Boutet Stéphane C, Disatnik Marie-Hélène, Chan Lauren S, Iori Kevin, Rando Thomas A
Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, CA 94305, USA.
Cell. 2007 Jul 27;130(2):349-62. doi: 10.1016/j.cell.2007.05.044.
Pax3 and Pax7 play distinct but overlapping roles in developmental and postnatal myogenesis. The mechanisms involved in the differential regulation of these highly homologous proteins are unknown. We present evidence that Pax3, but not Pax7, is regulated by ubiquitination and proteasomal degradation during adult muscle stem cell activation. Intriguingly, only monoubiquitinated forms of Pax3 could be detected. Mutation of two specific lysine residues in the C-terminal region of Pax3 reduced the extent of its monoubiquitination and susceptibility to proteasomal degradation, whereas introduction of a key lysine into the C-terminal region of Pax7 rendered that protein susceptible to monoubiquitination and proteasomal degradation. Monoubiquitinated Pax3 was shuttled to the intrinsic proteasomal protein S5a by interacting specifically with the ubiquitin-binding protein Rad23B. Functionally, sustained expression of Pax3 proteins inhibited myogenic differentiation, demonstrating that Pax3 degradation is an essential step for the progression of the myogenic program. These results reveal an important mechanism of Pax3 regulation in muscle progenitors and an unrecognized role of protein monoubiquitination in mediating proteasomal degradation.
Pax3和Pax7在发育及出生后的成肌过程中发挥着不同但又相互重叠的作用。这些高度同源蛋白的差异调控所涉及的机制尚不清楚。我们提供的证据表明，在成年肌肉干细胞激活过程中，Pax3受泛素化和蛋白酶体降解的调控，而Pax7则不然。有趣的是，只能检测到单泛素化形式的Pax3。Pax3 C末端区域中两个特定赖氨酸残基的突变降低了其单泛素化程度以及对蛋白酶体降解的敏感性，而在Pax7的C末端区域引入一个关键赖氨酸则使该蛋白易于发生单泛素化和蛋白酶体降解。单泛素化的Pax3通过与泛素结合蛋白Rad23B特异性相互作用而被转运至内在蛋白酶体蛋白S5a。在功能上，Pax3蛋白的持续表达抑制了成肌分化，表明Pax3的降解是成肌程序进展的关键步骤。这些结果揭示了肌肉祖细胞中Pax3调控的重要机制以及蛋白质单泛素化在介导蛋白酶体降解中未被认识的作用。
