0.7 The fluic mosaic model - the fluid lipid matrix  (Page 2/2)

1. Carefully examine the figure legend, axes labels and data. Summarize the results of this experiment as depicted in the figure above in a sentence or two. Be sure to describe only what the figure shows.

2. What do their results suggest about the position of proteins within the cell membrane over time? Why? Please explain.

3. Review the experimental control described in the paragraph above. Please explain why the control is important to the interpretation of the results in Figure 1.

4. Singer and Nicolson (1972) cite this study as evidence that the membrane behaves as a fluid through which proteins can diffuse. Review the results above, do you think they can be unequivocally interpreted as evidence for a fluid cell membrane or could these same results be caused by alternative cellular mechanisms? Please explain. If you conclude that alternative explanations are possible, please describe them.

5. Choose one of the models (explanations) you described in question 4 and briefly outline an experiment to test it. Be sure to explain what variable(s) you will measure and what results will support or refute the model.

Ultimately, Frye and Edidin (1970) were able to convincingly support the model that mosaic formation was due to the diffusion of stained proteins and thus, by implication, that the membrane behaved as a fluid as opposed to a rigid structure. To do so, they had to systematically eliminate several other models offering alternative mechanisms for the observed redistribution of proteins. These included the possibilities that

• new proteins were rapidly synthesized and inserted into the membrane over the course of the experiment.
• proteins were being removed from the surface in one location and reinserted in another.
• proteins synthesized in both the mouse and human cell prior to cell fusion were being inserted into the membrane after fusion.

To eliminate these alternative models, Frye and Edidin (1970) conducted additional experiments in which they treated the cells with chemicals that inhibited protein or ATP synthesis either before fusion (to test model 3) or after fusion (to test models 1 and 2). None of these treatments impeded protein redistribution and thus, mosaic formation, as expected if the alternative models were correct.

Temperature at which the cells were incubated did, however, affect rates of mosaic formation (Frye and Edidin, 1970). Was this outcome consistent with the explanation that protein redistribution results from diffusion through a fluid membrane? Answer the questions below to find out.

1. Consider the process of diffusion. How would you expect temperature to affect the rate at which molecules diffuse? Why? Please explain with reference to the process by which diffusion works.

2. Given the expectation articulated in question 1, predict how you would expect temperature to affect the number of mosaic cells formed after 40 minutes of incubation if protein movement was diffusion driven. To do this, imagine incubating a set of newly formed heterokaryons at a constant, preselected temperature and counting the number of mosaics after 40 minutes and then repeating this procedure for a series of predetermined temperatures between 0 and 37 degrees Celsius.

3. Convert your prediction from question 2 into a figure (graph) with temperature on the x-axis and % of cells exhibiting mosaicism after 40 minutes of incubation on the y-axis.

4. Consider the expected relationship between temperature and mosaic formation described in your answer to question 2. What types of results (relationships between temperature and mosaic formation) would not support this model? Please explain.

5. Sketch the null models described in question 4 on the figure produced for question 2.

Frye and Edidin's (1970) results describing the relationship between temperature of incubation and mosaic formation appear in Figure 2.

1. Carefully examine the figure legend, axes labels and data. Summarize the results of this experiment as depicted in the figure above in a sentence or two. Be sure to describe only what the figure shows.

2. Compare the data to the figures produced for questions 3 and 5 above. Which of your predictions, if any, do these results appear to support? Why? Please explain.

3. Do these results support a fluid model of the cell membrane in which proteins move by diffusion? Yes or no? Please explain.

Works cited

• 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.
• Nicolson, G.L., R. Hyman, and S.J. Singer. 1971. The two-dimensional topographic distribution of H-2 histocompatibility alloantigens on mouse red blood cell membranes. The Journal of Cell Biology . 50:905-910.
• Nicolson, G.L., S.P. Masouredis, and S.J. Singer. 1971. Quantitative two-dimensional ultrastructural distribution of Rho(D) antigenic sites on human erthryocyte membranes. Proceedings of the National Academy of Sciences. 68:1416-1420.
• Singer, S.J. and G. L. Nicolson. 1972. The fluid mosaic model of the structure of cell membranes. Science . 175: 720-731.