Roles of ADAM family proteins and egg integrins in mouse sperm-egg plasma membrane adhesion and fusion

Date of Completion

January 1998


Biology, Cell




Peptide mimetics from the predicted binding sites in the disintegrin domains of the five testis-expressed ADAMs(A Disintegrin And Metalloprotease), fertilin $\alpha$, fertilin $\beta$, cyritestin, ADAM 4 and ADAM 5, were tested in a sperm-egg plasma membrane adhesion and fusion assay. The active site peptide from cyritestin strongly inhibited (80-90%) sperm-egg adhesion and fusion and was a more potent inhibitor than the fertilin $\beta$ active site peptide, which showed 55-60% of inhibition for sperm-egg fusion. Antibodies generated against the active site region of either cyritestin or fertilin $\beta$ also strongly inhibited (80-90%) both sperm-egg adhesion and fusion. The active site peptide from fertilin $\alpha$ only showed limited inhibition ($\sim$30%). The active site peptides from ADAM 4 and ADAM 5 did not show any inhibition.^ Fertilin $\alpha$, fertilin $\beta$, cyritestin and ADAM 4 were expressed as large precursors and proteolytically processed into mature forms during sperm maturation. ADAM 5 was only detected as the processed form and only in testicular cells. Cyritestin and fertilin $\beta$ was localized by indirect immunofluorescence on the inner acrosomal membrane (IAM) and the equatorial region of acrosome reacted sperm, respectively.^ The minimal binding sequence of ADAMs is not as restricted as the RGD sequence in PII SVMP disintegrins. For fertilin $\beta$, at least a five amino acid sequence QDECD is required for the maximal function. The additional cysteine residue in the predicted binding loop of ADAMs seems to be a free thiol. Inhibition of sperm-egg adhesion and fusion with antibodies against integrins indicates $\alpha$6 and $\beta$1 containing integrins on egg plasma membrane might be the egg receptors for sperm.^ In summary, three ADAMs, fertilin $\alpha$, fertilin $\beta$ and cyritestin, are involved in sperm-egg plasma membrane adhesion and fusion. Sperm-egg plasma membrane interactions might be mediated by the disintegrin-like domains of ADAMs on sperm and integrins on egg surface. ^