A Novel Ca2+ Binding β Hairpin Loop Better Resembles Integrin Sequence Motifs Than the EF Hand

and residue 12 is one turn into the following ␣ helix (Strynadka and James, 1989). In a variant motif, observed in galactose binding protein (Vyas et al., 1987), Putative Ca 2ϩ binding sequence repeats in integrin ␣ the Ca 2ϩ binding loop occurs between an ␣ helix and a subunits have attracted great interest. Depending on ␤ strand (Figure 2C). The Ca 2ϩ ion is coordinated by side the integrin, Ca 2ϩ can either support ligand binding or chains of residues 1, 3, 5, and 9 and the main chain antagonize Mg 2ϩ-dependent ligand binding (Hynes, carbonyl of residue 7, but the role of residue 12 in the 1992). The putative Ca 2ϩ binding sequences in integrins EF hand is substituted by an acidic side chain from a mimic those found in a structure known as the " EF hand " neighboring, parallel ␤ strand. (Tuckwell et al., 1992) which is present in many Ca 2ϩ In the ␤ hairpin motif, both the sequence and the binding proteins, including troponins, parvalbumin, and backbone conformation of residues 1–7 are remarkably calmodulin (Fig.1). Therefore, the sequence repeats similar to those in the EF hand (Figures 1, 2A, and 2D). in integrins are widely referred to as EF hand–like. In Interestingly, however, residue 9 shifts by several ang-EF hands, Ca 2ϩ is bound in a loop between two ␣ hel-stroms and delivers two carboxylate oxygen atoms to ices, with one ␣-helical residue contributing to the ligation position Ϫz (Figure 2D). Thus, it fulfills the same Ca 2ϩ coordination (Strynadka and James, 1989) (Fig-function as residue 12 in the EF hand, as previously ure 2B). predicted for integrins (Oxvig and Springer, 1998). The The integrin Ca 2ϩ binding motifs are located in re-resulting coordination of the Ca 2ϩ ion is similar to the peats of about 60 residues each in the N-terminal portion EF hand, including the water molecule often observed of the ␣ subunit; the structure of these repeats is contro-at position Ϫx. However, this water is not coordinated versial. There are seven repeats in total; repeats 5–7 by another residue as usually observed in the EF hand, and also in some integrins repeat 4 each contain one and this may allow larger ions, such as Sr 2ϩ , to bind to Ca 2ϩ binding sequence motif. Starting with the similarity ␤ hairpin loops with affinity comparable to Ca 2ϩ , as to …