The lateral mobility of
alkaline phosphatase (AP) in the plasma membrane of osteoblastic and nonosteoblastic cells was estimated by fluorescence redistribution after photobleaching in embryonic and in
tumor cells, in cells that express AP naturally, and in cells transfected with an expression vector containing AP
cDNA. The diffusion coefficient (D) and the mobile fraction, estimated from the percent recovery (%R), were found to be cell-type dependent ranging from (0.58 +/- 0.16) X 10(-9) cm2s-1 and 73.3 +/- 10.5 in rat
osteosarcoma cells ROS 17/2.8 to (1.77 +/- 0.51) X 10(-9) cm2s-1 and 82.8 +/- 2.5 in rat
osteosarcoma cells UMR106. Similar values of D greater than or equal to 10(-9) cm2s-1 with approximately 80% recovery were also found in fetal rat calvaria cells, transfected skin fibroblasts, and transfected AP-negative
osteosarcoma cells ROS 25/1. These values of D are many times greater than "typical" values for
membrane proteins, coming close to those of
membrane lipid in fetal rat calvaria and ROS 17/2.8 cells (D = [4(-5)] X 10(-9) cm2s-1 with 75-80% recovery), estimated with the
hexadecanoyl aminofluorescein probe. In all cell types,
phosphatidylinositol (PI)-specific
phospholipase C released 60-90% of native and transfection-expressed AP, demonstrating that, as in other tissue types, AP in these cells is anchored in the membrane via a linkage to PI. These results indicate that the transfected cells used in this study possess the machinery for AP insertion into the membrane and its binding to PI. The fast AP mobility appears to be an intrinsic property of the way the
protein is anchored in the membrane, a conclusion with general implications for the understanding of the slow diffusion of other
membrane proteins.