Site‐Specific Ubiquitination Determines Lysosomal Sorting and Signal Attenuation of the Granulocyte Colony‐Stimulating Factor Receptor

Abstract

Ubiquitination of cytokine receptors controls intracellular receptor routing and signal duration, but the underlying molecular determinants are unclear. The suppressor of cytokine signaling protein SOCS3 drives lysosomal degradation of the granulocyte colony-stimulating factor receptor (G-CSFR), depending on SOCS3-mediated ubiquitination of a specific lysine located in a conserved juxtamembrane motif. Here, we show that, despite ubiquitination of other lysines, positioning of a lysine within the membrane-proximal region is indispensable for this process. Neither reallocation of the motif nor fusion of ubiquitin to the C-terminus of the G-CSFR could drive lysosomal routing. However, within this region, the lysine could be shifted 12 amino acids toward the Cterminus without losing its function, arguing against the existence of a linear sorting motif and demonstrating that positioning of the lysine relative to the SOCS3 docking site is flexible. G-CSFR ubiquitination peaked after endocytosis, was inhibited by methyl-β-cyclodextrin as well as hyperosmotic sucrose and severely reduced in internalization-defective G-CSFR mutants, indicating that ubiquitination mainly occurs at endosomes. Apart from elucidating structural and spatio-temporal aspects of SOCS3-mediated ubiquitination, these findings have implications for the abnormal signaling function of G-CSFR mutants found in severe congenital neutropenia, a hematopoietic disorder with a high leukemia risk.

Peroxiredoxin-controlled G-CSF signalling at the endoplasmic reticulum–early endosome interface

“…S3). By contrast, colocalization of the lysosomal routing defective G-CSFR mutant K5R persisted at 60 minutes after G-CSF stimulation, confirming that Prdx4 interacts with G-CSFR localized in early endosomes Wölfler et al, 2009) (supplementary material Fig. S4a).…”

“…Intriguingly, the G-CSFR truncation mutants found in SCN and AML respond differentially to suppressor of cytokine signaling 3 (SOCS3) in terms of STAT3 versus STAT5 inhibition: SOCS3-mediated inhibition of STAT5 activation is abolished as a result of the G-CSFR truncation, whereas inhibition of STAT3 remained largely intact (van de Geijn et al, 2004). This discrepancy relates to the fact that SOCS3-induced STAT5 inhibition entirely depends on SOCS box-mediated ubiquitylation of G-CSFR, whereas SOCS3-induced STAT3 inhibition is less dependent on this process van de Geijn et al, 2004;Wölfler et al, 2009). Together with these previous results our current data suggest that, whereas attenuation of STAT5 mainly depends on lysosomal degradation of the activated G-CSFR, STAT3 inhibition is mediated mainly by dephosphorylation of STAT3-binding tyrosine motifs controlled by the kinase inhibitory region of SOCS3 and by Ptp1b, when the G-CSFR resides in the early endosome.…”

“…A major mechanism implicated in the perturbed signalling functions of these truncated G-CSFR mutants is the loss of appropriate receptor endocytosis and lysosomal routing. These processes are controlled by a dileucine-based internalization motif (Hunter and Avalos, 1999;Ward et al, 1999) and by receptor ubiquitylation involving the suppressor of cytokine signalling protein SOCS3 Hermans et al, 2003;Hortner et al, 2002;Irandoust et al, 2007;Wölfler et al, 2009).…”

The SOCS box—Adapting proteins for ubiquitination and proteasomal degradation

“…For instance, ES cells expressing endogenous levels of SOCS3 lacking the SOCS box express elevated levels of phosphorylated Jak1 in response to LIF (46). In contrast, a series of studies from Irandoust and coworkers demonstrated that SOCS3 ubiquitinates lysine residues on the G-CSF receptor cytoplasmic domain, facilitating its trafficking to the lysosome (47,48). Thus, while SOCS3 potentially regulates G-CSF and gp130 receptor signalling through the combination of its KIR and SH2 domain, the precise ubiquitination of the different proteins in these receptor complexes may provide an additional level of regulation.…”

“…It requires a three-step enzymatic cascade that transfers the small ubiquitin molecule from an E1-activating enzyme to an E2-conjugating enzyme, followed by the covalent attachment of ubiquitin to the e-amino group of a target lysine residue by an E3 ubiquitin ligase. Ubiquitin itself can be ubiquitinated on lysine, and at least eight distinct modifications have now been identified that include monoubiquitination, multimonoubiquitination and linear and branched polyubiquitination on one or more of the seven lysines found in ubiquitin (lysine 48 and lysine 63-linked polyubiquitination are the best characterised) (1). Increasingly, deregulation of the ubiquitination pathway components, including negative regulators such as deubiquitinating enzymes, is seen as having an important role in the aetiology of human diseases including cancer, autoimmunity and other chronic inflammatory diseases (2)(3)(4)(5).…”