The influence of lipid state on the planar distribution of membrane proteins in Acholeplasma laidlawii

Abstract

The distribution of surface proteins within the plane of the cell membrane was investigated using a biotin-avidin-ferritin reagent. This reagent reacts specifically, in the case of Acholeplasma laidlawii , with proteins, and permits their visualization in the electron microscope. A. laidlawii membranes were prepared which had different lipid phase transition-temperature ranges as determined by X-ray diffraction. These membranes were labeled at temperatures above, below and in the middle of their transitions, where the states of the lipids were, respectively, smectic, paracrystalline or a mixture of the two phases. In membranes labeled in either the smectic or paracrystalline phase (above or below the transition), the labeled sites were relatively dispersed, whereas in membranes labeled at temperatures in the mixed phase (mid-transition), patches of high and low density of label were found. In all cases, the distribution of the proteins was dependent upon the temperature relative to the lipid transition rather than the absolute temperature. In addition, the patching phenomenon was found to be freely reversible in experiments in which the temperature was shifted after labeling. To explain these observations, a model is proposed which suggests a phase separation of lipid and the preferential association of protein with either the smectic or paracrystalline state. In order that such a change in protein distribution can occur, protein molecules must be mobile through both the paracrystalline and smectic lipid phases over the time-scale of the experiments. These results suggest that temperature-induced changes in the physical state of membrane lipids can cause changes in the relative location of protein components parallel to the membrane surface.