SQDGs have been found to be closely associated with certain membrane proteins. In some cases the (electrostatic) interactions may be very strong, as suggested by the inability of saturated SQDG molecules associated with purified chloroplast CF0-CF1 ATPase to exchange with other acidic lipids. It was shown also that SQDGs protect CF1 against cold inactivation in the presence of some ATP. CF1 bound to membranes was found to be much more resistant to heat and cold than solubilised protein. Mitochondrial coupling factor F1 is similarly protected by phospholipids and SQDGs although, in that case, both were equally effective.