Structural Insights into Aquaporin-0 Array Formation in Sphingomyelin and Cholesterol-Rich Membranes and Implications for Lipid Raft Organization

Core Concepts

The presence of cholesterol at the interface between adjacent aquaporin-0 (AQP0) tetramers in lipid membranes increases the mechanical stability of the AQP0 arrays, which may contribute to the formation and stabilization of lipid rafts in biological membranes.

Abstract

The study investigates the structural organization of aquaporin-0 (AQP0) in membranes composed of sphingomyelin and cholesterol, which are the main lipid components of lens membranes where AQP0 is highly abundant.
Key highlights:

AQP0 forms 2D crystal arrays in membranes with varying sphingomyelin to cholesterol ratios, consistent with the organization of AQP0 in native lens membranes.
Electron crystallographic structures of AQP0 in 2:1 and 1:2 sphingomyelin:cholesterol membranes reveal distinct cholesterol localization patterns around AQP0.
Molecular dynamics simulations show that the positions of cholesterol around AQP0 observed in the crystal structures represent the preferred cholesterol binding sites on the AQP0 surface.
A deep-binding cholesterol molecule is observed sandwiched between two adjacent AQP0 tetramers in the 1:2 sphingomyelin:cholesterol structure.
The presence of this deep-binding cholesterol increases the mechanical force required to separate the associated AQP0 tetramers, suggesting it stabilizes the AQP0 arrays.
The enhanced mechanical stability provided by the deep-binding cholesterol may contribute to the formation and stabilization of lipid rafts in biological membranes.

Stats

The molar ratio of cholesterol to other phospholipids in the human lens core plasma membrane ranges from 2.02 to 2.52, the highest of all human tissues.
AQP0 makes up over 60% of the total protein content in lens membranes.

Quotes

"Cholesterol has a planar four-aromatic ring structure with a short isooctyl alkyl chain and a small 3-β-hydroxyl head group that can form a hydrogen bond with a polar group."
"Mammalian lens membranes contain a high percentage of sphingomyelin and cholesterol, lipids that are characteristic for lipid rafts."
"The presence of cholesterol at the interface between adjacent AQP0 tetramers increases the force required to laterally detach two AQP0 tetramers, not only due to protein–protein contacts but also due to increased lipid–protein complementarity."

Key Insights Distilled From

Structure and dynamics of cholesterol-mediated aquaporin-0 arrays and implications for lipid rafts

by Chiu,P.-L., ... at www.biorxiv.org 05-18-2023

https://www.biorxiv.org/content/10.1101/2023.05.16.540959v3

Deeper Inquiries

How might the principles of AQP0 array formation and stabilization by cholesterol be applied to engineer protein clustering in synthetic membrane systems

The principles of AQP0 array formation and stabilization by cholesterol could be applied to engineer protein clustering in synthetic membrane systems by incorporating specific lipid compositions that promote protein-protein interactions and array formation. By designing membranes with a high cholesterol content and specific lipid arrangements, similar to those found in lens membranes, it may be possible to induce the clustering of target membrane proteins. The cholesterol molecules could act as "glue" to stabilize protein-protein interactions, similar to how they stabilize AQP0 tetramers in the lens membranes. Additionally, understanding the role of lipid-protein complementarity in stabilizing protein arrays could guide the design of synthetic membranes to promote protein clustering. By mimicking the lipid environment of AQP0 arrays, synthetic membrane systems could be engineered to facilitate the formation of protein clusters with specific spatial organization and functional properties.

What other membrane proteins, besides AQP0, might exhibit similar cholesterol-mediated clustering behavior, and how could this impact their biological functions

Other membrane proteins that might exhibit similar cholesterol-mediated clustering behavior include proteins that are known to interact with cholesterol or are found in lipid raft domains. Proteins such as caveolins, flotillins, and certain G protein-coupled receptors are known to associate with cholesterol-rich lipid rafts in cell membranes. These proteins could potentially form clusters or arrays in the presence of cholesterol, similar to AQP0. The impact of cholesterol-mediated clustering on the biological functions of these proteins could be significant, as it could influence their localization, interactions with other proteins, and signaling pathways. Understanding the role of cholesterol in promoting protein clustering could provide insights into the regulation of membrane protein function and organization in various cellular processes.

Could the deep-binding cholesterol observed between AQP0 tetramers play a role in regulating water permeability or other functional properties of the AQP0 arrays in the lens

The deep-binding cholesterol observed between AQP0 tetramers could play a role in regulating water permeability and other functional properties of the AQP0 arrays in the lens. The presence of cholesterol at the interface between adjacent AQP0 tetramers increases the force required to separate them, suggesting that cholesterol may contribute to the mechanical stability of the arrays. This deep-binding cholesterol could potentially affect the structural integrity of the AQP0 arrays and their ability to form tight junctions in the lens membrane. Additionally, the specific interactions between cholesterol and AQP0 may modulate the water permeability of the arrays, influencing the flow of water through the lens and maintaining lens transparency. Further research is needed to fully understand the functional implications of deep-binding cholesterol in regulating the properties of AQP0 arrays in the lens membrane.