On intact cells, a protease reaches only the outer face
- The idea
- Predict
- See it
- Why
- Check
The idea
“A protease added to the cells can cut any part of the protein, inside or out.”
You added the enzyme to the cells, so it seems it should reach every part of them. A protease is a big protein, though, and an intact membrane keeps it out.
The short version
The mistake: A protease added to the cells can cut any part of the protein, inside or out.
What's actually true: A protein reagent added to intact cells reaches only the outer face. It reaches the cytosolic face once the membrane is opened.
- Proteases, antibodies and other protein reagents are too large and polar to pass through the bilayer. They stay on the side where you add them.
- So on intact cells, anything such a reagent cuts or labels faces outside.
- Leaky cells let it reach both faces. A part that changes only then faces the cytosol.
On the exam: A reagent that can't cross membranes touches only the outer face of intact cells. Open them to reach inside.