Question

This is for BIOCHEM..

1. How can it be determined if a protein spans a membrane?

2. How is membrane asymmetry preserved?

Answer 1

1. Integral proteins are transmembrane proteins which have one or more segments that are embedded in the phospholipid bilayer. The technique which is commonly used to visualize distribution of membrane proteins is freeze-fracture technique. In this technique, the memrbane is practically split apart and the the proteins can be visualized by freezing it to near liquid nitrogen temperatures (-196 degree celsius) and then fracturing it with a microtome knife.
2. The lipids in the plasma membrane are asymmetrically distributed across the lipid bilayer; the amine-containing phospholipids are enriched on the cytoplasmic side of the plasma membrane, while the choline-containing and sphingolipids on the outer surface. Perhaps, the most significant contributors to the maintenance of dissipation of transbilayer lipid asymmetry are proteins that catalyze the movement of lipids across the membrane. These proteins are flippases and floppases. Flippases transport lipids from the outer monolayer to the cytoplasmic surface of the plasma membrane. Floppases, on the other hand, transport lipids in the opposite direction.