Effect of proteins on the motion of spin-labeled fatty acids in mycoplasma membranes

Spin-labeled fatty acids have been incorporated as structural probes into Acholeplasma laidlawii and Mycoplasma hominis native membranes and into pronase-treated membranes from which up to 75% of the protein had been removed. The electron paramagnetic resonance (EPR) spectra showed the mobility of the spin labels in the native and pronase-digested membranes to be temperature dependent, increasing as the nitroxide radical is moved away from the polar head group of the fatty acid. The changes were more pronounced in A. laidlawii than in M. hominis membranes. The mobility of the spin-labeled fatty acids in the pronase-digested membranes increased with protein digestion which was accompanied by a decrease in the isopycnic density of the membranes. On binding lysozyme or cytochrome c to the pronase-digested membranes, the mobility of the spin-labeled fatty acids decreased with a corresponding increase in membrane density. These changes were almost completely reversed when the soluble proteins were removed by 1 M NaCl. Our findings indicate that membrane proteins, including those bound electrostatically to membrane lipids, influence the physical state of membrane lipids.