A Fragment of Prosaposin (SGP‐1) from Rooster Sperm Promotes Sperm‐Egg Binding and Improves Fertility in Chickens

Abstract

ABSTRACT: A protein isolated from the supernatant of cryopreserved rooster sperm was found to increase the capability of cryopreserved rooster sperm to bind in vitro to the perivitelline membrane of a chicken egg and substantially raise fertility after artificial insemination (AI). That activity was partially purified and termed universal primary sperm‐egg binding protein (UPSEBP). Insufficient protein remained from 6 × 1011 sperm, despite retention of bioactivity, to allow sequencing. We deduced that the protein may be related to prosaposin (also termed SGP‐1, for sulfated glycoprotein‐1), and we used published amino acid sequences of prosaposin as a guide for synthesis of peptides. Certain peptides were found to increase in vitro sperm‐egg binding and increase fertility of frozen‐thawed or fresh rooster sperm, in a manner similar to semipurified UPSEBP. Active epitopes were in a 60 amino acid sequence, reflecting the intervening sequence between saposins A and B, plus short extensions into saposins A and B. Highest activity was found when this synthetic peptide was oxidized to form a disulfide bond between terminal cysteines. Antibody against a synthetic peptide consisting of 58 of these 60 amino acids bound to a 7–9 kilodalton protein in UPSEBP. Collectively, the data support the conclusion that UPSEBP is a fragment of prosaposin. Because prosaposin is in semen in humans and animals, these observations have broad implications for possible cause and therapy of one type of subfertility.