Filamin A (FLNa), a dimeric actin crosslinking and scaffold protein with numerous intracellular binding partners, anchors the platelet adhesion glycoprotein (GP) Ib-IX-V receptor to actin cytoskeleton. We mapped the GPIb␣ binding site to a single domain of FLNa and resolved the structure of this domain and its interaction complex with the corresponding GPIb␣ cytoplasmic domain. This is the first atomic structure of this class of membrane glycoprotein-cytoskeleton connection. GPIb␣ binds in a groove formed between the C and D ␤ strands of FLNa domain 17. The interaction is strikingly similar to that between the ␤7 integrin tail and a different FLNa domain, potentially defining a conserved motif for FLNa binding. Nevertheless, the structures also reveal specificity of the interfaces, which explains different regulatory mechanisms. To verify the topology of GPIb-FLNa interaction we also purified the native complex from platelets and showed that GPIb interacts with the C-terminus of FLNa, which is in accordance with our biochemical and structural data. (Blood. 2006;107:1925-1932

Filamins are essential in cell motility and many developmental processes. They are large actin cross linking proteins that contain actin binding domains in their N termini and a long rod region constructed from 24 tandem Ig domains. Dimerization is crucial for the actin crosslinking function of filamins and requires the most C-terminal Ig domain. We describe here the crystal structure of this 24th Ig domain (Ig24) of human filamin C and show how it mediates dimerization. The dimer interface is novel and quite different to that seen in the Dictyostelium discoideum filamin analog. The sequence signature of the dimerization interface suggests that the C-terminal domains of all vertebrate filamins share the same dimerization mechanism. Furthermore, we show that point mutations in the dimerization interface disrupt the dimer and that the dissociation constant for recombinant Ig24 is in the micromolar range.

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