Spermatogenesis is a unique system of differentiation involving cellular remodeling and the biogenesis of sperm-specific organelles. To study the biogenesis of one such organelle, the acrosome, we have been examining the gene expression, biosynthesis, and targeting of specific acrosomal proteins during mammalian spermatogenesis. An acrosomal marker that we recently purified and began characterizing is acrogranin, a 67,000-molecular-weight glycoprotein originally isolated from guinea pig testes. This glycoprotein is detected in pachytene spermatocytes and is found later in the acrosomes of developing spermatids and sperm. Immunoblotting of several tissues and immunofluorescent localization in frozen sections of guinea pig testes suggested that acrogranin was a germ cell-specific glycoprotein that was expressed meiotically and post-meiotically. However, Northern blot analysis demonstrated that the mRNA for acrogranin was ubiquitously expressed in all guinea pig and mouse tissues examined. Furthermore, the primary structures of guinea pig and mouse acrogranins, deduced from the cDNA sequences, reveal that this glycoprotein is a cysteine-rich molecule with a motif that is tandemly repeated seven times, very similar to that of the human epithelin/granulin precursor. We conclude that guinea pig and mouse acrogranins are homologues of the precursor of the human and rat epithelin/granulin peptides previously demonstrated to have growth-modulating properties.

Mouse genomic clones encoding the epithelin/granulin gene and its 5'-and 3'-flanking regions have been isolated and sequenced. This gene was found to be a single-copy gene, and contained 13 exons interrupted by 12 introns. Eight out of the 12 introns are classified as phase 0, and are located within the central part of each of the tandem repeats in the amino acid sequence of the epithehn/granulin precursor. The first intron is unique because of the interruption of the 5'-untranslated region and its fairly large size (approximately 2.4 kbp). Consensus sequences for several of the potential regulatory elements are present in the 5'-flanking sequence, including a common CCAAT sequence.