The E3 ubiquitin ligase Itch regulates sorting nexin 9 through an unconventional substrate recognition domain.

The level of intracellular proteins is mainly regulated through modifications by ubiquitin ligases that target them for degradation. Members of the NEDD4 family of E3 ubiquitin ligases, such as Itch (atrophin-1 interacting protein 4), possess up to four WW domains for specific association with PY motif-containing substrates. We have identified sorting nexin 9 (SNX9), a protein involved in endocytic processes, as a new substrate of Itch. Itch ubiquitylates SNX9 and regulates intracellular SNX9 levels. Using truncated proteins, we found that the interaction with SNX9 is mediated by the proline-rich domain (PRD) of Itch, a domain distinct from the conventional WW recognition domain, and the SH3 domain of SNX9. Interaction with the PRD of Itch is essential for SNX9 ubiquitylation and degradation. Furthermore, this effect is specific for Itch, as NEDD4, a related PRD-containing E3 ligase, does not bind SNX9. SNX18, a second member of the SNX family containing an SH3 domain, was also found to bind to Itch. Our results indicate that the pool of substrates of NEDD4 family E3 ubiquitin ligases extends beyond proteins containing PY motifs.

The SHB adapter protein is required for normal maturation of mesoderm during in vitro differentiation of embryonic stem cells.

Definitive mesoderm arises from a bipotent mesendodermal population, and to study processes controlling its development at this stage, embryonic stem (ES) cells can be employed. SHB (Src homology 2 protein in beta-cells) is an adapter protein previously found to be involved in ES cell differentiation to mesoderm. To further study the role of SHB in this context, we have established ES cell lines deficient for one (SHB+/-) or both SHB alleles (SHB-/-). Differentiating embryoid bodies (EBs) derived from these ES cell lines were used for gene expression analysis. Alternatively, EBs were stained for the blood vessel marker CD31. For hematopoietic differentiation, EBs were differentiated in methylcellulose. SHB-/- EBs exhibited delayed down-regulation of the early mesodermal marker Brachyury. Later mesodermal markers relatively specific for the hematopoietic, vascular, and cardiac lineages were expressed at lower levels on day 6 or 8 of differentiation in EBs lacking SHB. The expression of vascular endothelial growth factor receptor-2 and fibroblast growth factor receptor-1 was also reduced in SHB-/- EBs. SHB-/- EBs demonstrated impaired blood vessel formation after vascular endothelial growth factor stimulation. In addition, the SHB-/- ES cells formed fewer blood cell colonies than SHB+/+ ES cells. It is concluded that SHB is required for appropriate hematopoietic and vascular differentiation and that delayed down-regulation of Brachyury expression may play a role in this context.

Ubiquitylation of a melanosomal protein by HECT-E3 ligases serves as sorting signal for lysosomal degradation.

The production of pigment by melanocytic cells of the skin involves a series of enzymatic reactions that take place in specialized organelles called melanosomes. Melan-A/MART-1 is a melanocytic transmembrane protein with no enzymatic activity that accumulates in vesicles at the trans side of the Golgi and in melanosomes. We show here that, in melanoma cells, Melan-A associates with two homologous to E6-AP C-terminus (HECT)-E3 ubiquitin ligases, NEDD4 and Itch, and is ubiquitylated. Both NEDD4 and Itch participate in the degradation of Melan-A. A mutant Melan-A lacking ubiquitin-acceptor residues displays increased half-life and, in pigmented cells, accumulates in melanosomes. These results suggest that ubiquitylation regulates the lysosomal sorting and degradation of Melan-A/MART-1 from melanosomes in melanocytic cells.

The FRK/RAK-SHB signaling cascade: a versatile signal-transduction pathway that regulates cell survival, differentiation and proliferation.

Recent experiments have unravelled novel signal transduction pathways that involve the SRC homology 2 (SH2) domain adapter protein SHB. SHB is ubiquitously expressed and contains proline rich motifs, a phosphotyrosine binding (PTB) domain, tyrosine phosphorylation sites and an SH2 domain and serves a role in generating signaling complexes in response to tyrosine kinase activation. SHB mediates certain responses in platelet-derived growth factor (PDGF) receptor-, fibroblast growth factor (FGF) receptor-, neural growth factor (NGF) receptor TRKA-, T cell receptor-, interleukin-2 (IL-2) receptor- and focal adhesion kinase- (FAK) signaling. Upstream of SHB in some cells lies the SRC-like FYN-Related Kinase FRK/RAK (also named BSK/IYK or GTK). FRK/RAK and SHB exert similar effects when overexpressed in rat phaeochromocytoma (PC12) and beta-cells, where they both induce PC12 cell differentiation and beta-cell proliferation. Furthermore, beta-cell apoptosis is augmented by these proteins under conditions that cause beta-cell degeneration. The FRK/RAK-SHB responses involve FAK and insulin receptor substrates (IRS) -1 and -2. Besides regulating apoptosis, proliferation and differentiation, SHB is also a component of the T cell receptor (TCR) signaling response. In Jurkat T cells, SHB links several signaling components with the TCR and is thus required for IL-2 production. In endothelial cells, SHB both promotes apoptosis under conditions that are anti-angiogenic, but is also required for proper mitogenicity, spreading and tubular morphogenesis. In embryonic stem cells, dominant-negative SHB (R522K) prevents early cavitation of embryoid bodies and reduces differentiation to cells expressing albumin, amylase, insulin and glucagon, suggesting a role of SHB in development. In summary, SHB is a versatile signal transduction molecule that produces diverse biological responses in different cell types under various conditions. SHB operates downstream of GTK in cells that express this kinase.

IL-2 receptor signaling through the Shb adapter protein in T and NK cells.

Interleukin-2 induces heterodimerization of the IL-2 receptor beta and gamma subunits. This study addresses a role of the Shb adapter protein in IL-2 receptor signaling in T and NK cells. The IL-2Rbeta and gamma chains were found to co-immunoprecipitate with Shb, when each alone was co-expressed with Shb in COS cells. Using fusion proteins, the Shb SH2 domain was found to associate in a phosphotyrosine-dependent manner with the IL-2 receptor beta and gamma subunits upon IL-2 stimulation in primary T cells and the NK cell line NK-92. The main binding site of the Shb SH2 domain was phosphorylated Tyr-510 in the IL-2Rbeta chain. Shb was also phosphorylated upon IL-2 stimulation when overexpressed together with IL-2Rbeta (in pre-B cells, which express the gamma chain constitutively). These cells were also less apoptotic in the presence of IL-2 than cells overexpressing a mutant Shb (with a defect SH2 domain) or cells expressing a mutant IL-2Rbeta, with the Shb binding sites mutated to phenylalanine (Y392F, Y510F). JAK1 and JAK3 were also found to associate with Shb, but in contrast to the Shb-IL-2 receptor association, JAK1 and 3 appear to associate with the proline-rich regions of Shb. In conclusion, Shb links the IL-2 receptor to other signaling proteins and mediates the regulation of apoptosis in the presence of IL-2.

Shb links SLP-76 and Vav with the CD3 complex in Jurkat T cells.

This study addresses the interactions between the adaptor protein Shb and components involved in T cell signalling, including SLP-76, Gads, Vav and ZAP70. We show that both SLP-76 and ZAP70 co-immunoprecipitate with Shb in Jurkat T cells and that Shb and Vav co-immunoprecipitate when cotransfected in COS cells. We also demonstrate, utilizing fusion protein constructs, that SLP-76, Gads and Vav associate independently of each other to different domains or regions, of Shb. Overexpression of an SH2 domain-defective Shb causes diminished phosphorylation of SLP-76 and Vav and consequently decreased activation of c-Jun kinase upon T cell receptor (TCR) stimulation. Shb was also found to localize to glycolipid-enriched membrane microdomains (GEMs), also called lipid rafts, after TCR stimulation. Our results indicate that upon TCR stimulation, Shb is targeted to these lipid rafts where Shb aids in recruiting the SLP-76-Gads-Vav complex to the T cell receptor zeta-chain and ZAP70.