MDC-15 (ADAM-15, metargidin), a membrane-anchored metalloprotease/disintegrin/cysteine-rich protein, is expressed on the surface of a wide range of cells and has an RGD tripeptide in its disintegrin-like domain. MDC-15 is potentially involved in cell-cell interactions through its interaction with integrins. We expressed a recombinant MDC-15 disintegrin-like domain as a fusion protein with glutathione S-transferase (designated D-15) in bacteria and examined its binding function to integrins using mammalian cells expressing different recombinant integrins. We found that D-15 specifically interacts with ␣v3 but not with the other integrins tested (␣21, ␣31, ␣41, ␣51, ␣61, ␣64, ␣v1, ␣IIb3, and ␣L2). Mutation of the tripeptide RGD to SGA totally blocked binding of D-15 to ␣v3, suggesting that D-15-␣v3 interaction is RGD-dependent. When the sequence RPTRGD is mutated to NWKRGD, D-15 is recognized by both ␣IIb3 and ␣v3, suggesting that the receptor binding specificity is mediated by the sequence flanking the RGD tripeptide, as in snake venom disintegrins. These results indicate that the disintegrin-like domain of MDC-15 functions as an adhesion molecule and may be involved n ␣v3-mediated cell-cell interactions.Metalloprotease/disintegrin/cysteine-rich proteins (MDCs, also called ADAMs) 1 are membrane-anchored proteins with several domains including a metalloprotease domain, a disintegrin-like domain, a cysteine-rich sequence, an epidermal growth factor-like sequence, a transmembrane domain, and a short cytoplasmic domain (1). The biological functions of MDCs are not clear; however, we do know that fertilins (MDC-1 and -2) (2) are involved in sperm-egg binding and fusion (3), meltrins (MDC-12) (4) are involved in myoblast fusion during muscle development, and KUZ (a Drosophila MDC protein) (5) assists in neurogenesis. The MDC cytoplasmic domain has a proline-rich potential SH3 binding motif, suggesting that MDCcounter receptor interaction may induce signal transduction.Integrins are a family of cell adhesion receptors that bind to a variety of ligands, including extracellular matrix proteins and other cell surface molecules (6 -10). MDCs are potential ligands for integrins, since most snake venom disintegrins interact with integrins ␣IIb3 and ␣v3 (reviewed in Ref. 11 and references therein). However, little is known about the receptor specificity of MDCs, except that mouse egg integrin ␣61 has been proposed as a receptor for fertilin (2). Evans et al. (12) recently expressed recombinant fertilin fragments in bacteria as fusion proteins with maltose-binding protein (12). The recombinant fertilin- fragment has been shown to bind to the egg membrane to which sperm bind and to block sperm from binding to the egg. These results suggest that the disintegrinlike domains of MDCs may be properly folded in bacteria, that glycosylation of the disintegrin-like domain may not be required for interaction with receptors, and that a strategy using recombinant MDC proteins is a viable alternative to those using puri...