Physical studies of lipid organization and dynamics in mycoplasma membranes.

It should be clear from this summary that we currently know a great deal about the organization and dynamics of the lipids in mycoplasma membranes in general, and in the cell membrane of A. laidlawii in particular. In fact, research on mycoplasma membranes has been important in unambiguously establishing the fundamental lipid bilayer structure of all biological membranes and in elucidating some of the major properties of bilayers in biomembranes, such as their thermotropic phase behavior and interactions with cholesterol and membrane proteins. Although a great deal has been learned, a number of issues have not been fully resolved. In particular, the concept of membrane lipid fluidity must be refined and quantitated, and the relationship between orientational order and rates of motion better understood. This will require that the apparent discrepancies between some of the results obtained, for example, by the various spectroscopic techniques, be resolved. In particular, the nature of the boundary lipid surrounding integral membrane proteins will require further study, as will the question of the specificity of lipid-protein interactions. Also, accurate quantitative measurements for the lateral and rotational mobilities of the various lipid components in the mycoplasma membranes have not yet been made. Although not reviewed in this chapter, the related questions of the in vivo rate of phospholipid, glycolipid, and cholesterol transverse diffusion (flip-flop), and the possible asymmetric transbilayer distribution of these components in mycoplasma membranes, are still not well understood. Although much remains to be done, particularly with respect to our understanding of protein structure and function in mycoplasma membranes, a solid basis for further advances has now been laid. The many natural advantages of mycoplasma for biochemical and biophysical investigations of membrane structure and function should continue to make these organisms very useful for membrane studies for years to come.