Restricted lateral diffusion of surface membrane components in Tetrahymena thermophila

The mechanisms whereby proteins are spatially oriented in the membrane bilayer remain the object ofintense speculation. Since membrane proteins would be expected to diffuse rapidly in the lateral plane unless restricted [ 11, experiments have concentrated on determining how the lateral movement of proteins is controlled, and in particular whether this could occur by specific binding to cytoskeletal elements. If the lateral mobility of proteins is controlled by binding to cytoskeletal elements, then variations in the lateral mobility would not be expected to occur if the membrane fluidity were changed isothermally. A mutant auxotrophe of Tetrahymena thermophila, an organism with a highly organised cortical morphology [2], has been isolated. In this mutant the membrane fluidity can be altered over a wide range by supplementation with various fatty acids [3]. Before examining the lateral mobility of surface components of the mutant, it was necessary to study the wild-type organism, and establish whether variations in the lateral mobility occurred under normal experimental conditions. Reported here are fluorescence photo-bleaching recovery (FPR) measurements which demonstrate that a large fraction of the surface proteins are immobile, both when intact antibodies and monovalent Fab fragments are used as probe. However, starvation of cells leads to an increase in the fraction of mobile surface components. Furthermore, there are indications that, as in other cell systems [4,5], variations in the lateral diffusion occur during the cell cycle.