Analysis of molecular mechanisms on integrin-family specific adhesion

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Leukocyte integrin LFA-1 plays a critical role in leukocyte migration and immunological synapse formation. In these processes, Rap1 are involved in avidity regulation of LFA-1 upon stimulation with chemokines and antigen. Avidity changes of integrins are regulated through their cytoplasmic regions. To identify specific residues that are involved in these processes, we have reconstituted human LFA-1 bearing truncated and point-mutated αL and β2 cytoplasmic regions and examined their effects on Rap1 -dependent adhesion to ICAM-1. Truncation of the αL, but not β2 subunit cytoplasmic region abolished Rap1 -dependent adhesion to ICAM-1. The two lysine residues (KK1097/1099) in the αL subunit were found to be critical in the Rap-1 dependent adhesion to ICAM-1. The KK1097/1099AA mutation was also defective in binding to ICAM-1 triggered by SDF-1. On the other hand, the substitution of tyrosine to alanine (Y735A) of the β2 subunit inhibited internalization of LFA-1, and impaired detachment, which resulted in elongated cell shape. Our findings have revealed the primary role of the αL cytoplasmic region in the response to the inside-out signaling of Rap1 and the β2 subunit in the subsequent post-adhesion events.