Visualization of Protein Interactions in Living Cells Using Bimolecular Fluorescence Complementation ( Bi FC ) Analysis

106

Reversible ubiquitination of activated receptor complexes signals their sorting between recycling and degradation and thereby dictates receptor fate. The deubiquitinating enzyme ubiquitin-specific protease 8 (USP8/UBPy) has been previously implicated in the regulation of the epidermal growth factor receptor (EGFR); however, the molecular mechanisms governing its recruitment and activity in this context remain unclear. Herein, we investigate the role of USP8 in countering ligandinduced ubiquitination and down-regulation of EGFR and characterize a subset of protein-protein interaction determinants critical for this function. USP8 depletion accelerates receptor turnover, whereas loss of hepatocyte growth factor-regulated substrate (Hrs) rescues this phenotype, indicating that USP8 protects EGFR from degradation via an Hrs-dependent pathway. Catalytic inactivation of USP8 incurs EGFR hyperubiquitination and promotes receptor localization to endosomes marked by high ubiquitin content. These phenotypes require the central region of USP8, containing three extended Arg-X-XLys (RXXK) motifs that specify direct low affinity interactions with the SH3 domain(s) of ESCRT-0 proteins, STAM1/2. The USP8⅐STAM complex critically impinges on receptor ubiquitination status and modulates ubiquitin dynamics on EGFR-positive endosomes. Consequently, USP8-mediated deubiquitination slows progression of EGFR past the earlyto-recycling endosome circuit in a manner dependent upon the RXXK motifs. Collectively, these findings demonstrate a role for the USP8⅐STAM complex as a protective mechanism regulating early endosomal sorting of EGFR between pathways destined for lysosomal degradation and recycling.

Section: Methodsmentioning

confidence: 99%

Regulation of Epidermal Growth Factor Receptor Ubiquitination and Trafficking by the USP8·STAM Complex

Berlin¹,

Schwartz

Nash

2010

106

“…To investigate the cellular interactions between USP8 and the ESCRT-0 complex, BiFC microscopy was employed. BiFC takes advantage of a GFP variant, Venus fluorescent protein (VFP) to enable visualization of protein-protein complexes as they form in living cells (35,38,39). In this assay, the VFP fluorophore is separated into N-and C-terminal fragments (VN and VC, respectively), each fused to one of two proteins of interest (Fig.…”

Section: Usp8 Modulates the Integrity Of The Endocytic Compartment Atmentioning

confidence: 99%

The Deubiquitinating Enzyme USP8 Promotes Trafficking and Degradation of the Chemokine Receptor 4 at the Sorting Endosome

Berlin¹,

Higginbotham

Dise

et al. 2010

Reversible ubiquitination orchestrated by the opposition of ubiquitin ligases and deubiquitinating enzymes mediates endocytic trafficking of cell surface receptors for lysosomal degradation. Ubiquitin-specific protease 8 (USP8) has previously been implicated in endocytosis of several receptors by virtue of their deubiquitination. The present study explores an indirect role for USP8 in cargo trafficking through its regulation of the chemokine receptor 4 (CXCR4). Contrary to the effects of USP8 loss on enhanced green fluorescent protein, we find that USP8 depletion stabilizes CXCR4 on the cell surface and attenuates receptor degradation without affecting its ubiquitination status. In the presence of ligand, diminished CXCR4 turnover is accompanied by receptor accumulation on enlarged early endosomes and leads to enhancement of phospho-ERK signaling. Perturbation in CXCR4 trafficking, resulting from USP8 inactivation, occurs at the ESCRT-0 checkpoint, and catalytic mutation of USP8 specifically targeted to the ESCRT-0 complex impairs the spatial and temporal organization of the sorting endosome. USP8 functionally opposes the ubiquitin ligase AIP4 with respect to ESCRT-0 ubiquitination, thereby promoting trafficking of CXCR4. Collectively, our findings demonstrate a functional cooperation between USP8, AIP4, and the ESCRT-0 machinery at the early sorting phase of CXCR4 and underscore the versatility of USP8 in shaping trafficking events at the earlyto-late endosome transition.Endocytosis and trafficking of cell surface receptors is essential for organized signal transduction and maintenance of homeostasis. Malfunctions along the molecular pathways governing endocytosis can lead to a wide range of human pathologies including metabolic disorders, autoimmune diseases, and cancer (1, 2). Ubiquitination, a reversible post-translational modification of proteins (3), mediates spatial and temporal aspects of endocytosis by dictating macromolecular complex assembly and cargo fate (4). Although polyubiquitination linked through Lys 48 of ubiquitin targets substrates for proteasomal degradation (5), mono-and Lys 63 -linked ubiquitination signal cargo trafficking (6), and modulate function of endocytic sorting machinery (7). The reversible nature of ubiquitination is imparted by deubiquitinating enzymes (DUBs) 4 and enables dynamic regulation of these ubiquitin-dependent processes in the cell (8).Trafficking of endocytosed cargo for degradation by the lysosome occurs in a series of discrete selection stages that utilize the endosomal sorting complexes required for transport (ESCRTs) -0, -I, -II, and -III. The ESCRT-0 complex, consisting of the adaptor proteins hepatocyte growth factor-regulated substrate (Hrs) and signal transducing adaptor molecule (STAM), resides at the sorting endosome and functions in the first step of cargo selection. ESCRT-0 partitions the endocytosed material arriving on the early endosomes between recycling and the multivesicular body (9 -11) and interacts with downstream ESCRT machinery (12, 13) respon...

“…1a). As described previously for BiFC (30), expression of each construct individually does not produce fluorescence, although expression of both halves, when paired to interacting proteins, will generate a functional fluorescent protein. Fig.…”

Section: Bimolecular Fluorescencementioning

confidence: 99%

Na+/H+ Exchanger Regulatory Factor-1 Is Involved in Chemokine Receptor Homodimer CCR5 Internalization and Signal Transduction but Does Not Affect CXCR4 Homodimer or CXCR4-CCR5 Heterodimer

Hammad

Kuang

Yan

et al. 2010

Chemokine receptors are members of the G protein-coupled receptor (GPCR) family. CCR5 is also the principal co-receptor for macrophage-tropic strains of human immunodeficiency virus, type 1 (HIV-1), and efforts have been made to develop ligands to inhibit HIV-1 infection by promoting CCR5 receptor endocytosis. Given the nature of GPCRs and their propensity to form oligomers, one can consider ligand-based therapies as unselective in terms of the oligomeric composition of complexes. For example, a ligand targeting a CCR5 homomer could likely induce signal transduction on a heteromeric CCR5-CXCR4. Other avenues could therefore be explored. We identified a receptor adaptor interacting specifically with one receptor complex but not others. NHERF1, an adaptor known for its role in desensitization, internalization, and regulation of the ERK signaling cascade for several GPCRs, interacts via its PDZ2 domain with the CCR5 homodimer but not with the CXCR4-CCR5 heterodimer or CXCR4 homodimer. To further characterize this interaction, we also show that NHERF1 increases the CCR5 recruitment of arrestin2 following stimulation. NHERF1 is also involved in CCR5 internalization, as we demonstrate that coexpression of constructs bearing the PDZ2 domain can block CCR5 internalization. We also show that NHERF1 potentiates RANTES (regulated on activation normal T cell expressed and secreted)-induced ERK1/2 phosphorylation via CCR5 activation and that this activation requires NHERF1 but not arrestin2. Taken together, our results suggest that oligomeric receptor complexes can associate specifically with partners and that in this case NHERF1 could represent an interesting new target for the regulation of CCR5 internalization and potentially HIV infection.