= Protein cell biology
\b[Protein cell biology] studies how <proteins> function within <biological cells>: where they are delivered, which partners they encounter, and how these interactions affect cellular processes. A <protein>'s <protein primary structure> and abundance do not by themselves specify its location or available partners. <RNA-binding proteins>, <G proteins> and <protein targeting> illustrate different aspects of that problem.
The <microsomal protein import experiment> makes location experimentally testable. In 1975, <Günter Blobel> and <Bernhard Dobberstein> combined <cell-free translation> components with dog-pancreas <microsomes>. An <immunoglobulin light chain> product made while suitable membranes were present became processed and resistant to external <proteolysis>, whereas <globin> did not enter the same protected compartment. Adding membranes only after synthesis did not reproduce the effect in this preparation.<Transfer of Proteins across Membranes II by Günter Blobel and Bernhard Dobberstein>[Abstract and pp.856–862, light-chain/globin comparisons and timing of membrane addition]{ref}
The important observation is controlled accessibility, not just recovery near a membrane. A <protein> can bind a vesicle's outside without crossing it; enclosure can protect it from an external <protease>. <Protease protection assays> therefore complement measurements of total product and <membrane topology>.
For an original two-compartment example, suppose a fixed total of100 competent cargo molecules is divided between20 enclosed and80 exposed molecules. Redistributing the same total to70 enclosed and30 exposed changes accessibility even though total abundance stays100. If every enclosed molecule is protected and every exposed molecule is digested in the stipulated model, the protected fraction changes from0.20 to0.70. Real measurements also need checks for membrane integrity, protease activity and product recovery; the fractions alone do not identify every transport mechanism. This is why cellular organization belongs alongside the isolated molecule's chemistry.
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