Growth Factor-regulated Tyrosine Kinase Research Articles

O-GlcNAcylation regulates epidermal growth factor receptor intracellular trafficking and signaling

SignificanceEpidermal growth factor receptor (EGFR) is one of the most important membrane receptors that transduce growth signals into cells to sustain cell growth, proliferation, and survival. EGFR signal termination is initiated by EGFR internalization, followed by trafficking through endosomes, and degradation in lysosomes. How this process is regulated is still poorly understood. Here, we show that hepatocyte growth factor regulated tyrosine kinase substrate (HGS), a key protein in the EGFR trafficking pathway, is dynamically modified by a single sugar N-acetylglucosamine. This modification inhibits EGFR trafficking from endosomes to lysosomes, leading to the accumulation of EGFR and prolonged signaling. This study provides an important insight into diseases with aberrant growth factor signaling, such as cancer, obesity, and diabetes.

Tyrosine kinase Syk interacts with Hrs after TLR3 stimulation in murine microglial cells.

Abstract TLR3 (Toll-like receptor 3) belongs to the family of receptors involved in innate immune response. Present in endosomal compartment in cells, TLR3 recognizes the dsRNA (double-stranded RNA), viral nucleic acid or intermediate during viral replication and initiates signal transduction leading to inflammatory response. However, the first steps of signaling cascade occurring in cells immediately after TLR3 stimulation are still not well understood and require careful attention. In this paper we demonstrate that after poly(I:C) (dsRNA mimetic) stimulation of murine microglial cells (C8D1A cell line) tyrosine kinase Syk interacts with Hrs (hepatocyte growth factor regulated tyrosine kinase substrate), one of the ESCRT-0 (endosomal sorting complex required for transportation-0) components. This protein complex was recently implicated in trafficking of other endosomal TLRs, TLR7 and TLR9, and plays a possible role in the TLR3 signaling. Phosphorylation of Hrs, which is a very likely result of Hrs-Syk interaction, also takes place after poly(I:C) stimulation, strongly suggesting that an association between activated TLR3 and ESCRT-0 exists. Therefore, Hrs may influence TLR3 signaling pathways, but this requires further investigation.

A study of cellular localization of HRS and other endosome markers during spermatogenesis in Drosophila melanogaster using chimeric GFP constructs

Acrosome is a specialized organelle in spermatozoids necessary for fertilization of oocyte. Acrosome is formed, according to various theories, either from vesicles of the Golgi apparatus or from endosomes and lysosomes. Hepatocyte growth factor regulated tyrosine kinase substrate (Hrs) is a multidomain protein component of ESCRT-0 complex, that participates in sorting of proteins absorbed during endocytosis. It has been shown that, in mammals, Hgs protein (Hrs homologue) possesses the ability to bind the acrosome, but whether Drosophila Hrs has a similar capacity has not been determined. We found that two forms of Hrs protein are expressed in Drosophila testes: a long form, Hrs-B, and a truncated form, Hrs-A. The later lacks a VHS domain and a portion of FYVE domain, both directly involved in Hrs anchoring to endosomes. We also determined that, unlike mammal Hgs, Drosophila Hrs-B isoform and an almost identical truncated form, Hrs8 ([290–760]), are localized not to acrosome, but to spermatocyte fusomes. This localization requires a portion of the protein molecule located between amino acid residues 383 and 472, which presumably correspond to NF2- and/or Stam-binding domains. In situ hybridization for hrs mRNA showed that the gene is expressed during early spermatogenesis. This is consistent with our data that the Hrs protein binds to spermatocyte fusomes and is absent during later spermatogenesis stages. In addition, we have shown that Hrs is not involved in regulation or implementation of cytokinesis in spermatocytes. Finally, despite the absence of Hrs on Drosophila acrosomes, we detected endosome markers Rab4, Rab7, and Rab11 on this organelle. Thus, our data support the endosomal hypothesis of acrosome biogenesis.

Hepatocyte growth factor regulated tyrosine kinase substrate in the peripheral development and function of B-cells

Hepatocyte growth factor (HGF)-regulated tyrosine kinase substrate (Hrs) is a vesicular sorting protein that functions as one of the endosomal-sorting proteins required for transport (ESCRT). Hrs, which binds to ubiquitinated proteins through its ubiquitin-interacting motif (UIM), contributes to the lysosomal transport and degradation of ubiquitinated membrane proteins. However, little is known about the relationship between B-cell functions and ESCRT proteins in vivo. Here we examined the immunological roles of Hrs in B-cell development and functions using B-cell-specific Hrs-deficient (Hrsflox/flox;mb1cre/+:Hrs-cKO) mice, which were generated using a cre-LoxP recombination system. Hrs deficiency in B-cells significantly reduced T-cell-dependent antibody production in vivo and impaired the proliferation of B-cells treated in vitro with an anti-IgM monoclonal antibody but not with LPS. Although early development of B-cells in the bone marrow was normal in Hrs-cKO mice, there was a significant decrease in the number of the peripheral transitional B-cells and marginal zone B-cells in the spleen of Hrs-cKO mice. These results indicate that Hrs plays important roles during peripheral development and physiological functions of B lymphocytes.

Plasticity of Polyubiquitin Recognition as Lysosomal Targeting Signals by the Endosomal Sorting Machinery

Lysosomal targeting is fundamental for the regulated disposal of ubiquitinated membrane proteins from the cell surface. To elucidate ubiquitin (Ub) configurations that are necessary and sufficient as multivesicular body (MVB)/lysosomal-sorting motifs, the intraendosomal destination and transport kinetics of model transmembrane cargo molecules bearing monoubiquitinated, multi-monoubiquitinated, or polyubiquitinated cytoplasmic tails were determined. Monomeric CD4 chimeras with K63-linked poly-Ub chains and tetrameric CD4-mono-Ub chimeras were rapidly targeted to the lysosome. In contrast, lysosomal delivery of CD4 chimeras exposing K48-linked Ub chains was delayed, whereas delivery of monoubiquitinated CD4 chimeras was undetectable. Similar difference was observed in the lysosomal targeting of mono- versus polyubiquitinated invariant chain and CD4 ubiquitinated by the MARCH (membrane-associated RING-CH) IV Ub ligase. Consistent with this, Hrs (hepatocyte growth factor regulated tyrosine kinase phosphorylated substrate), an endosomal sorting adaptor, binds preferentially to K63-Ub chain and negligibly to mono-Ub. These results highlight the plasticity of Ub as a sorting signal and its recognition by the endosomal sorting machinery, and together with previous data, suggest a regulatory role for assembly and disassembly of Ub chains of specific topology in lysosomal cargo sorting.

A Novel Sorting Sequence in the β2-Adrenergic Receptor Switches Recycling from Default to the Hrs-dependent Mechanism

Plasma membrane recycling of G protein-coupled receptors can occur by at least two distinct mechanisms as follows: a "default" mechanism that occurs nonselectively, and a specifically sorted mechanism that requires the endosome-associated protein Hrs. In this study we have defined a sequence in the beta2-adrenergic receptor cytoplasmic tail that confers Hrs dependence on receptor recycling. This sequence resembles acidic dileucine class motifs found in other membrane proteins but is structurally and functionally distinct from previously identified sorting sequences. Mutation of the novel sorting sequence rendered plasma membrane recycling independent of Hrs and independent of a distal PDZ ligand required for Hrs-dependent recycling. We propose that the novel sorting sequence functions to "switch" endocytic trafficking between mechanistically distinct recycling modes, thereby explaining failure of the wild type beta2-adrenergic receptor to recycle efficiently by default.

STAM–AMSH interaction facilitates the deubiquitination activity in the C-terminal AMSH

Signal transducing adaptor molecule (STAM) complexed with hepatocyte growth factor regulated tyrosine kinase substrate (Hrs) works on sorting of cargo proteins in multivesicular body (MVB) pathway. Associated molecule with SH3 domain of STAM (AMSH), a zinc-containing ubiquitin isopeptidase, is thought to play a role in regulation of ubiquitin-mediated degradation by binding to STAM. We have found that AMSH requires the conformation of Px(V/I)(D/N)RxxKP sequence to bind SH3 domain of STAM with ∼7 μM affinity, and that the isolated C-terminal domain of AMSH contains the isopeptidase activity. Deubiquitination by AMSH was assisted when ubiquitins were bound to STAM which can bind to AMSH simultaneously. With the specificity toward K63-linked ubiquitins, this facilitated ubiquitin processing activity of AMSH may imply a distinct regulatory mechanism for sorting and degradation through STAM binding.

The UIM domain of Hrs couples receptor sorting to vesicle formation.

Hepatocyte growth factor regulated tyrosine kinase substrate (Hrs), a main component of the 'bilayered' clathrin coat on sorting endosomes, was originally identified as a substrate of activated tyrosine kinase receptors. We have analysed Hrs phosphorylation in response to epidermal growth factor (EGF) stimulation and show that the evolutionary conserved tyrosines Y329 and Y334 provide the principal phosphorylation sites. Hrs is proposed to concentrate ubiquitinated receptors within clathrin-coated regions via direct interaction with its UIM (ubiquitin interaction motif) domain. We show that the same UIM domain is necessary for EGF-stimulated tyrosine phosphorylation of Hrs. Over-expression of wild-type Hrs or a double mutant, Y329/334F, defective in EGF-dependent phosphorylation, both substantially retard EGF receptor (EGFR) degradation by inhibiting internal vesicle formation and thereby preventing EGFR incorporation into lumenal vesicles of the multivesicular bodies. In contrast, mutation or deletion of the Hrs-UIM domain strongly suppresses this effect. In addition the UIM-deletion and point mutants are also observed on internal membranes, indicating a failure to dissociate from the endosomal membrane prior to incorporation of the receptor complex into lumenal vesicles. Our data suggest a role for the UIM-domain of Hrs in actively retaining EGFR at the limiting membrane of endosomes as a prelude to lumenal vesicle formation.

Hrs and endocytic sorting of ubiquitinated membrane proteins.

Endocytosed receptors are either recycled to the plasma membrane or trapped within intralumenal vesicles of multi-vesicular bodies for subsequent degradation in lysosomes. How the cell is able to sort receptors in endosomes has so far been largely unknown. The hepatocyte growth factor regulated tyrosine kinase substrate, Hrs, is an essential protein that has been implicated in cell signalling and intracellular membrane trafficking. Very recently, several reports have demonstrated a role for Hrs in endocytic sorting of ubiquitinated membrane proteins. Here, we review current knowledge about how Hrs recognises ubiquitinated cargo that is destined for lysosomal degradation, and how Hrs may act as a key regulator of the molecular machinery involved in receptor sorting and multivesicular body formation.