= Protein targeting
{wiki}
<Protein targeting> directs a <protein> to its cellular destination. Synthesis in the <cytosol> does not imply that the <protein> works there.
For entry into the <endoplasmic reticulum>, distinguish three questions: does the <nascent chain> reach the membrane, does part of it cross into the <ER lumen>, and does a segment remain embedded as a <transmembrane protein>? <Cotranslational protein translocation> couples these events to <biological translation>. A <signal peptide> can initiate delivery without specifying the eventual destination of the mature <protein>.
\Image[/protein-targeting-er-pathway-and-topology.png]
{title=<Protein targeting> to the <endoplasmic reticulum>: delivery and three possible outcomes}
{description=Original pathway and <membrane topology> schematic. The upper row separates <SRP> recognition, membrane docking and passage through a <translocon>. Lower cartoons stipulate a soluble cargo, a cleaved <signal peptide> plus retained <stop-transfer sequence>, and an uncleaved internal <signal anchor>. Each drawn terminus is labeled; the internal-anchor orientation is one possibility. Shapes and lengths are illustrative, with no molecular coordinates or measured timings.}
This distinction matters experimentally. Finding a <protein> in a membrane-containing <centrifugation pellet> could reflect surface binding or <protein aggregation>. The <microsomal protein import experiment> instead asks whether an intact membrane prevents an external <protease> from reaching the product. That tests accessibility across a barrier, rather than proximity to it.
Back to article page
