= Secretory protein routing
<Secretory protein routing> connects entry into the <endoplasmic reticulum>, <protein folding> and sorting, transfer to the <Golgi apparatus>, and delivery onward. In the conventional soluble-cargo route, <transport vesicles> carry lumenal contents between compartments; <exocytosis> releases cargo outside the <cell membrane>. Resident <endoplasmic reticulum> <proteins> can be retained or retrieved, and defective cargo can be held back. Many <cytosol> <proteins> never enter this route.<The Cell: A Molecular Approach, 2nd edition by Geoffrey M. Cooper/The Endoplasmic Reticulum>[The Cell: A Molecular Approach, 2nd edition by Geoffrey M. Cooper; Protein Folding and Processing in the ER, Figure 9.14; Export of Proteins and Lipids from the ER, Figures 9.20 and 9.21]{ref}
\Image[/secretory-protein-routing-and-membrane-sides.png]
{title=<Secretory protein routing> preserves membrane sides}
{description=Original conceptual route for soluble cargo and a <membrane protein>. Intermediate compartments and return routes are omitted. Arrows show connectivity, not measured timing, fluorescence or <electron microscopy>.}
The useful geometric rule is preservation of membrane sides: during budding and fusion, a lumen-facing region of a <membrane protein> stays lumen-facing, then faces the exterior after <exocytosis>; its <cytosol>-facing region stays cytosolic.<The Cell: A Molecular Approach, 2nd edition by Geoffrey M. Cooper/The Endoplasmic Reticulum>[The Cell: A Molecular Approach, 2nd edition by Geoffrey M. Cooper; Insertion of Proteins into the ER Membrane, Figure 9.10; Export of Proteins and Lipids from the ER: preservation of orientation]{ref}
Work through the schematic using two distinguishable labels A and B on a stipulated <membrane protein>. Start with A facing the <ER lumen> and B facing the <cytosol>. At each budding or fusion step, carry the surrounding membrane together with the labels; do not turn the <protein> over. The resulting pair is A facing the <Golgi apparatus> interior and B facing the <cytosol>, then A facing a <transport vesicle> interior and B facing the <cytosol>, and finally A facing the exterior and B facing the <cytosol>. This is a stepwise argument about the depicted operations, not a claim that the compartments have identical chemical contents. Likewise, once soluble cargo is inside the <ER lumen>, this schematic reaches the exterior with zero further crossings through a <lipid bilayer>: enclosed spaces join when membranes fuse. Delivery therefore need not mean repeatedly pushing a folded <protein> through a membrane.
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