BARRIERS TO DIFFUSION OF PLASMA-MEMBRANE PROTEINS FORM EARLY DURING GUINEA-PIG SPERMIOGENESIS

The plasma membrane of the mature guinea pig sperm is segregated into at least four domains of different composition. Previous studies have shown that some proteins localized within these domains are free to di ffuse laterally, suggesting that barriers to protein diffusion are res ponsible for maintaining the nonuniform distribution of at least some surface proteins in mature sperm. The different membrane domains appea r sequentially during sperm morphogenesis in the testis and during lat er passage through the epididymis. To determine when diffusion barrier s become functional during sperm development, we examined the diffusio n of two proteins that are expressed on the cell surface of developing spermatids and become segregated to different plasma membrane domains during the course of spermiogenesis. Both proteins exhibited rapid la teral diffusion throughout spermiogenesis, even after they become loca lized to specific regions of the surface membrane. These results sugge st that barriers to membrane diffusion form concomitantly with membran e domains during spermiogenesis.