Figure 5.1 Rapid Diffusion of Membrane Proteins
The fluid mosaic model of cell membranes, described by Singer and Nicolson (1972), was critical to understanding biological membranes as proteins floating in a phospholipid matrix. Integral to this model was earlier work by Frye and Edidin (1970). These researchers examined the movement of proteins within the cell membrane by constructing heterokaryons, cells comprised of nuclei from both mice and humans. By using fluorescent stains (red or green) that were specific either to the mouse or human proteins (antigens), Frye and Edidin observed that after 40 minutes, the antigens were totally intermixed. To explain these results, the researchers hypothesized four mechanisms including new protein synthesis, movement of proteins from cytoplasm into the cell membrane, movement of proteins from the cell membrane into cytoplasm and then to a different location on the cell membrane, and diffusion of proteins within the cell membrane. In order to test these various mechanisms, the researchers conducted additional treatments involving inhibition of protein or ATP synthesis. The researchers also used a temperature gradient to test the effect of temperature on the intermixing of human and mouse proteins. The inhibitory treatments had no effect on the rate at which the proteins were intermixed, suggesting that proteins were neither being newly created nor actively transported. By contrast, decreased temperature reduced the rate of protein intermixing, as would be expected if the proteins were moving by diffusion. Thus, it was shown that the membrane surface is fluid, allowing for free diffusion of proteins within this surface.
Original paper
Frye, L. D., and M. Edidin. 1970. The rapid intermixing of cell surface antigens after formation of mouse-human heterokaryons. Journal of Cell Science 7: 319–335.
Links
Singer, S. J. and G. L. Nicolson. 1972. The fluid mosaic model of the structure of cell membranes. Science 175: 720–731.
Animation: Protein Diffusion in the Plane of the Membrane
Martin, L.: The Fluid Mosaic Model - The Fluid Lipid Matrix
The Johns Hopkins University: Diffusion Process
Georgia State University: Diffusion and Osmosis
University of Vermont: Lab Books—Diffusion, Osmosis, and Nernst Equation
Home page of Dr. Michael Edidin