Binary and ternary complexes of FLNa-Ig21 with cytosolic tails of αMβ2 integrin reveal dual role of filamin mediated regulation

Background: Cytoskeletal protein filamin A is critical for the outside-in signaling of integrins. Although molecular mechanisms of filamin-integrin interactions are not fully understood. Mostly, the membrane distal (MD) part of the cytosolic tail (CT) of beta subunit of integrin is known to interact with filamin A domain 21 (FLNa-Ig2). However, binary and ternary complexes of full-length CTs of leucocyte specific beta 2 integrins with FLNa-Ig21 are yet to be elucidated. Methods: Binding interactions of the CTs of integrin alpha M beta 2 with FLNa-Ig21 are extensively investigated by NMR, ITC, cell-based functional assays and computational docking. Results: The alpha M CT demonstrates interactions with FLNa-Ig21 forming a binary complex. Filamin/alpha M interface is mediated by sidechain-sidechain interactions among non-polar and aromatic residues involving MP helix of alpha M and the canonical CD face of FLNa-Ig21. Functional assays delineated an interfacial residue Y1137 of alpha M CT is critical for in-cell binding to FLNa-Ig2. In addition, full-length beta 2 CT occupies two distinct binding sites in complex with FLNa-Ig21. A ternary complex of FLNa-Ig21 with CTs has been characterized. In the ternary complex, alpha M CT moves away to a distal site of FLNa-Ig21 with fewer interactions. Conclusion: Our findings demonstrate a plausible dual role of filamin in integrin regulation. The molecular interactions of the ternary complex are critical for the resting state of integrins whereas stable FLNa-Ig21/alpha M CT binary complex perhaps be required for the activated state. General significance: Filamin binding to both alpha and beta CTs of other integrins could be essential in regulating bidirectional signaling mechanisms.

Automated summary

The study reveals a potential dual role of filamin in integrin regulation, where interactions within the ternary complex are critical for the resting state of integrins and stable binary complexes are possibly required for the activated state. Filamin binding to both alpha and beta CTs of other integrins could play an essential role in regulating bidirectional signaling mechanisms.