Common activation mechanism of beta 2 integrins (P5073)

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Abstract Beta 2 integrins are α/β heterodimeric adhesion molecules on leukocytes consisting of αLβ2, αMβ2, αXβ2 and αDβ2. They undergo significant conformational changes to bind to their ligands at the domain called the α I domain of the α subunit. It has been postulated that the domain called the β I domain of the β subunit undergoes conformational changes and crosstalks with the α I domain in favor for ligand binding. However, the regions important for the β I domain activation have not been determined yet. Our unpublished and published αXβ2 structures suggested that the α1 helix of the β I domain was important in the activation of the β I domain. To examine the importance of the α1 helix, alanine scanning mutagenesis was performed on this region and transient transfectants in 293T cells were used for the experiments. Our β2 V124A and L132A mutations were constitutively active in all αL, αM, and αX integrins as demonstrated by ligands’ binding (αLβ2 to ICAM-1, αMβ2 to fibrinogen, ICAM-1, and iC3b, αXβ2 to iC3b) and activation-sensitive epitope exposures (KIM127, m24 and MEM148) even without activation. On the other hand, β2 L127A and L135A mutations were deactivating in all αL, αM and αX integrins as shown by diminished ligands’ binding and activation-sensitive epitope exposures even in an activating condition. Our data suggested that b2 integrins shared the common activation pathway of the β I domain, which would subsequently relay to the αI domain.