Proteolytic Regulation of the Zinc Finger Transcription Factor YY1, a Repressor of Muscle-restricted Gene Expression

Abstract

Regulated proteolysis has been postulated to be critical for proper control of cell functions. Muscle development, in particular, involves a great deal of structural adaptation and remodeling mediated by proteases. The transcription factor YY1 represses muscle-restricted expression of the sarcomeric ␣-actin genes. Consistent with this repressor function of YY1, the nuclear regulator is down-regulated at the protein level during skeletal as well as cardiac muscle cell differentiation. However, the YY1 message remains relatively unaltered throughout the myoblast-myotube transition, implicating a post-translational regulatory mechanism. We show that YY1 can be a substrate for cleavage by the calciumactivated neutral protease calpain II (m-calpain) and the 26 S proteasome. The calcium ionophore A23187 destabilized YY1 in cultured myoblasts, and the decrease in YY1 protein levels could be prevented by calpain inhibitor II and calpeptin. Treatment with the proteasome inhibitors MG132 and lactacystin resulted in the stabilization of YY1 protein, which is consistent with the finding that YY1 is readily polyubiquitinated in reticulocyte lysates. We further show that proteolytic targeting by calpain II and the proteasome involves different structural elements of YY1. This study thus illustrates two proteolytic pathways through which the transcriptional regulator can be differentially targeted under different cell growth conditions. Many cellular processes are known to be controlled by shortlived proteins, including products of the proto-oncogenes, cell cycle regulators, and developmentally regulated transcription factors (1-3). The fast turnover of these regulatory proteins reflects a metabolic requirement for rapidly changing their concentrations and is presumably mediated by a complex interplay among various proteases and protease inhibitors. Selective degradation of transcriptional activators and repressors, in particular, may provide efficient regulatory mechanisms contributing to the rapid shut-off and turn-on of gene activity, respectively. The recent study of the pleiotropic transcription factor NF-B has revealed that the ubiquitinproteasome pathway can function not only in the complete degradation of proteins but also in the regulated processing of precursors into active transcription factors (4, 5). Calpains represent the other major class of nonlysosomal proteases functioning in a calcium-dependent fashion (6, 7). Interestingly, both the proteasome and calpains have been found to play a regulatory role in the function and/or stability of c-Fos and the tumor suppressor protein p53 (8 -11). Thus, rapid degradation or processing of specific transcription factors can underlie a wide range of dynamic cellular and developmental processes.Muscle development involves a great deal of structural adaptation and remodeling mediated by induced protein synthesis and degradation (12)(13)(14). Although protein turnover must be highly selective if it is to be developmentally useful, little is known concerning the regulatory mech...

Regulatory polymorphisms in the bovine Ankyrin 1 gene promoter are associated with tenderness and intramuscular fat content

“…Adamowicz and co-workers [47] also reported that a mutation in the Sp1 binding site of the bovine leptin gene affects its expression level. The presence of a G allele of this SNP (rSNP4) created a binding site for the zinc finger nuclear protein, YY1 (Yin Yang 1), also known as NF-E1, UCRBP or CF1, which is known to have fundamental roles in repressing and activating a diverse group of promoters; it not only initiates transcription but also activates or represses it [48,49]. YY1 is also shown to repress muscle specific expression of sarcomeric alpha-actin and c-myc genes [49].…”

“…The presence of a G allele of this SNP (rSNP4) created a binding site for the zinc finger nuclear protein, YY1 (Yin Yang 1), also known as NF-E1, UCRBP or CF1, which is known to have fundamental roles in repressing and activating a diverse group of promoters; it not only initiates transcription but also activates or represses it [48,49]. YY1 is also shown to repress muscle specific expression of sarcomeric alpha-actin and c-myc genes [49]. Walowitz and co-workers also reported that calpains and the 26 S proteasome are involved in YY1 stability in rat muscle indicating that myogenic transcription may be inactivated by developmentally regulated proteolysis to promote muscle development.…”

Posttranslational regulation of the retinoblastoma gene family member p107 by calpain protease

“…Alternatively, in response to di erent stimuli these proteins may be subject to degradation by either calpain or the proteasome. An example of this dual regulation was shown recently in myoblasts where the transcription factor YY1 was degraded by calpain during skeletal muscle cell di erentiation, and by the proteasome in response to serum starvation (Walowitz et al, 1998). As discussed above, a considerable body of evidence has accumulated showing that RB and p107 function, although partially overlapping, may also be divergent.…”

“…Reported calpain substrates include cytokines, cytoskeletal-associated proteins, enzymes, including focal adhesion kinase (Cooray et al, 1996) and protein kinase C (Croall and Demartino, 1991;Eto et al, 1995), and the transcriptional activators c-fos, c-Jun, YY1, and p53 (Hirai et al, 1991;Kavita and Mizel, 1995;Walowitz et al, 1998).…”

“…p107 levels are low in G0 and early G1, and then rise as cells approach the G1/S transition (Whyte, 1995;Moberg et al, 1996). An important determinant of cell cycle protein periodicity is protein stability, and ubiquitin-dependent proteolysis, catalyzed by the 26S proteasome, has been implicated in the posttranslational regulation of a wide range of cell cycle regulatory proteins including cyclins B1, A, D1, and E, p53, p27, YY1, c-Jun, c-fos and c-myc (Glotzer et al, 1991;Ciechanover et al, 1991;Treier et al, 1994;King et al, 1995;Sudakin et al, 1995;Pagano et al, 1995;Crook et al, 1996;Clurman et al, 1996;Maki et al, 1996;Choi et al, 1997;Walowitz et al, 1998).…”