Leukocyte integrin αLβ2 transmembrane association dynamics revealed by coarse-grained molecular dynamics simulations

Integrins are transmembrane (TM) proteins that mediate bidirectional mechanical signaling between the extracellular matrix and the cellular cytoskeletal network. Each integrin molecule consists of non-covalently associated alpha- and beta-subunits, with each subunit consisting of a large ectodomain, a single-pass TM helix, and a short cytoplasmic tail. Previously we found evidence for a polar interaction (hydrogen bond) in the outer membrane clasp (OMC) of the leukocyte integrin alpha L beta 2 TMs that is absent in the platelet integrin alpha IIb beta 3 OMC. Here, we compare the self-assembly dynamics of alpha L beta 2 and alpha IIb beta 3 TM helices in a model membrane using coarse-grained molecular dynamics simulations. We found that although alpha IIb beta 3 TM helices associate more easily, packing is suboptimal. In contrast, alpha L beta 2 TM helices achieve close-to-optimal packing. This suggests that alpha L beta 2 TM packing is more specific, possibly due to the interhelix hydrogen bond. Theoretical association free energy profiles show a deeper minimum at a smaller helix-helix separation for alpha L beta 2 compared with alpha IIb beta 3. The alpha IIb beta 3 profile is also more rugged with energetic barriers whereas that of alpha L beta 2 is almost without barriers. Disruption of the interhelix hydrogen bond in alpha L beta 2 via the beta 2T686G mutation results in poorer association and a similar profile as alpha IIb beta 3. The OMC polar interaction in alpha L beta 2 thus plays a significant role in the packing of the TM helices.