Human filamins (Flns) are multifunctional cytoskeletal proteins that are involved in cell motility, cytoskeletal regulation and cellular signaling (Popowicz et al.
; Zhou et al.
). There are three known filamin isoforms: FlnA, FlnB and FlnC. FlnA and FlnB are expressed in many tissues and cell types, while FlnC expression is primarily restricted to muscle and cardiac tissues (van der Flier & Sonnenberg, 2001
). Filamins are ~280 kDa proteins comprised of an amino-terminal actin-binding domain and 24 immunoglobulin-like domains (Ig repeats). The 24 Ig repeats have been categorized into classes A, B, C and D (Ithychanda et al.
) based on sequence similarity. The FlnA-Ig10 structure presented here represents the first structure of a class D Ig repeat from FlnA to be deposited in the Protein Data Bank (PDB).
A major structural role of the filamins is to cross-link actin filaments, adding mechanical strength and flexibility to the actin cytoskeleton under load (Popowicz et al.
). Filamins, however, also play a direct role in linking the actin cytoskeleton to signaling and cell-surface proteins (Kim & McCulloch, 2011
; Feng & Walsh, 2004
). The physiological importance of the filamins is emphasized by the fact that missense mutations in FlnA and FlnB cause developmental malformations in bone, limbs and a variety of organs (Feng & Walsh, 2004
). The effects of these missense mutations vary greatly and depend upon the Fln domain in which the mutation is located. Missense mutations in FlnA-Ig10 are correlated with the otopalatodigital syndrome spectrum disorders frontometaphyseal dysplasia (FMD) and Melnick–Needles syndrome (MNS). Both FMD and MNS are skeletal dysplasias found predominantly in females, as both disorders are typically embryonically or perinatally lethal to males (Robertson, 2007
). MNS exhibits variable manifestations in diagnosed females ranging from mild skeletal dysplasias to significant skeletal abnormalities leading to thoracic restriction (Robertson, 2004
). FMD in females is characterized by substantial skeletal dysplasia, which can include supraorbital hyperostosis, vertebral fusions and phalangeal contractures (Robertson et al.
). All known missense mutations correlated with MNS are located in FlnA-Ig10, while two of the three known missense mutations correlated with lethal FMD are located in FlnA-Ig10 (Robertson et al.
Here, we present the 2.44 Å resolution crystal structure of FlnA-Ig10. The structure exhibits a canonical immunoglobulin-like domain fold resembling the structures of other FlnA Ig repeats. The structure features a disulfide-linked crystal lattice, a β-mercaptoethanol thio-adduct on a single surface-exposed cysteine and an acetate ion coordinated by both side-chain-mediated and backbone-mediated hydrogen bonds. The FlnA-Ig10 crystal structure allows analysis of the potential structural consequences of missense mutations correlated with otopalatodigital syndrome spectrum disorders.