We've seen the fat, so where's the beef?!
Historically, membrane lipid has been more easily studied than membrane protein, because the latter cannot easily be examined by the extraction and assay techniques used to characterize lipid; typically, membrane proteins precipitate irreversible in organic solvents. In the late 1960's, however, our knowledge of membrane proteins was advanced dramatically when it was discovered they could be solubilized by charged detergents and separated electrophoretically according to their molecular size and their molecular weights inferred empirically by reference to the separation of standard proteins with known molecular weights. The most common form of the technique involves the use of the detergent, sodium dodecyl sulfate, and subsequent separation by electrophoretic passage through a gel of polymerized acrylamide: sodium-dodecyl-sulfate polyacryamide-gel-electrophoresis or SDS-PAGE for short.
When membrane proteins of mammalian erythocyte "ghosts" (cells washed free of their cytoplasm) are examined by SDS-PAGE, banding patterns similar to those on the opposite page are produced. The proteins range in Mr's from approximately 240 kDa (kiloDalton, 1000 times the mass of hydrogen) to less than 20 kDa, with three proteins - those of 240, 220 and 100 kDa - accounting for almost two-thirds of all the protein present: so-called electrophoretic Bands 1, 2, and 3. (Note the relative staining densities of these bands!) The pattern is very similar for red cell membrane proteins isolated from a variety of different mammals, but it differs significantly from those produced for membrane proteins of different tissues (even within the same organism!).
As valuable as such patterns are, they don't tell us much about membrane protein structure or how the proteins are organized in membranes. How might SDS-PAGE be used to address this problem? Let's examine <next/5bmembprot> the results of a simple extraction experiment.