Surface proteins of ejaculated porcine sperm and sperm incubated in the uterus

Proteins exposed on the surface of ejaculated and in utero-incubated porcine spermatozoa were labeled with 125 I. Following iodination, plasma membrane was isolated from the sperm and solubilized with sodium dodecyl sulfate. Proteins in the membrane were then separated by gel electrophoresis. Under these iodination conditions, ∼5-6 x 10 5 atoms of iodide per sperm were removed from solution. Hyaluronidase, an internal protein, was isolated following iodination. This protein was not labeled, indicating that the iodination method was specific for surface proteins. Of the 11 prominent proteins detected in the plasma membrane of ejaculated and in utero-incubated sperm, two shifted to lower molecular weights during incubation in utero for 90 min. A 14,000 dalton protein band comprised 68% of the plasma membrane protein of ejaculated sperm, but only 44% of incubated sperm. Although six proteins were labeled in both ejaculated and incubated sperm, the iodination profiles were different. In both cases, the 14,000 dalton protein was associated with the majority of the radioactivity. The radioactivity per unit weight of membrane was significantly greater for plasma membrane isolated from in utero-incubated sperm than for plasma membrane isolated from ejaculated sperm. These results indicate that the prominent protein in the plasma membrane of porcine sperm is a 14,000 dalton protein exposed on the surface of the sperm cell. In addition, this protein is released from sperm during incubation in the uterus increasing the accessibility of other membrane proteins to the surface.