Structural function of the basic nuclear proteins in ram spermatids.

The function of the cysteine-containing spermatidal proteins and of protamine in the packaging and stabilization of chromatin during ram spermiogenesis was investigated. Extractions of the histones and spermatidal proteins from the nonround spermatid nuclei decreases the nuclear stability (sonication resistance), decondenses the chromatin, and reduces the diameter of the largest chromatin threads (100-200 A vs. 380 A in the control nuclei). Extractions by acid, salt, or heparin have no effect on the protamine-containing electron-opaque chromatin. In contrast, treatment by dithiothreitol alone decondenses all the nonround spermatid nuclei at a rate which decreases with the maturation state of the nuclei. The electron-opaque chromatin is then resolved in 35-A-thick filaments. Experimentally induced fluctuations of the level of SS bonding appear to influence the chromatin stabilization and ultrastructure in most of the nonround spermatid nuclei. These data evidence that noncovalent interactions play a main structural role at the beginning of chromatin reorganization, and SS bonding between spermatidal proteins and then between protamine molecules increases progressively and becomes mainly responsible for the chromatin stabilization in the protamine-containing nuclei.