The Sperm‐surface glycoprotein, SGP, is necessary for fertilization in the frog, Xenopus laevis

Abstract

To identify a molecule involved in sperm-egg plasma membrane binding at fertilization, a monoclonal antibody against a sperm-surface glycoprotein (SGP) was obtained by immunizing mice with a sperm membrane fraction of the frog, Xenopus laevis, followed by screening of the culture supernatants based on their inhibitory activity against fertilization. The fertilization of both jellied and denuded eggs was effectively inhibited by pretreatment of sperm with intact anti-SGP antibody as well as its Fab fragment, indicating that the antibody recognizes a molecule on the sperm's surface that is necessary for fertilization. On Western blots, the anti-SGP antibody recognized large molecules, with molecular masses of 65-150 kDa and minor smaller molecules with masses of 20-28 kDa in the sperm membrane vesicles. SGP was distributed over nearly the entire surface of the sperm, probably as an integral membrane protein in close association with microfilaments. More membrane vesicles containing SGP bound to the surface were found in the animal hemisphere compared with the vegetal hemisphere in unfertilized eggs, but the vesicle-binding was not observed in fertilized eggs. These results indicate that SGP mediates sperm-egg membrane binding and is responsible for the establishment of fertilization in Xenopus.

The electrical block to polyspermy induced by an intracellular Ca2+ increase at fertilization of the clawed frogs, Xenopus laevis and Xenopus tropicalis

“…In this connection, the HPX domain of MMP‐2 on the sperm surface is involved in the activation of egg accompanied by [Ca 2+ ] i in a voltage‐dependent manner (Iwao et al, ). A partial amino acid sequence of the HPX domain of X. tropicalis (AMSQIRGETFFFK, pI: 10.20) is very similar to that of X. laevis (GMSQIRGETFFFK, pI: 10.15); this domain is important for voltage‐dependent fertilization, and the small difference in the voltage required for proper fertilization between these species might be due to the difference in the amount of HPX domain localized on the sperm and/or the molecules associated with MMP‐2, such as a sperm‐surface glycoprotein (SGP, pI; 5.5–6.0, Iwao et al, ; Nagai et al, ). Investigating the differences in the distribution of MMP‐2 and SGP on the sperm between these species will be of interest.…”

Egg activation in physiological polyspermy

“…We have also recently demonstrated that SGP on the sperm membrane has a bifunctional role in sperm binding to both the vitelline envelope and the egg plasma membrane at Xenopus fertilization (Nagai et al 2009, Kubo et al 2010. Further investigation of the function of citrate synthase, not only in fertilization but also in early embryonic development, will provide insight into Ca 2C signaling and cell cycle regulation.…”

“…Sperm induces transient phosphorylation of Uroplakin III (UPIII) on the egg membrane dependent on Src kinase . It has been postulated that the sperm protease associated with sperm surface glycoprotein (SGP, Nagai et al 2009) cleaves UPIII to activate Src kinase. UPIII seems to serve as a primary sperm receptor for the Ca 2C release from cortical ER ( Fig.…”

Sperm acrosin is responsible for the sperm binding to the egg envelope during fertilization in Japanese quail (Coturnix japonica)

“…Immunization of mice with sperm plasma membrane, cell fusion of lymphocytes from immunized mice with myeloma PAI, and establishment of an antibody library against sperm plasma membrane components were carried out as described previously (Nagai et al 2009). …”

“…Recently, by the aid of an antibody library raised against the sperm plasma membrane of X. laevis (Nagai et al 2009), Kubo et al (2010) identified a component that can bind to the vitelline envelope (VE) using a newly developed method, the dot blot assay. As a result, the authors successfully obtained a mAb specific to the sperm membrane protein and showed that the antigen protein interacts with the VE component gp37, a mammalian ZP1 homolog in X. laevis, as shown by farwestern blotting.…”