Membrane Proteins


Biological membranes consist of lipids and proteins. The lipids are arranged as a fluid bilayer and the proteins are either embedded in the lipid or are bound to the membrane surface. Research on membranes has moved from early studies of the lipids and membrane models to the determination of the molecular structure of membrane proteins.

Keywords: membrane proteins; lipid bilayer; α helix; hydrophobic; detergent

Figure 1.

Model of a biological membrane showing a phospholipid bilayer and intrinsic membrane proteins. The lipid bilayer consists of a hydrophobic core made up of the acyl chains of the lipids and interface regions made up of the polar headgroups of the lipids. Single‐span or bitopic membrane proteins cross the membrane once and are of two types. Type I membrane proteins have their carboxyl‐terminus on the cytosolic side of the membrane and their amino‐terminus on the extracellular side, while type II membrane proteins have the opposite orientation. Multispan (polytopic) membrane proteins cross the membrane a number of times. The transmembrane segments (red boxes) are α‐helical in conformation. N, amino‐terminus; C, carboxyl‐terminus.

Figure 2.

Structure of bacteriorhodopsin. The protein spans the membrane with seven helical segments that are bundled together to form a globular structure: (a) view from the inside of the cell, (b) side view from the lipid bilayer.

Figure 3.

Structure of bacterial porin. Porin forms a β barrel consisting of 16 antiparallel β strands that form a transmembrane pore. Surface loops connecting the strands fold into the barrel: (a) view from the inside of the cell; (b) side view from the outer membrane.


Further Reading

Cowan SW (1993) Bacterial porins: lessons from three high‐resolution structures. Current Opinion in Structural Biology 3: 501–507.

Deisenhofer J and Michel H (1989) The photosynthetic reaction center from the purple bacterium Rhodopseudomonas viridis. Science 245: 1463–1473.

Gennis RB (1989) Biomembranes. Molecular Structure and Function. Berlin: Springer‐Verlag.

Reithmeier RAF (1995) Characterization and modeling of membrane proteins using sequence analysis. Current Opinion in Structural Biology 5: 491–500.

Singer SJ and Nicolson GL (1972) The fluid mosaic model of the structure of cell membranes. Science 175: 720–731.

Unwin PNT and Henderson R (1984) The structure of proteins in biological membranes. Scientific American 250: 78–94.

Yeagle PL (1992) The Structure of Biological Membranes. Boca Raton, FL: CRC Press.

Yeagle PL (1993) The Membranes of Cells, 2nd edn. Orlando, FL: Academic Press.

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Reithmeier, Reinhart AF(Apr 2001) Membrane Proteins. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0000624]