Mammalian filamins (FLNs) are a family of three large actin-binding proteins. FLNa, the founding member of the family, was implicated in migration by cell biological analyses and the identification of FLNA mutations in the neuronal migration disorder periventricular heterotopia. However, recent knockout studies have questioned the relevance of FLNa to cell migration. Here we have used shRNA-mediated knockdown of FLNa, FLNb or FLNa and FLNb, or, alternatively, acute proteasomal degradation of all three FLNs, to generate FLN-deficient cells and assess their ability to migrate. We report that loss of FLNa or FLNb has little effect on migration but that knockdown of FLNa and FLNb, or proteolysis of all three FLNs, impairs migration. The observed defect is primarily a deficiency in initiation of motility rather than a problem with maintenance of locomotion speed. FLN-deficient cells are also impaired in spreading. Re-expression of full length FLNa, but not re-expression of a mutated FLNa lacking immunoglobulin domains 19 to 21, reverts both the spreading and the inhibition of initiation of migration.Our results establish a role for FLNs in cell migration and spreading and suggest that compensation by other FLNs may mask phenotypes in single knockout or knockdown cells. We propose that interactions between FLNs and transmembrane or signalling proteins, mediated at least in part by immunoglobulin domains 19 to 21 are important for both cell spreading and initiation of migration.

The ubiquitin-proteasome system is a central mechanism for controlled proteolysis that regulates numerous cellular processes in eukaryotes. As such, defects in this system can contribute to disease pathogenesis. In this pathway, E3 ubiquitin ligases provide platforms for binding specific substrates, thereby coordinating their ubiquitylation and subsequent degradation by the proteasome. Despite the identification of many E3 ubiquitin ligases, the identities of their specific substrates are still largely unresolved. The ankyrin repeat-containing protein with a suppressor of cytokine signaling box 2 (ASB2) gene that we initially identified as a retinoic acid-response gene in acute promyelocytic leukemia cells encodes the specificity subunit of an E3 ubiquitin ligase complex that is involved in hematopoietic cell differentiation. We have recently identified filamin A and filamin B as the first ASB2 targets and shown that ASB2 triggers ubiquitylation and proteasome-mediated degradation of these proteins. The ubiquitin-proteasome system (UPS) 1 plays an essential role in the regulation of protein stability in eukaryotic cells.Degradation of a protein by the UPS entails two successive steps: the covalent attachment of multiple ubiquitin molecules to the protein substrate and its degradation by the 26 S proteasome (1, 2). Ubiquitylation of protein substrates occurs through the sequential action of distinct enzymes: a ubiquitinactivating enzyme, E1; a ubiquitin-conjugating enzyme, E2; and a ubiquitin ligase, E3, responsible for the specific recognition of substrates. Increasing attention has been recently given to the UPS leading to the identification of hundreds of E3 ubiquitin ligases (E3s). Two major classes of E3s have been described: (i) E3s of the HECT (homologous to the E6-associated protein carboxyl terminus) domain family that function as ubiquitin carriers (3, 4) and (ii) E3s of the RING (really interesting new gene) or of the U box families that have no inherent catalytic activity but recruit an E2 enzyme toward substrates (5-7).Classical approaches to identify substrates of E3s are based on the identification of interacting proteins. Although these have successfully led to the identification of a number of substrates of monomeric E3s, identification of substrates of multimeric E3s is very challenging because of the weak affinity of substrates for their requisite specificity subunit and because of the labile nature of the substrate complexed with the specificity subunit (8).Acute promyelocytic leukemia (APL) is associated with six reciprocal translocations always involving the retinoic acid receptor ␣ (RAR␣) gene (9 -11). The RAR␣ protein is a member of the nuclear receptor superfamily that stimulates myeloid differentiation in the presence of its ligand, all-transretinoic acid (RA). In more than 95% of APL, the t(15;17) translocation between the promyelocytic leukemia (PML) gene on chromosome 15 and the RAR␣ gene on chromosome 17 produces the PML-RAR␣ fusion protein (12). The PML-RAR␣ protein enhances the rep...

No abstract

By providing contacts between hematopoietic cells and the bone marrow microenvironment, integrins are implicated in cell adhesion and thereby in control of cell fate of normal and leukemia cells. The ASB2 gene, initially identified as a retinoic acid responsive gene and a target of the promyelocytic leukemia retinoic acid receptor ␣ oncoprotein in acute promyelocytic leukemia cells, encodes two isoforms, a hematopoietic-type (ASB2␣) and a muscle-type (ASB2␤) that are involved in hematopoietic and myogenic differentiation, respectively. ASB2␣ is the specificity subunit of an E3 ubiquitin ligase complex that targets filamins to proteasomal degradation. To examine the relationship of the ASB2␣ structure to E3 ubiquitin ligase function, functional assays and molecular modeling were performed. We show that ASB2␣, through filamin A degradation, enhances adhesion of hematopoietic cells to fibronectin, the main ligand of ␤1 integrins. Furthermore, we demonstrate that a short N-terminal region specific to ASB2␣, together with ankyrin repeats 1 to 10, is necessary for association of ASB2␣ with filamin A. Importantly, the ASB2␣ N-terminal region comprises a 9-residue segment with predicted structural homology to the filamin-binding motifs of migfilin and ␤ integrins. Together, these data provide new insights into the molecular mechanisms of ASB2␣ binding to filamin.