Membrane protein
= Membrane protein
{wiki}
A <membrane protein> is a <protein> associated with a biological membrane. Its association, orientation and mobility are separate properties: being embedded does not imply spanning, and spanning does not imply moving freely. <Membrane proteins> supply routes for <membrane transport> and recognition for <signal transduction>.<The Cell: A Molecular Approach, 2nd edition by Geoffrey M. Cooper/Cell Membranes>[Cooper, The Cell: A Molecular Approach, 2nd edition, Cell Membranes, opening paragraph and Membrane Proteins]{ref}
An <ion channel> supplies a pathway for <ions>; a <carrier protein> changes which side can reach a bound <solute>; a <cell-surface receptor> connects an external <ligand> to intracellular <signal transduction>. These examples distinguish passage from communication: a signal can cross the boundary as a change of <protein> activity without the <ligand> itself crossing.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/Membrane Proteins>[Alberts and others, Molecular Biology of the Cell, 4th edition, Membrane Proteins Can Be Associated with the Lipid Bilayer in Various Ways: association reflects function]{ref}
\Image[/membrane-protein-topology.png]
{title=Membrane association and the orientation of a spanning chain}
{description=Original schematic cross-sections of a <lipid bilayer>, followed by a constructed three-crossing <polypeptide chain>. Colored paths represent placement, not atomic <protein structures>. The <N-terminus> starts outside; each complete crossing switches sides, leaving the <C-terminus> inside. Drawn dimensions are unscaled.}
The same boundary can therefore retain substances and communicate selectively with its surroundings. <Membrane topology> tells us which parts can meet a given <ligand>; <lateral mobility of membrane proteins> tells us whether two parts can approach each other along the boundary. A diagram of a <cell membrane> becomes more useful when it answers both questions.