Functional and Structural Insights into ASB2α, a Novel Regulator of Integrin-dependent Adhesion of Hematopoietic Cells

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 alpha oncoprotein in acute promyelocytic leukemia cells, encodes two isoforms, a hematopoietic-type (ASB2 alpha) and a muscle-type (ASB2 alpha) that are involved in hematopoietic and myogenic differentiation, respectively. ASB2 alpha is the specificity subunit of an E3 ubiquitin ligase complex that targets filamins to proteasomal degradation. To examine the relationship of the ASB2 alpha structure to E3 ubiquitin ligase function, functional assays and molecular modeling were performed. We show that ASB2 alpha, through filamin A degradation, enhances adhesion of hematopoietic cells to fibronectin, the main ligand of beta 1 integrins. Furthermore, we demonstrate that a short N-terminal region specific to ASB2 alpha, together with ankyrin repeats 1 to 10, is necessary for association of ASB2 alpha with filamin A. Importantly, the ASB2 alpha N-terminal region comprises a 9-residue segment with predicted structural homology to the filamin-binding motifs of migfilin and beta integrins. Together, these data provide new insights into the molecular mechanisms of ASB2 alpha binding to filamin.