Biochemical and ultrastructural study of the sperm chromatin from Mytilus galloprovincialis

Protein composition and ultrastructure of the mature spermatozoa of the mussel Mytilus galloprovincialis were studied upon gradual decondensation of the nuclei with increasing NaCl concentration. Three types of protein were found, associated with the sperm DNA: (1) the sperm-specific proteins S1, S2 and S3 (80% of the acid-soluble proteins); (2) the four core histones (20%); (3) three non-histone proteins tightly bound to DNA (about 4 μg protein per 100 μg DNA). The sperm-specific protein S3 was the first to dissociate at about 0.5 M NaCl and electron micrographs of spread nuclei indicated its participation in the final compaction of the nucleus. Hypotonically treated sperm nuclei revealed the presence of 21-25 nm large granules irregularly scattered along some of the DNA fibers. These granules correspond to the 'superbeads' of histone-containing chromatins. The tightly bound non-histone proteins were represented by a triplet in the range 60-80 kD. They formed 30-60 nm large annular bodies holding DNA fibers and resisting high salt-detergent treatment.