A frequent allele codes for a truncated variant of semenogelin I, the major protein component of human semen coagulum

Human semen coagulum predominantly consists of high molecular mass complexes of the seminal vesicle secreted semenogelin I (SgI) and semenogelin II (SgII). Here we describe a previously unknown variant of the SgI gene that is present at an allele frequency of approximately 3% in the Swedish population. It gives rise to a protein with a molecular mass of 43 kDa, SgI(43), which compared with the 50 kDa variant, SgI(50), is lacking a tandem repeat of 60 amino acid residues that was probably deleted by homologous recombination. In spite of the size difference, SgI(43) has many properties in common with SgI(50), such as a very high iso-electric point and susceptibility to proteolytic degradation by prostate-specific antigen. Heterozygous carriers of the SgI(43) allele neither show impaired fertility nor do they significantly differ from individuals homozygous for SgI(50) with respect to sperm parameters such as semen volume, sperm count and fraction of motile spermatozoa.