The integrin Mac-1 (αMβ2, CD11b/CD18) is an important adhesion receptorexpressed on macrophages and neutrophils. It plays a crucial role in phagocytosis, cell-cell fusion, and cell migration. αMβ2 is also the most promiscuous integrin with over 100 known ligands that span a broad range of physical and chemical attributes, many of which bind to the inserted (I) domain from the αM subunit. The interaction of αMI-domain with cytokine pleiotrophin (PTN) were determine. PTN is a cationic protein known to induce Mac-1- mediated adhesion and migration in cells. The data showed that PTN’s interaction with αMI-domain contains both divalent cation-dependent and independent mechanisms. In particular, PTN’s N-terminal domain has weak interactions with the N/C-termini side of αMI-domain using a metal-independent mechanism. However, stronger interaction is achieved through the chelation of the divalent cation in the metal ion-dependent adhesion site of active αMI-domain by PTN’s acidic residues. Although many acidic residues in PTN can act as the chelator, active αMI-domain’s interaction with PTN’s E98 plays an especially important role. NOE, chemical shift perturbation (CSP) data, and mutagenesis studies showed residues near E98 are at the binding interface and the E98 mutation greatly reduced binding affinity between two proteins. Interestingly, the CSP and MD simulation data showed the binding interface can be supported by the interaction of PTN’s H95 with the acidic clusters D242, E244, and D273 from αMI-domain, while PTN’s E66 form electrostatic interaction with R208 and K245 from αMI-domain. The determined recognition motif of αMI-domain for its ligands is (H/R/K)xxE. The ability to accommodate the longer distance between E and (H, R, K) compared to the zwitterionic motif RGDii explained how αMβ2 can interact with a large repertoire of ligands and be versatile in its functional portfolio.