particular their membrane curvature. Insertion of proteins into a lipid bilayer and the shape of protein trans-membrane domains also affect the trans-membrane asymmetry between surface areas of luminal and cytosol leaflets of the membrane. In the cases where lipid molecules with a specific shape are not predominant, the shape of lipids (cylindrical, conical, or wedge-like) is less important for the regulation of membrane curvature, due to the flexibility of their acyl chains and their high ability to diffuse.

Highlights

Biological membranes consist of two lipid monolayers, attached to each other through acyl chains
We demonstrated that (i) inhibition of the SNARE machinery alone reduced trans-membrane area asymmetry (TAA) of the Golgi cisternae, and induced a narrowing of the cisternal perforations, followed by invagination of cisternal membranes; (ii) inhibition of the ARF/COPI machinery alone increased the TAA of the Golgi cisternae, and induced a widening of the cisternal perforations, followed by the Golgi tubulation; and (iii) inhibition of both machineries did not change Golgi shape significantly
Membrane curvature plays a significant role in phase transition facilitating fission of membrane tubules and making lipid and protein sorting easier

Summary

Introduction

Biological membranes consist of two lipid monolayers, attached to each other through acyl chains. When one measures the level of membrane curvature by means of electron microscopy, the results depend on the sample preparation, namely, fixation and staining procedures. In living cells the trans-membrane area asymmetry (TAA; the ratio between the surface area of two membrane leaflets) of the coatomer (COP) I-dependent vesicles is 1.4, whereas after fixation by osmium OsO4 the measurement of this parameter would give 1.5. The mean diameter of intra-luminal vesicles inside multi-vesicular bodies is 42 nm Their TAA is equal to 1.53; after the preparation of samples for electron microscopy TAA will be equal to 2.61 [4] (Figure 1A,B). The diameter of the plasmalemma surface area used for the filopodium formation is equal to 4000. The surface area is equal to 1.32 × 107 nm (R—radius of the plasmalemma area; r—radius of the tip and filopodium; h—height)

Factors Determining the Membrane Curvature

Membrane Geometry and Organelle Dynamics

Findings

Conclusions and Perspectives