A novel ubiquitin-binding protein ZNF216 functioning in muscle atrophy

The ubiquitin–proteasome system (UPS) is critical for specific degradation of cellular proteins and plays a pivotal role on protein breakdown in muscle atrophy. Here, we show that ZNF216 directly binds polyubiquitin chains through its N-terminal A20-type zinc-finger domain and associates with the 26S proteasome. ZNF216 was colocalized with the aggresome, which contains ubiquitinylated proteins and other UPS components. Expression of Znf216 was increased in both denervation- and fasting-induced muscle atrophy and upregulated by expression of constitutively active FOXO, a master regulator of muscle atrophy. Mice deficient in Znf216 exhibited resistance to denervation-induced atrophy, and ubiquitinylated proteins markedly accumulated in neurectomized muscle compared to wild-type mice. These data suggest that ZNF216 functions in protein degradation via the UPS and plays a crucial role in muscle atrophy

The Cytosolic Domain of Pex22p Stimulates the Pex4p-Dependent Ubiquitination of the PTS1-Receptor

Peroxisomal biogenesis is an ubiquitin-dependent process because the receptors required for the import of peroxisomal matrix proteins are controlled via their ubiquitination status. A key step is the monoubiquitination of the import receptor Pex5p by the ubiquitin-conjugating enzyme (E2) Pex4p. This monoubiquitination is supposed to take place after Pex5p has released the cargo into the peroxisomal matrix and primes Pex5p for the extraction from the membrane by the mechano-enzymes Pex1p/Pex6p. These two AAA-type ATPases export Pex5p back to the cytosol for further rounds of matrix protein import. Recently, it has been reported that the soluble Pex4p requires the interaction to its peroxisomal membrane-anchor Pex22p to display full activity. Here we demonstrate that the soluble C-terminal domain of Pex22p harbours its biological activity and that this activity is independent from its function as membrane-anchor of Pex4p. We show that Pex4p can be functionally fused to the trans-membrane segment of the membrane protein Pex3p, which is not directly involved in Pex5p-ubiquitination and matrix protein import. However, this Pex3(N)-Pex4p chimera can only complement the double-deletion strain pex4Δ/pex22Δ and ensure optimal Pex5p-ubiquitination when the C-terminal part of Pex22p is additionally expressed in the cell. Thus, while the membrane-bound portion Pex22(N)p is not required when Pex4p is fused to Pex3(N)p, the soluble Pex22(C)p is essential for peroxisomal biogenesis and efficient monoubiquitination of the import receptor Pex5p by the E3-ligase Pex12p in vivo and in vitro. The results merge into a picture of an ubiquitin-conjugating complex at the peroxisomal membrane consisting of three domains: the ubiquitin-conjugating domain (Pex4p), a membrane-anchor domain (Pex22(N)p) and an enhancing domain (Pex22(C)p), with the membrane-anchor domain being mutually exchangeable, while the Ubc- and enhancer-domains are essential

Exposure of bipartite hydrophobic signal triggers nuclear quality control of Ndc10 at the endoplasmic reticulum/nuclear envelope

Degradation of mutant Ndc10 is mediated by the E3 ligase Doa10 at the endoplasmic reticulum/nuclear envelope membrane. An autonomous degradation motif was localized to the C-terminal region of Ndc10. The motif is composed of two indispensable elements: a hydrophobic surface of an amphipathic helix and a loosely structured, hydrophobic C-terminal tail