The murine heterochromatin protein M31 is associated with the chromocenterin round spermatids and is a component of mature spermatozoa

In mature sperm the normal nucleosomal packaging of DNA found in somatic an d meiotic cells is transformed into a highly condensed form of chromatin wh ich consists mostly of nucleoprotamines. Although sperm DNA is highly conde nsed it is nevertheless packaged into a highly defined nuclear architecture which may be organized by the heterochromatic chromocenter, One major comp onent of heterochromatin is the heterochromatin protein 1 which is involved in epigenetic gene silencing. In order to investigate the possible involve ment of heterochromatin protein in higher order organization of sperm DNA w e studied the localization of the muring homologue of heterochromatin prote in 1, M31, during chromatin reorganization in male germ cell differentiatio n. Each cell type in the testis showed a unique distribution pattern of M31 , Colocalization to the heterochromatic regions were found in Sertoli cells , in midstage pachytene spermatocytes, and in round spermatids in which M31 localizes to the centromeric chromocenter. M31 cannot be detected in elong ated spermatids or mature spermatozoa immunocytologically, but could be det ected in mature spermatozoa by Western blotting, We suggest that M31, a nuc lear protein involved in the organization of chromatin architecture, is inv olved in higher order organization of sperm DNA (C) 2000 Academic Press.