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Change in conformation of integral membrane protein X to wedge the membrane.

Fig. 3 Different mechanisms of membrane bending in red cells and other cell types. (a) Alteration of lipid composition of one leaflet of the bilayer. The double grey line denotes the bilayer. In this cartoon, a new lipid is added to the inner leaflet, causing inward membrane bending. (b) Bending by contraction of the underlying cytoskeleton. The cytoskeleton, denoted by a spring-shaped meshwork underlying the membrane, contracts, in this case causing outward bending, which would be manifest in the red cell as acanthocytosis. (c) Bending by assembly into membrane of new cytoskeletal proteins that associate via electrostatic interactions with integral proteins. Clathrin is the paradigm for this mechanism (d) Bending by reversible association of 'BAR domain' protein with one leaflet of the membrane. This protein has angled ends that directly associate with the lipid bilayer of the membrane, joined by an angled crank. As it binds to the membrane, it causes inward bending. (e) Membrane bending due to change in conformation of an integral membrane protein. The membrane-spanning protein denoted by a rectangle changes conformation to become wedge-shaped, changing the shape of the cell. This has never been shown directly but is theoretically feasible

Change in conformation of integral membrane protein X to wedge the membrane.

Flat bilayer

Fig. 3 Different mechanisms of membrane bending in red cells and other cell types. (a) Alteration of lipid composition of one leaflet of the bilayer. The double grey line denotes the bilayer. In this cartoon, a new lipid is added to the inner leaflet, causing inward membrane bending. (b) Bending by contraction of the underlying cytoskeleton. The cytoskeleton, denoted by a spring-shaped meshwork underlying the membrane, contracts, in this case causing outward bending, which would be manifest in the red cell as acanthocytosis. (c) Bending by assembly into membrane of new cytoskeletal proteins that associate via electrostatic interactions with integral proteins. Clathrin is the paradigm for this mechanism (d) Bending by reversible association of 'BAR domain' protein with one leaflet of the membrane. This protein has angled ends that directly associate with the lipid bilayer of the membrane, joined by an angled crank. As it binds to the membrane, it causes inward bending. (e) Membrane bending due to change in conformation of an integral membrane protein. The membrane-spanning protein denoted by a rectangle changes conformation to become wedge-shaped, changing the shape of the cell. This has never been shown directly but is theoretically feasible groups at position C24 on the side chain. The nuclear part of the sterol is identical to that in cholesterol. These phytosterols, which are found in the 'oily' plants such as olive and avocado, circulate in the blood and can partition into the red cell membrane. The shape change caused by phytosterols begs the question, what role do sterols play in membrane bending? This is very hard to answer. There is a major technical difficulty; exchange of cholesterol between the inner and outer leaflets is very fast, quite beyond measurement by current methods [63]. Is it pumped? This is impossible to know. Is there more cholesterol associated with the inner leaflet than the outer? Again, this is very difficult to know, but this was the conclusion reached by Devaux and others some years ago, using spin-labelled cholesterol [59].

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