Membranprotein

Membranprotein, also known as membrane protein, is a protein that interacts with biological membranes. They can be integral (intrinsic) proteins embedded within the lipid bilayer, often spanning the membrane with one or more transmembrane segments; or peripheral (extrinsic) proteins attached to the membrane surface by electrostatic interactions or through covalent lipid anchors. Some proteins are monotopic, associating with only one side of the bilayer, while lipid-anchored proteins are tethered to the membrane by covalently attached lipid molecules.

Structure and topology: Integral proteins typically contain hydrophobic transmembrane domains forming alpha-helices or beta-barrels that create pores or transport pathways. Peripheral proteins usually have soluble regions that interact with membrane lipids or other proteins. The arrangement and post-translational modifications influence localization and function.

Functions: Membrane proteins mediate transport of ions and small molecules (channels and carriers), catalyze membrane-associated reactions (enzymes), and participate in signal transduction as receptors. They also contribute to cell adhesion, membrane fusion, and maintenance of cell shape and polarity. Many membrane proteins are targets for pharmacological intervention.

Examples: Aquaporins (water channels), voltage-gated ion channels, solute carriers, the Na+/K+-ATPase, G protein-coupled receptors, and cytochrome c oxidase complex. In organelles, porins in mitochondrial membranes are beta-barrel proteins.

Study and challenges: Structural characterization often uses X-ray crystallography, cryo-EM, or NMR, with detergents, liposomes, or nanodiscs. Bioinformatic predictions identify transmembrane segments and topology.

See also: UniProt membrane protein entries, Pfam membrane protein families, membrane proteomics.