Characterization of the plasma membrane of Mycoplasma laidlawii. VII. Phase transitions of membrane lipids

Abstract

Use of a very sensitive differential calorimeter has revealed two major regions of heat absorption in purified plasma membranes of Mycoplasma laidlawii. These consist of a rapidly increasing rate of heat absorption showing an inflection point at approx. 40°; this is followed at higher temperatures by a slightly decreased rate of heat uptake. Within this latter region it is barely possible to distinguish a second inflection point. These transitions were found also to be properties of whole cells. Experimental results presented in this paper support the conclusion that the first region of heat absorption is due to a reversible transition in the lipid component of the membrane, and that the second region is due to an irreversible denaturation of the protein component. These results also provide evidence that the lipid transition is essentially unaffected by the protein component of the membrane. The calorimetric transition curves indicate that the apparent heat capacity of the membrane lipids increases during the transition. This most likely arises from new modes of freedom available to the fatty acid hydrocarbon chains in the melted state. By means of a viable cell titer taken before and after cells were submitted to a calorimetric run extending to 40°, the lipid transition was demonstrated unambiguously to be a property of living cells. The T m of the lipid transition was found to be a function of the conditions of growth of the cells. In all cases studied the temperature at which the cells grew lies within the range of the lipid transition. Results suggest that the cells attempt to regulate their position in the lipid transition.