 Protein cell biology

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.^[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.