RESUMO
Laminins are major cell-adhesive proteins of basement membranes that interact with integrins in a divalent cation-dependent manner. Laminin-511 consists of α5, ß1, and γ1 chains, of which three laminin globular domains of the α5 chain (α5/LG1-3) and a Glu residue in the C-terminal tail of chain γ1 (γ1-Glu1607) are required for binding to integrins. However, it remains unsettled whether the Glu residue in the γ1 tail is involved in integrin binding by coordinating the metal ion in the metal ion-dependent adhesion site of ß1 integrin (ß1-MIDAS), or by stabilizing the conformation of α5/LG1-3. To address this issue, we examined whether α5/LG1-3 contain an acidic residue required for integrin binding that is as critical as the Glu residue in the γ1 tail; to achieve this, we undertook exhaustive alanine substitutions of the 54 acidic residues present in α5/LG1-3 of the E8 fragment of laminin-511 (LM511E8). Most of the alanine mutants possessed α6ß1 integrin binding activities comparable with wild-type LM511E8. Alanine substitution for α5-Asp3198 and Asp3219 caused mild reduction in integrin binding activity, and that for α5-Asp3218 caused severe reduction, possibly resulting from conformational perturbation of α5/LG1-3. When α5-Asp3218 was substituted with asparagine, the resulting mutant possessed significant binding activity to α6ß1 integrin, indicating that α5-Asp3218 is not directly involved in integrin binding through coordination with the metal ion in ß1-MIDAS. Given that substitution of γ1-Glu1607 with glutamine nullified the binding activity to α6ß1 integrin, these results, taken together, support the possibility that the critical acidic residue coordinating the metal ion in ß1-MIDAS is Glu1607 in the γ1 tail, but no such residue is present in α5/LG1-3.
