No abstract

Ubiquitin-mediated protein degradation is the main mechanism for controlled proteolysis, which is crucial for muscle development and maintenance. The ankyrin repeat-containing protein with a suppressor of cytokine signaling box 2 gene (ASB2) encodes the specificity subunit of an E3 ubiquitin ligase complex involved in differentiation of hematopoietic cells. Here, we provide the first evidence that a novel ASB2 isoform, ASB2b, is important for muscle differentiation. ASB2b is expressed in muscle cells during embryogenesis and in adult tissues. ASB2b is part of an active E3 ubiquitin ligase complex and targets the actin-binding protein filamin B ( The ubiquitin-proteasome system (UPS) is one of the major mechanisms for controlled proteolysis, which is a crucial determinant of many cellular events in eukaryotes. Degradation of a protein by the ubiquitin-proteasome pathway entails two successive events: the covalent attachment of ubiquitin chains to lysine residues in a substrate protein leading to its recognition and ATP-dependent proteolysis by the proteasome. Ubiquitylation of protein substrates occurs through the sequential action of distinct enzymes: a ubiquitin-activating enzyme E1, a ubiquitin-conjugating enzyme E2 and a ubiquitin ligase E3 responsible for the specific recognition of substrates through dedicated interaction domains. 1 ASB2 is one of 18 members of the ankyrin repeat-containing suppressor of cytokine signaling (SOCS) box protein family (ASB) that are characterized by variable numbers of N-terminal ankyrin repeats. 2 The ASB2 gene was originally identified as an retinoic acid-inducible gene involved in induced differentiation of myeloid leukemia cells. 3,4 We have previously demonstrated that, by interacting with the elongin BC complex, ASB2 can assemble with a Cullin5/Rbx module to form an E3 ubiquitin ligase complex that stimulates polyubiquitylation by the E2 ubiquitin-conjugating enzyme UbcH5a. 5,6 This strongly suggests that ASB2 targets specific proteins to destruction by the proteasome during differentiation of hematopoietic cells. We have recently shown that ASB2 ubiquitin ligase activity drives proteasome-mediated degradation of the ubiquitously expressed actin-binding protein filamins (FLNs), FLNa and b, and can regulate integrin-mediated cell spreading. 6 During muscle development, dramatic changes in protein expression and cell morphology rely on the turnover of regulatory and structural components. Indeed, myogenic transcription factors such as MyoD and its E2A partner or negative Id regulator as well as myofibrillar proteins were shown to be degraded by the UPS. 7-11 Although some E3 ubiquitin ligases active during myogenesis have been identified, 12-23 a precise understanding of the function of ubiquitylation in muscle development and the identities of specific ubiquitin ligases and their potential substrates is lacking.Here we show that ASB2 expression is not restricted to hematopoietic cells but is also expressed and regulated in muscle cells during mouse and chick embryogenesis.