ASB2 Is an Elongin BC-interacting Protein That Can Assemble with Cullin 5 and Rbx1 to Reconstitute an E3 Ubiquitin Ligase Complex

Abstract

The ankyrin repeat-containing protein with a suppressor of cytokine signaling box-2 (ASB2) gene was identified as a retinoic acid-response gene and a target of the promyelocytic leukemia-retinoic acid receptor-␣ oncogenic protein characteristic of acute promyelocytic leukemia. Expression of ASB2 in myeloid leukemia cells inhibits growth and promotes commitment, recapitulating an early step known to be critical for differentiation. Here we show that ASB2, by interacting with the Elongin BC complex, can assemble with Cullin5⅐Rbx1 to form an E3 ubiquitin ligase complex that stimulates polyubiquitination by the E2 ubiquitin-conjugating enzyme Ubc5. This is a first indication that a member of the ASB protein family, ASB2, is a subunit of an ECS (Elongin C-Cullin-SOCS box)-type E3 ubiquitin ligase complex. Altogether, our results strongly suggest that ASB2 targets specific proteins to destruction by the proteasome in leukemia cells that have been induced to differentiate.

ASB2α, an E3 Ubiquitin Ligase Specificity Subunit, Regulates Cell Spreading and Triggers Proteasomal Degradation of Filamins by Targeting the Filamin Calponin Homology 1 Domain

“…1A). The SOCS box mediates interaction with a cullin family member (Cullin 5) and RING finger proteins (Rbx1/2) by interacting with elongin BC to form an E3 ubiquitin ligase complex (3,4). ASB2␣ was therefore proposed to regulate myeloid cell proliferation and differentiation by targeting a specific regulatory protein, or proteins, for degradation (1,3).…”

“…ASB2␣ is the specificity subunit of an E3 ubiquitin ligase complex (3,4) and contains an N-terminal region followed by 15 predicted ankyrin repeats and a C-terminal suppressor of cytokine signaling (SOCS) box (Fig. 1A).…”

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

“…ASB2 proteins belong to the family of ASB proteins that harbor a variable number of ankyrin repeats (ANK) followed by a suppressor of cytokine signaling box located at the C-terminal end of the protein (23). These proteins are the specificity subunits of E3 ubiquitin ligase complexes (21,22). Indeed, suppressor of cytokine signaling box-mediated interactions with the Elongin B-Elongin C (EloBEloC) complex and the Cul5/Rbx2 module allow ASB2 proteins to assemble a multimeric E3 ubiquitin ligase complex, and so regulate the turnover of specific proteins involved in cell differentiation.…”

“…Importantly, Notch activation stimulated ASB2 expression (20). ASB2 encodes two isoforms, a hematopoietic-type (ASB2␣) and a muscle-type (ASB2␤) that are involved in hematopoietic and myogenic differentiation, respectively (21,22). ASB2 proteins belong to the family of ASB proteins that harbor a variable number of ankyrin repeats (ANK) followed by a suppressor of cytokine signaling box located at the C-terminal end of the protein (23).…”

Protein Interaction Screening for the Ankyrin Repeats and Suppressor of Cytokine Signaling (SOCS) Box (ASB) Family Identify Asb11 as a Novel Endoplasmic Reticulum Resident Ubiquitin Ligase

“…In contrast, we have very little information about the biological and molecular function of the ASBs. Only a few ASB family members have been tested for E3 ligase activity (8,9), and just a small subset of substrates have been identified to date. For example, ASB2 is up-regulated by retinoic acid in acute promyelocytic cells where it targets filament A and B for proteasomal degradation, thereby regulating hematopoetic cell differentiation (10).…”