Functional classification of ADAMs based on a conserved motif for binding to integrin α9β1. Implications for sperm-egg binding and other cell interactions

ADAMs (a disintegrin and metalloproteases) are members of the metzincin superfamily of metalloproteases. Among integrins binding to disintegrin domains of ADAMs are α_vβ₁ and α_vβ₃ and they bind in an RGD-independent and an RGD-dependent manner, respectively. Human ADAM15 is the only ADAM with the RGD motif in the disintegrin domain. Thus, both integrin α₉β₁ and α_vβ₃ recognize the ADAM15 disintegrin domain. We determined how these integrins recognize the ADAM15 disintegrin domain by mutational analysis. We found that the Arg⁴⁸¹ and the Asp-Leu-Pro-Glu-Phe residues (residues 488-492) were critical for α₉β₃ binding, but the RGD motif (residues 484-486) was not. In contrast, the RGD motif was critical for α_vβ₃ binding, but the other residues flanking the RGD motif were not. As the RX₆DLPEF α₉β₁ recognition motif (residues 481-492) is conserved among ADAMs, except for ADAM10 and 17, we hypothesized that α₉β₁ may recognize disintegrin domains in all ADAMs except ADAM10 and 17. Indeed we found that α₉β₁ bound avidly to the disintegrin domains of ADAM1, 2, 3, and 9 but not to the disintegrin domains of ADAM10 and 17. As several ADAMs have been implicated in sperm-oocyte interaction, we tested whether the functional classification of ADAMs, based on specificity for integrin α₉β₁, applies to sperm-egg binding. We found that the ADAM2 and 15 disintegrin domains bound to oocytes, but the ADAM17 disintegrin domain did not. Furthermore, the ADAM2 and 15 disintegrin domains effectively blocked binding of sperm to oocytes, but the ADAM17 disintegrin domain did not. These results suggest that oocytes and α₉β₁ have similar binding specificities for ADAMs and that α₉β₁, or a receptor with similar specificity, may be involved in sperm-egg interaction during fertilization. As α₉β₁ is a receptor for many ADAM disintegrins and α₉β₁ and ADAMs are widely expressed, α₉β₁-ADAM interaction may be of a broad biological importance.