Guinea Pig Fertilin Exhibits Restricted Lateral Mobility in Epididymal Sperm and Becomes Freely Diffusing during Capacitation

The guinea pig sperm protein fertilin functions in sperm–egg plasma membrane binding. Fertilin is initially present in the plasma membrane of the whole head in testicular sperm, then becomes concentrated into the posterior head domain during epididymal passage. Fertilin remains localized to the posterior head plasma membrane following the acrosome reaction, when it functions in sperm–egg interaction. Fluorescence redistribution after photobleaching was used to examine the lateral mobility of fertilin in both acrosome-intact and acrosome-reacted sperm. Fertilin exhibited highly restricted lateral mobility in both testicular and epididymal sperm (D < 10−10 cm2/s). However, fertilin in acrosome-reacted sperm was highly mobile within the membrane bilayer (D = 1.8 × 10−9 cm2/s and %R = 84). Measurement of the lateral mobility of fertilin in capacitated, acrosome-intact sperm revealed two populations of cells. In approximately one-half of the cells, lateral mobility of fertilin was similar to sperm freshly isolated from the cauda epididymis; while in the other half fertilin was highly mobile. The release of fertilin from interactions that restrict its lateral mobility may regulate its function in sperm–egg interaction.