Using a ubiquitin ligase as an unfolded protein sensor.

A significant fraction of all proteins are misfolded and must be degraded. The ubiquitin-proteasome pathway provides an essential protein quality control function necessary for normal cellular homeostasis. Substrate specificity is mediated by proteins called ubiquitin ligases. In the endoplasmic reticulum (ER) a specialized pathway, the endoplasmic reticulum associated degradation (ERAD) pathway provides means to eliminate misfolded proteins from the ER. One marker used by the ER to identify misfolded glycoproteins is the presence of a high-mannose (Man5-8GlcNAc2) glycan. Recently, FBXO2 was shown to bind high mannose glycans and participate in ERAD. Using glycan arrays, immobilized glycoprotein pulldowns, and glycan competition assays we demonstrate that FBXO2 preferentially binds unfolded glycoproteins. Using recombinant, bacterially expressed GST-FBXO2 as an unfolded protein sensor we demonstrate it can be used to monitor increases in misfolded glycoproteins after physiological or pharmaceutical stressors.

EGF regulation of proximal tubule cell proliferation and VEGF-A secretion.

Proximal tubule cell (PTC) proliferation is critical for tubular regeneration and recovery from acute kidney injury. Epidermal growth factor (EGF) and vascular endothelial growth factor (VEGF-A) are important for the maintenance of tubulointerstitial integrity and can stimulate PTC proliferation. We utilized HK-2 cells, an immortalized human PTC line, to characterize the EGF-dependent regulation of VEGF-A secretion and proliferation in PTCs. We demonstrate that EGF stimulates VEGF-A secretion via the EGF receptor (EGFR) and stimulates cell proliferation via activation of the VEGF receptor, VEGFR-2. EGFR activation promotes MAPK (ERK1/2) activation and HIF-1α expression, which are required for basal and EGF-stimulated VEGF-A secretion. EGF also stimulates the phosphorylation of P70S6 kinase (P70S6K), the downstream target of mTORC1. Rapamycin decreased basal and EGF stimulated HIF-1α and enhanced MAPK (ERK1/2) activation, while MAPK (ERK/12) inhibition downregulated HIF-1α expression and the phosphorylation of p70S6K. EGF stimulation of p70S6K was also independent of p-AKT Inhibition of the mTORC1 pathway with rapamycin abolished phosphorylation of p70S6K but had no effect on VEGF-A secretion, indicating that EGF-stimulated VEGF-A secretion did not require mTORC1 pathway activation. We demonstrate evidence of a complex crosstalk between the MAPK/ERK and mTORC1 pathways, wherein MAPK (ERK1/2) activation stimulates p-P70S6K, while p-P70S6K activation seems to inhibit MAPK (ERK1/2) in EGF-treated HK-2 cells. Our results suggest that EGF stimulates MAPK (ERK1/2) in HK-2 cells, which in turn increases HIF-1α expression and VEGF-A secretion, indicating that VEGF-A mediates EGF-stimulated cell proliferation as an autocrine proximal tubular epithelial cell growth factor.

Diversity in tissue expression, substrate binding, and SCF complex formation for a lectin family of ubiquitin ligases.

Post-translational modification of proteins regulates many cellular processes. Some modifications, including N-linked glycosylation, serve multiple functions. For example, the attachment of N-linked glycans to nascent proteins in the endoplasmic reticulum facilitates proper folding, whereas retention of high mannose glycans on misfolded glycoproteins serves as a signal for retrotranslocation and ubiquitin-mediated proteasomal degradation. Here we examine the substrate specificity of the only family of ubiquitin ligase subunits thought to target glycoproteins through their attached glycans. The five proteins comprising this FBA family (FBXO2, FBXO6, FBXO17, FBXO27, and FBXO44) contain a conserved G domain that mediates substrate binding. Using a variety of complementary approaches, including glycan arrays, we show that each family member has differing specificity for glycosylated substrates. Collectively, the F-box proteins in the FBA family bind high mannose and sulfated glycoproteins, with one FBA protein, FBX044, failing to bind any glycans on the tested arrays. Site-directed mutagenesis of two aromatic amino acids in the G domain demonstrated that the hydrophobic pocket created by these amino acids is necessary for high affinity glycan binding. All FBA proteins co-precipitated components of the canonical SCF complex (Skp1, Cullin1, and Rbx1), yet FBXO2 bound very little Cullin1, suggesting that FBXO2 may exist primarily as a heterodimer with Skp1. Using subunit-specific antibodies, we further demonstrate marked divergence in tissue distribution and developmental expression. These differences in substrate recognition, SCF complex formation, and tissue distribution suggest that FBA proteins play diverse roles in glycoprotein quality control.

Cellular turnover of the polyglutamine disease protein ataxin-3 is regulated by its catalytic activity.

Ataxin-3, a deubiquitinating enzyme, is the disease protein in spinocerebellar ataxia type 3, one of many neurodegenerative disorders caused by polyglutamine expansion. Little is known about the cellular regulation of ataxin-3. This is an important issue, since growing evidence links disease protein context to pathogenesis in polyglutamine disorders. Expanded ataxin-3, for example, is more neurotoxic in fruit fly models when its active site cysteine is mutated (1). We therefore sought to determine the influence of ataxin-3 enzymatic activity on various cellular properties. Here we present evidence that the catalytic activity of ataxin-3 regulates its cellular turnover, ubiquitination, and subcellular distribution. Cellular protein levels of catalytically inactive ataxin-3 were much higher than those of active ataxin-3, in part reflecting slower degradation. In vitro studies revealed that inactive ataxin-3 was more slowly degraded by the proteasome and that this degradation occurred independent of ubiquitination. Slower degradation of inactive ataxin-3 correlated with reduced interaction with the proteasome shuttle protein, VCP/p97. Enzymatically active ataxin-3 also showed a greater tendency to concentrate in the nucleus, where it colocalized with the proteasome in subnuclear foci. Taken together, these and other findings suggest that the catalytic activity of this disease-linked deubiquitinating enzyme regulates several of its cellular properties, which in turn may influence disease pathogenesis.