Phosphoproteins are structural components of bull sperm outer dense fiber

To isolate the bull sperm outer dense fibers, the cells were treated at room temperature with 0.05% cetyltrimethylammonium bromide and 7 mM 2-mercaptoethanol. This treatment solubilized most of the sperm structure except for the sperm head and the fibrillar complex. The latter was purified by discontinuous sucrose gradient centrifugation. Electron microscopy showed that in the isolated complex each fiber remains attached to the corresponding segmented column of the connecting piece. Analysis of the fibrillar complex by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate showed three major components with molecular weights of 85,000, 33,000, and 11,000. These were purified, and their amino acid composition was found to be similar to that of the polypeptides of rat sperm outer dense fibers. Furthermore the components of 85,000 and 33,000 daltons contain about 2 mol of phosphoserine per mole of polypeptide, which indicates that they are phosphoproteins.