Changes in the organization of the sea urchin egg plasma membrane upon fertilization: Indications from the lateral diffusion rates of lipid-soluble fluorescent dyes

We have used fluorescence photobleaching recovery to measure the diffusion of lipid probes (1,2-acyl-2-(N-4-nitro-benzo-2-oxo-1,3-diazole)aminocaproylphosphatidylcholine (NBD-PC)) and the 3,3′-diacylindocarbocyanine iodides (C_ndiI) with even acyl chain lengths (n = 10 to 18) in the plasma membranes of the sea urchin egg Strongylocentrotus purpuratus before and after fertilization. At 15°C measured diffusion coefficients, D, ranged from (1 to 5) × 10^-9 cm²/sec, diffusing fractions from 36 to 77% for the C_ndI's. D shows a relative minimum at n = 12 for unfertilized eggs, a relative maximum at n = 14 for fertilized eggs. Thus the effect of fertilization on diffusion coefficient is dependent on probe structure. The fraction of molecules free to diffuse also depends upon acyl chain length; this fraction was smaller in fertilized eggs than in unfertilized eggs for all chain lengths. Our results indicate that different lipid analogs probe different microenvironments and suggest the existence of domains of lipids differing in composition or physical state from the average for the egg plasma membrane.