The calcium-and integrin-binding protein 1 (CIB1) is a ubiquitous Ca 2؉ -binding protein and a specific binding partner for the platelet integrin ␣IIb cytoplasmic domain, which confers the key role of CIB1 in hemostasis. CIB1 is also known to be involved in apoptosis, embryogenesis, and the DNA damage response. In this study, the solution structures of both Ca 2؉ -CIB1 and Mg 2؉ -CIB1 were determined using solution-state NMR spectroscopy. The methyl groups of Ile, Leu, and Val were selectively protonated to compensate for the loss of protons due to deuteration. The solution structure of Ca 2؉ -CIB1 possesses smaller opened EF-hands in its C-domain compared with available crystal structures. Ca 2؉ -CIB1 and Mg 2؉ -CIB1 have similar structures, but the N-lobe of Mg 2؉ -CIB1 is slightly more opened than that of Ca 2؉ -CIB1. Additional NMR experiments, such as chemical shift perturbation and methyl group solvent accessibility as measured by a nitroxide surface probe, were carried out to further characterize the structures of Ca 2؉ -CIB1 and Mg 2؉ -CIB1 as well as their interactions with the integrin ␣IIb cytoplasmic domain. NMR measurements of backbone amide proton slow motion (microsecond to millisecond) dynamics confirmed that the C-terminal helix of Ca 2؉ -CIB1 is displaced upon ␣IIb binding. The EF-hand III of both Ca 2؉ -CIB1 and Mg 2؉ -CIB1 was identified to be directly involved in the interaction of CIB1 with ␣IIb. Together, these data illustrate that CIB1 behaves quite differently from related EF-hand regulatory calcium-binding proteins, such as calmodulin or neuronal calcium sensor proteins.The calcium-and integrin-binding protein 1 (CIB1) is a member of the regulatory Ca 2ϩ -binding helix-loop-helix or EFhand protein family (1). CIB1 is a ubiquitous 191-residue (22 kDa) protein with several functions in cell signaling (2, 3). CIB1 has been reported to interact with a number of protein targets, including the platelet integrin ␣IIb subunit (4), sphingosine kinase 1 (5), p21-activated kinase (6), apoptosis signal-regulating kinase (7), polo-like protein kinases (8), and a recently reported new target in the regulation of cardiac hypertrophy, calcineurin B (9). The interaction of CIB1 with the integrin ␣IIb subunit has been studied extensively (10 -13). The integrin ␣IIb subunit usually associates with the integrin 3 subunit, and the heterodimeric ␣IIb3 protein is involved in both so-called "inside-out" and "outside-in" signaling pathways (3). CIB1 is believed to be capable of specifically binding to the ␣IIb cytoplasmic domain and dissociating the ␣IIb3 dimer, which in turn triggers integrin activation (11,12). The interactions between Ca 2ϩ -CIB1 and a fragment of the integrin ␣IIb subunit (residues 983-1008, hereafter referred to as the ␣IIb peptide) have been suggested to mainly involve a hydrophobic pocket in the C-domain of Ca 2ϩ -CIB1 with a dissociation constant in the submicromolar range (10).The calcium-bound CIB1 (Ca 2ϩ -CIB1) structure consists of four helix-loop-helix (EF-hand) Ca 2ϩ -binding m...