Download Figure 5.1 Rapid Diffusion of Membrane Proteins The fluid mosaic

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.
http://jcs.biologists.org/cgi/reprint/7/2/319
Links
Singer, S. J. and G. L. Nicolson. 1972. The fluid mosaic model of the structure of cell
membranes. Science 175: 720–731.
http://www.jstor.org/pss/1733071
Animation: Protein Diffusion in the Plane of the Membrane
http://course1.winona.edu/sberg/ANIMTNS/frey.htm
Martin, L.: The Fluid Mosaic Model - The Fluid Lipid Matrix
http://cnx.org/content/m15257/latest/
The Johns Hopkins University: Diffusion Process
http://www.jhu.edu/~virtlab/diffus/diff_txt.htm
Georgia State University: Diffusion and Osmosis
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/diffus.html
University of Vermont: Lab Books—Diffusion, Osmosis, and Nernst Equation
http://physioweb.med.uvm.edu/diffusion/
Home page of Dr. Michael Edidin
http://www.bio.jhu.edu/Faculty/Edidin/Default.html