WIP1 is a novel specific target for growth hormone action

DNA damage repair (DDR) is mediated by phosphorylating effectors ATM kinase, CHK2, p53, and γH2AX. We showed earlier that GH suppresses DDR by suppressing pATM, resulting in DNA damage accumulation. Here, we show GH acting through GH receptor (GHR) inducing wild-type p53-inducible phosphatase 1 (WIP1), which dephosphorylated ATM and its effectors in normal human colon cells and three-dimensional human intestinal organoids. Mice bearing GH-secreting xenografts exhibited induced colon WIP1 with suppressed pATM and γH2AX. WIP1 was also induced in buffy coats derived from patients with elevated GH from somatotroph adenomas. In contrast, decreased colon WIP1 was observed in GHR-/- mice. WIP1 inhibition restored ATM phosphorylation and reversed GH-induced DNA damage. We elucidated a novel GH signaling pathway activating Src/AMPK to trigger HIPK2 nuclear-cytoplasmic relocation and suppressing WIP1 ubiquitination. Concordantly, blocking either AMPK or Src abolished GH-induced WIP1. We identify WIP1 as a specific target for GH-mediated epithelial DNA damage accumulation.

A novel antifolate suppresses growth of FPGS-deficient cells and overcomes methotrexate resistance

Cancer cells make extensive use of the folate cycle to sustain increased anabolic metabolism. Multiple chemotherapeutic drugs interfere with the folate cycle, including methotrexate and 5-fluorouracil that are commonly applied for the treatment of leukemia and colorectal cancer (CRC), respectively. Despite high success rates, therapy-induced resistance causes relapse at later disease stages. Depletion of folylpolyglutamate synthetase (FPGS), which normally promotes intracellular accumulation and activity of natural folates and methotrexate, is linked to methotrexate and 5-fluorouracil resistance and its association with relapse illustrates the need for improved intervention strategies. Here, we describe a novel antifolate (C1) that, like methotrexate, potently inhibits dihydrofolate reductase and downstream one-carbon metabolism. Contrary to methotrexate, C1 displays optimal efficacy in FPGS-deficient contexts, due to decreased competition with intracellular folates for interaction with dihydrofolate reductase. We show that FPGS-deficient patient-derived CRC organoids display enhanced sensitivity to C1, whereas FPGS-high CRC organoids are more sensitive to methotrexate. Our results argue that polyglutamylation-independent antifolates can be applied to exert selective pressure on FPGS-deficient cells during chemotherapy, using a vulnerability created by polyglutamylation deficiency.

Small molecules to regulate the GH/IGF1 axis by inhibiting the growth hormone receptor synthesis

Growth hormone (GH) and insulin-like growth factor-1 (IGF1) play an important role in mammalian development, cell proliferation and lifespan. Especially in cases of tumor growth there is an urgent need to control the GH/IGF1 axis. In this study we screened a 38,480-compound library, and in two consecutive rounds of analogues selection, we identified active lead compounds based on the following criteria: inhibition the GH receptor (GHR) activity and its downstream effectors Jak2 and STAT5, and inhibition of growth of breast and colon cancer cells. The most active small molecule (BM001) inhibited both the GH/IGF1 axis and cell proliferation with an IC50 of 10-30 nM of human cancer cells. BM001 depleted GHR in human lymphoblasts. In preclinical xenografted experiments, BM001 showed a strong decrease in tumor volume in mice transplanted with MDA-MB-231 breast cancer cells. Mechanistically, the drug acts on the synthesis of the GHR. Our findings open the possibility to inhibit the GH/IGF1 axis with a small molecule.

Growth Hormone Receptor Regulation in Cancer and Chronic Diseases

The GHR signaling pathway plays important roles in growth, metabolism, cell cycle control, immunity, homeostatic processes, and chemoresistance via both the JAK/STAT and the SRC pathways. Dysregulation of GHR signaling is associated with various diseases and chronic conditions such as acromegaly, cancer, aging, metabolic disease, fibroses, inflammation and autoimmunity. Numerous studies entailing the GHR signaling pathway have been conducted for various cancers. Diverse factors mediate the up- or down-regulation of GHR signaling through post-translational modifications. Of the numerous modifications, ubiquitination and deubiquitination are prominent events. Ubiquitination by E3 ligase attaches ubiquitins to target proteins and induces proteasomal degradation or starts the sequence of events that leads to endocytosis and lysosomal degradation. In this review, we discuss the role of first line effectors that act directly on the GHR at the cell surface including ADAM17, JAK2, SRC family member Lyn, Ubc13/CHIP, proteasome, βTrCP, CK2, STAT5b, and SOCS2. Activity of all, except JAK2, Lyn and STAT5b, counteract GHR signaling. Loss of their function increases the GH-induced signaling in favor of aging and certain chronic diseases, exemplified by increased lung cancer risk in case of a mutation in the SOCS2-GHR interaction site. Insight in their roles in GHR signaling can be applied for cancer and other therapeutic strategies.

Fos-Zippered GH Receptor Cytosolic Tails Act as Jak2 Substrates and Signal Transducers

Members of the Janus kinase (Jak) family initiate the majority of downstream signaling events of the cytokine receptor family. The prevailing principle is that the receptors act in dimers: 2 Jak2 molecules bind to the cytosolic tails of a cytokine receptor family member and initiate Jak-signal transducer and activator of transcription signaling upon a conformational change in the receptor complex, induced by the cognate cytokine. Due to the complexity of signaling complexes, there is a strong need for in vitro model systems. To investigate the molecular details of the Jak2 interaction with the GH receptor (GHR), we used cytosolic tails provided with leucine zippers derived from c-Fos to mimic the dimerized state of GHR. Expressed together with Jak2, fos-zippered tails, but not unzippered tails, were stabilized. In addition, the Jak-signal transducer and activator of transcription signaling pathway was activated by the fos-zippered tails. The stabilization depended also on α-helix rotation of the zippers. Fos-zippered GHR tails and Jak2, both purified from baculovirus-infected insect cells, interacted via box1 with a binding affinity of approximately 40nM. As expected, the Jak kinase inhibitor Ruxolitinib inhibited the stabilization but did not affect the c-Fos-zippered GHR tail-Jak2 interaction. Analysis by blue-native gel electrophoresis revealed high molecular-weight complexes containing both Jak2 and nonphosphorylated GHR tails, whereas Jak2-dissociated tails were highly phosphorylated and monomeric, implying that Jak2 detaches from its substrate upon phosphorylation.

Automation of a Phospho-STAT5 Staining Procedure for Flow Cytometry for Application in Drug Discovery

Drug discovery often requires the screening of compound libraries on tissue cultured cells. Some major targets in drug discovery belong to signal transduction pathways, and PerFix EXPOSE* allows easy flow cytometry phospho assays. We thus investigated the possibility to further simplify and automate this assay, to allow the direct screening of drugs targeting signaling pathways. We show here the sensitivity of this fully automated assay on human growth hormone (hGH)-driven JAK/STAT5-activated IM-9 cells, and we discuss the throughput of this system, which is compatible with medium-throughput drug screening. Because the kit works directly on whole blood samples, ex-vivo assays are also possible with this approach, which could allow for the screening of drugs under more physiological conditions.

Multimeric growth hormone receptor complexes serve as signaling platforms

Growth hormone (GH) signaling is required for promoting longitudinal body growth, stem cell activation, differentiation, and survival and for regulation of metabolism. Failure to adequately regulate GH signaling leads to disease: excessive GH signaling has been connected to cancer, and GH insensitivity has been reported in cachexia patients. Since its discovery in 1989, the receptor has served a pivotal role as the prototype cytokine receptor both structurally and functionally. Phosphorylation and ubiquitylation regulate the GH receptor (GHR) at the cell surface: two ubiquitin ligases (SCF(βTrCP2) and CHIP) determine the GH responsiveness of cells by controlling its endocytosis, whereas JAK2 initiates the JAK/STAT pathway. We used blue native electrophoresis to identify phosphorylated and ubiquitylated receptor intermediates. We show that GHRs occur as ∼500-kDa complexes that dimerize into active ∼900-kDa complexes upon GH binding. The dimerized complexes act as platforms for transient interaction with JAK2 and ubiquitin ligases. If GH and receptors are made in the same cell (autocrine mode), only limited numbers of ∼900-kDa complexes are formed. The experiments reveal the dynamic changes in post-translational modifications during GH-induced signaling events and show that relatively simple cytokine receptors like GHRs are able to form higher order protein complexes. Insight in the complex formation of cytokine receptors is crucially important for engineering cytokines that control ligand-induced cell responses and for generating a new class of therapeutic agents for a wide range of diseases.

The Jak/STAT signaling pathway is downregulated at febrile temperatures

BACKGROUND

The Janus family of kinases (JAKs), Jak1, Jak2, Jak3, and Tyk2, constitute a subgroup of non-receptor protein tyrosine kinases. Upon cytokine binding, the receptor-associated kinases are activated and phosphorylate tyrosine residues in their cognate cytokine receptors. Their activities are controlled at several levels and include cellular concentration, auto-activation, and degradation.

PRINCIPAL FINDINGS

Our findings show that elevated temperatures in the fever range irreversibly aggregate Jak2 and considerably reduce functional Jak2 protein levels. Jak2 synthesis remains unaltered. We observed that also the protein level of the signal transducer and activator of transcription, STAT5b, is transiently decreased at temperatures above 37°C. Consequently, the signaling response, e.g. via the growth hormone receptor, is reduced.

CONCLUSIONS/SIGNIFICANCE

These findings predict that elevated body temperatures lower the responsiveness of cytokine receptors.

Identification of the ubiquitin ligase Triad1 as a regulator of endosomal transport

The ubiquitin system plays an important role in trafficking of signaling receptors from the plasma membrane to lysosomes. Triad1 is a ubiquitin ligase that catalyzes the formation of poly-ubiquitin chains linked via lysine-48 as well as lysine-63 residues. We show that depletion of Triad1 affects the sorting of both growth hormone and epidermal growth factor. Triad1-depleted cells accumulate both ligands in endosomes. While fluid phase transport to the lysosomes is reduced in the absence of Triad1, growth hormone receptor can recycle back to the plasma membrane together with transferrin. Using immune electron microscopy we show that Triad1 depletion results in enlarged endosomes with enlarged and irregular shaped intraluminal vesicles. The endosomes display prominent clathrin coats and show increased levels of growth hormone label. We conclude that Triad1 is required for the proper function of multivesicular bodies.

Ubc13 and COOH terminus of Hsp70-interacting protein (CHIP) are required for growth hormone receptor endocytosis

Growth hormone receptor (GHR) endocytosis is a highly regulated process that depends on the binding and activity of the multimeric ubiquitin ligase, SCF(βTrCP) (Skp Cullin F-box). Despite a specific interaction between β-transducin repeat-containing protein (βTrCP) and the GHR, and a strict requirement for ubiquitination activity, the receptor is not an obligatory target for SCF(βTrCP)-directed Lys(48) polyubiquitination. We now show that also Lys(63)-linked ubiquitin chain formation is required for GHR endocytosis. We identified both the ubiquitin-conjugating enzyme Ubc13 and the ubiquitin ligase COOH terminus of Hsp70 interacting protein (CHIP) as being connected to this process. Ubc13 activity and its interaction with CHIP precede endocytosis of GHR. In addition to βTrCP, CHIP interacts specifically with the cytosolic tails of the dimeric GHR, identifying both Ubc13 and CHIP as novel factors in the regulation of cell surface availability of GHR.

βTrCP controls GH receptor degradation via two different motifs

The physiological roles of GH are broad and include metabolism regulation and promotion of somatic growth. Therefore, the responsiveness of cells to GH must be tightly regulated. This is mainly achieved by a complex and well-controlled mechanism of GH receptor (GHR) endocytosis. GHR endocytosis occurs independently of GH and requires the ubiquitin ligase, SCF (βTrCP) that is recruited to the ubiquitin-dependent endocytosis (UbE) motif in the cytoplasmic tail of the GHR. In this study we report that, in addition to the UbE motif, a downstream degron, DSGRTS, binds to βTrCP. The WD40 residues on βTrCP involved in the interaction with this sequence are identical to the ones necessary for binding the classical motif, DSGxxS, in inhibitor of NFκB signalling, and β-catenin. Previously, we showed that this motif is not involved in GH-induced endocytosis. We show here that the DSGRTS sequence significantly contributes to GHR endocytosis/degradation in basal conditions, whereas the UbE motif is involved both in basal and GH-induced conditions. These findings explain the high rate of GHR degradation under basal conditions, which is important for regulating the responsiveness of cells to GH.

Jak2 is a negative regulator of ubiquitin-dependent endocytosis of the growth hormone receptor

Background

Length and intensity of signal transduction via cytokine receptors is precisely regulated. Degradation of certain cytokine receptors is mediated by the ubiquitin ligase SCF(βTrCP). In several instances, Janus kinase (Jak) family members can stabilise their cognate cytokine receptors at the cell surface.

Principal Findings

In this study we show in Hek293 cells that Jak2 binding to the growth hormone receptor prevents endocytosis in a non-catalytic manner. Following receptor activation, the detachment of phosphorylated Jak2 induces down-regulation of the growth hormone receptor by SCF(βTrCP). Using γ2A human fibroblast cells we show that both growth hormone-induced and constitutive growth hormone receptor endocytosis depend on the same factors, strongly suggesting that the modes of endocytosis are identical. Different Jak2 RNA levels in HepG2, IM9 and Hek293 cells indicate the importance of cellular concentration on growth hormone receptor function. Both Jak2 and βTrCP bind to neighbouring linear motifs in the growth hormone receptor tail without the requirement of modifications, indicating that growth hormone sensitivity is regulated by the cellular level of non-committed Jak2.

Conclusions/Significance

As signal transduction of many cytokine receptors depends on Jak2, the study suggests an integrative role of Jak2 in cytokine responses based on its enzyme activity as well as its stabilising properties towards the receptors.

Site of synthesis, intracellular transport and secretion of glycoprotein in exocrine cells

The site of attachment of the first sugar, N-acetylgalactosamine, to the seryl and threonyl residues of the protein chain is unknown in exocrine cells. The subsequent sugars of the carbohydrate side chains, galactose and N-acetylglucosamine alternately, and the end-group sugars, galactose, N-acetylgalactosamine and fucose, are attached in the Golgi complex. Sulphate too is attached in that structure. In the stomach, sulphate is probably transferred in the most mature cisterna of the Golgi stacks, galactose and fucose in other cisternae, suggesting a gradient in transferase activities along the stack. The possibilities of regulating the amount and relative sugar composition of the glycoproteins are discussed. The secretory product is stored in granules. Their polygonal, large and swollen appearance and complex formation by loss of bordering membranes, as observed in many kinds of glycoprotein-secreting cells ('mucous cells') might be caused by ineffective fixation techniques. Direct vascular perfusion results in a picture no different from what is found in non-mucous cells. Whether secretion is merely exocytotic, as in non-mucous cells, or whether it is accompanied by a loss of membrane and even cytoplasm needs thorough investigation, with the effects of various fixation techniques being compared.

Autocrine growth hormone: effects on growth hormone receptor trafficking and signaling

GH and GH receptor are expressed in many extrapituitary tissues, permitting autocrine/paracrine activity. Autocrine GH has regulatory functions in embryonic development and cellular differentiation and proliferation and is reported to be involved in the development and metastasis of tumor cells. To understand the principles of transport and signaling of autocrine GH and GH receptor, we used a model system to express both proteins in the same cell. Our experiments show that GH binds the GH receptor immediately after synthesis in the endoplasmic reticulum and facilitates maturation of GH receptor. The hormone-receptor complexes arrive at the cell surface where exogenously added GH is unable to bind these receptors. Autocrine GH activates the GH receptors, but signal transduction occurs only after exiting the endoplasmic reticulum. This model study explains why autocrine GH-producing cells may be insensitive for GH (antagonist) treatment and clarifies autocrine signaling events.

The ubiquitin ligase SCF(betaTrCP) regulates the degradation of the growth hormone receptor

SCF ubiquitin ligases play a pivotal role in the regulation of cell division and various signal transduction pathways, which in turn are involved in cell growth, survival, and transformation. SCF(TrCP) recognizes the double phosphorylated DSGPhiXS destruction motif in beta-catenin and IkappaB. We show that the same ligase drives endocytosis and degradation of the growth hormone receptor (GHR) in a ligand-independent fashion. The F-box protein beta-TrCP binds directly and specifically with its WD40 domain to a novel recognition motif, previously designated as the ubiquitin-dependent endocytosis motif. Receptor degradation requires an active neddylation system, implicating ubiquitin ligase activity. GHR-TrCP binding, but not GHR ubiquitination, is necessary for endocytosis. TrCP2 silencing is more effective on GHR degradation and endocytosis than TrCP1, although overexpression of either isoform restores TrCP function in silenced cells. Together, these findings provide direct evidence for a key role of the SCF(TrCP) in the endocytosis and degradation of an important factor in growth, immunity, and life span regulation.

Disulfide bonds determine growth hormone receptor folding, dimerisation and ligand binding

The growth hormone receptor contains seven cysteine residues in its extracellular domain. The six in the growth hormone binding domain form disulfide bonds, and help the receptor to gain its correct three-dimensional structure. In this study we replaced the cysteine for serine and alanine residues and investigated their role in growth hormone receptor folding, dimerisation and signal transduction. Folding and growth hormone binding capacity of the wild-type growth hormone receptor require less than two minutes for completion. Although less efficient, all mutant receptors arrive at the cell surface as pre-formed dimers. Disulfide bond C38-C48 is important for efficient maturation. The middle disulfide-bond, C83-C94, is important for ligand binding. Removing disulfide bond C108-C122 has little effect without affecting signalling. When two or all disulfide bonds are changed, ligand binding and activation are blocked. Dimerisation is delayed when all disulfide bonds are destroyed.

Hypoxia-mediated degradation of Na,K-ATPase via mitochondrial reactive oxygen species and the ubiquitin-conjugating system

We set out to determine whether cellular hypoxia, via mitochondrial reactive oxygen species, promotes Na,K-ATPase degradation via the ubiquitin-conjugating system. Cells exposed to 1.5% O2 had a decrease in Na,K-ATPase activity and oxygen consumption. The total cell pool of alpha1 Na,K-ATPase protein decreased on exposure to 1.5% O2 for 30 hours, whereas the plasma membrane Na,K-ATPase was 50% degraded after 2 hours of hypoxia, which was prevented by lysosome and proteasome inhibitors. When Chinese hamster ovary cells that exhibit a temperature-sensitive defect in E1 ubiquitin conjugation enzyme were incubated at 40 degrees C and 1.5% O2, the degradation of the alpha1 Na,K-ATPase was prevented. Exogenous reactive oxygen species increased the plasma membrane Na,K-ATPase degradation, whereas, in mitochondrial DNA deficient rho(0) cells and in cells transfected with small interfering RNA against Rieske iron sulfur protein, the hypoxia-mediated Na,K-ATPase degradation was prevented. The catalase/superoxide dismutase (SOD) mimetic (EUK-134) and glutathione peroxidase overexpression prevented the hypoxia-mediated Na,K-ATPase degradation and overexpression of SOD1, but not SOD2, partially inhibited the Na+ pump degradation. Accordingly, we provide evidence that during hypoxia, mitochondrial reactive oxygen species are necessary to degrade the plasma membrane Na,K-ATPase via the ubiquitin-conjugating system.

Sorting nexin 17 facilitates LRP recycling in the early endosome

The low-density lipoprotein (LDL) receptor-related protein (LRP) is a multiligand endocytic receptor and a member of the LDL receptor family. Here we show that sorting nexin 17 (Snx 17) is part of the cellular sorting machinery that regulates cell surface levels of LRP by promoting its recycling. While the phox (PX) domain of Snx 17 interacts with phosphatidylinositol-3-phosphate for membrane association, the FERM domain and the carboxyl-terminal region participate in LRP binding. Immunoelectron microscopy shows that the membrane-bound fraction of Snx 17 is localized to the limiting membrane and recycling tubules of early endosomes. The NPxY motif, proximal to the plasma membrane in the LRP cytoplasmic tail, is identified as the Snx 17-binding motif. Functional mutation of this motif did not interfere with LRP endocytosis, but decreased LRP recycling from endosomes, resulting in increased lysosomal degradation. Similar effects are found after knockdown of endogenous Snx 17 expression by short interfering RNA. We conclude that Snx 17 binds to a motif in the LRP tail distinct from the endocytosis signals and promotes LRP sorting to the recycling pathway in the early endosomes.

ATPase-deficient hVPS4 impairs formation of internal endosomal vesicles and stabilizes bilayered clathrin coats on endosomal vacuoles

Epidermal growth factor receptors (EGFRs) destined for lysosomal degradation are sorted in the early endosomal vacuole into small, lumenal vesicles that arise by inward budding of the limiting membrane. We have previously shown that, before their incorporation into internal vesicles, EGFRs are concentrated in flat bilayered-clathrin coats on the endosomal vacuole. Here, we show that an ATPase-deficient mutant of hVPS4 (hVPS4(EQ)) increases the association of bilayered coats with endosomal vacuoles. In addition, hVPS4(EQ) leads to a reduction in the number of internal vesicles in early and late endosomal vacuoles, and retention of EGFRs at the limiting membrane. Interestingly, hVPS4(EQ) was predominantly found on non-coated regions of endosomal vacuoles, often at the rim of a coated area. In line with published data on Vps4p function in yeast, these results suggest that hVPS4 is involved in the release of components of the bilayered coat from the endosomal membrane. Moreover, our data suggest that disassembly of the coat is required for the formation of internal vesicles.

The growth hormone receptor interacts with its sheddase, the tumour necrosis factor-alpha-converting enzyme (TACE)

Proteolysis of the GHR (growth hormone receptor) occurs at the cell surface and results in the release of its extracellular domain, the GHBP (growth hormone-binding protein). TACE (tumour necrosis factor-alpha-converting enzyme) has been identified as a putative protease responsible for GHR cleavage. However, GHR-TACE interaction has not been observed until now. Here, we identified TACE in Chinese hamster cells and confirmed processing and cell-surface expression. Interaction between GHR and TACE was only observed after growth hormone binding. As the growth hormone-GHR(2) complex is a poor substrate for TACE, we conclude that the GHR-TACE interaction precedes proteolysis, and is transient. Furthermore, we demonstrate that TACE is present in endosomes, where it partly co-localizes with endocytosed growth hormone ligand.

Ubiquitin system-dependent regulation of growth hormone receptor signal transduction

The growth hormone (GH) receptor is a key regulator of cellular metabolism. Unlike most growth factor receptors, its downregulation is not initiated by its ligand. Like many growth factor receptors, specific molecular mechanisms guarantee that a receptor can signal only once in its lifetime. Three features render the GH receptor unique: (a) an active ubiquitination system is required for both uptake (endocytosis) and degradation in the lysosomes; (b) uptake of the receptor is a continuous process, independent of both GH binding and Jak2 signal transduction; (c) only the cell surface expression of dimerised GH receptors is controlled by the ubiquitin system. This system enables two independent regulatory mechanisms for the endocrinology of the GH/GHR axis: the pulsatile secretion of GH by the pituitary and the GH sensitivity of individual cells of the body by the effects of the ubiquitin system on GH receptor availability.

Small glutamine-rich tetratricopeptide repeat-containing protein (SGT) interacts with the ubiquitin-dependent endocytosis (UbE) motif of the growth hormone receptor

Endocytosis of the growth hormone receptor (GHR) is regulated by the ubiquitin-conjugating system. A cytosolic 10 amino acid motif, referred to as the ubiquitin-dependent endocytosis (UbE) motif, is involved in the ubiquitination as well as in the endocytosis of the receptor. Proteins that are implicated in one of these processes have not been identified so far. Using a glutathione S-transferase (GST)-pulldown assay with a GST fusion protein encompassing the UbE motif of the GHR, a 35 kDa protein was purified. The protein was identified by MS as small glutamine-rich tetratricopeptide repeat (TPR)-containing protein (SGT). We found that GHR interacts with SGT. In vivo, both the precursor and the mature form of the receptor interacted with SGT. Inactivation of the ubiquitin-conjugating system did not affect the GHR-SGT interaction. Binding studies showed that the first TPR motif of SGT interacts with the UbE motif of the GHR. Taken together, these data show that SGT is a GHR-interacting protein, which binds independent of the ubiquitin-conjugating system.

Dimerization and signal transduction of the growth hormone receptor

GH binding to cell surface-localized GH receptors (GHRs) induces a conformational change of the dimerized receptors, resulting in activation of Janus kinase 2 and downstream signaling pathways. Interactions between the extracellular subdomain 2 of adjacent GHR polypeptides result in a 500-A2 contact interface, which has previously been suggested to stabilize the GH-(GHR)2 complex. In this study, we investigated further the role of subdomain 2 in GHR function. Amino acids that participate in (e.g. aspartic acid 152, tyrosine 200, or serine 201) or lie close to (e.g. asparagine 143 or cysteine 241) the contact interface were mutated in rabbit GHR. Surprisingly, none of the mutations affected GHR dimerization, as demonstrated by coimmunoprecipitation of a truncated, epitope-tagged GHR. However, signal transduction of GHR(D152H), GHR(Y200D), and GHR(S201K) mutants was precluded. More insight into the molecular mechanism of the signaling defect was obtained when we examined the effect of the mutations on the integrity of the GH-(GHR)2 complex in a protease-protection assay. In contrast to wild-type GHR, GHR(N143K), and GHR(C241S), the GHR(D152H), GHR(Y200D), and GHR(S201K) mutants were not protected against protease digestion by GH, indicating that a structural change is prevented. Together, we provide new evidence for a critical role of aspartic acid 152, tyrosine 200, and serine 201 of the GHR contact interface in the GH-induced conformational change to a signaling-competent complex rather than in GHR dimerization.

A method to increase the number of growth hormone receptors at the surface of cells

The number of growth hormone receptors (GHRs) per cell are regulated and this feature plays a major role in the hormone responsiveness of the body. We previously observed in transfected Chinese hamster lung cells that GHR availability is determined by three factors: endocytosis (75%), shedding (10%), and other undetermined mechanisms (15%). The endocytosis depends on an active ubiquitin conjugation system. In addition, this process is ligand-independent. Here, we show that this principle is useful to increase the abundance of GHRs at the cell surface of cells using a combination of inhibitors. In theory, an inhibitor that targets the ubiquitin conjugation specific for the GHR, would suffice, as almost 80% of the removal rate depends on this mechanism. As the molecular mechanism is unknown yet, we used a general inhibitor of proteasome action. Unfortunately, such an inhibitor stimulates the shedding process severalfold. Our data show that the combination of a general proteasome inhibitor and a matrix metalloprotease inhibitor results in an almost twofold increase in functional GHRs at the cell surface, and generate new perspectives to increase the sensitivity of cells for growth hormone.

The ubiquitin-proteasome pathway and the regulation of growth hormone receptor availability

The number of growth hormone receptors (GHR) per cell are regulated and this feature plays a major role in the hormone responsiveness of the body. This article deals with the regulatory mechanisms underlying the availability of GHR for serum growth hormone. The availability of membrane proteins at the cell surface can be regulated at different locations within the cell: (1) The amount of protein synthesized in the endoplasmic reticulum (ER) is largely controlled by gene transcription. In addition, the ER quality control system regulates the exiting of properly folded proteins from the ER. (2) In the trans-Golgi network, proteins can either be diverted directly to the lysosomes or be transported to the cell surface. (3) At the plasma membrane, the endocytic machinery can select proteins for endocytosis via clathrin-coated pits or proteins may be subject to proteolysis, resulting in shedding of the extracellular domain. (4) In endosomes, internalized proteins are either recycled back to the plasma membrane or targeted to the lysosome for degradation. At each of these cellular locations the ubiquitin-proteasome pathway can specifically regulate protein levels via different mechanisms. In transfected Chinese hamster lung cells, GHR availability is determined by three factors: endocytosis (75%), shedding (10%), and other undetermined mechanisms (15%). As outlined in this article the level of GHR at the cell surface, defined as GHR availability, is mainly regulated by the ubiquitin-proteasome pathway.

Multiple internalization motifs differentially used by prolactin receptor isoforms mediate similar endocytic pathways

Prolactin (PRL) regulates a variety of physiological processes, including mammary gland growth and differentiation, modulation of behavior, and immune function. A long PRL receptor (lPRLR) and short (sPRLR) isoform were identified in ruminants and rodents, which differ in their distal cytoplasmic domains and possess markedly distinct signaling capacities. Here we compared endocytosis of the bovine isoforms and found that the lPRLR internalized faster than the sPRLR, which would contribute to short-term down-regulation of lPRLR signaling at targets expressing both isoforms. Multiple motifs were required to mediate internalization of the lPRLR, including a phenylalanine (F290) plus a nearby dileucine, and three dileucines proximal to amino acid 272. This is different from the closely related GH receptor that requires only the phenyl-alanine-containing motif for endocytosis. Truncated lPRLR (cT272), which is the same length as the sPRLR and contained the proximal three dileucines, internalized at the same rate as the full-length lPRLR. Finally, the two dileucines shared by the sPRLR were able to mediate similar endocytic pathways as the lPRLR, as revealed by overexpression of mutant dynamin and clathrin hub, despite the slower rate. These studies define the basis of cellular trafficking of PRLR isoforms and increase our understanding of control of target cell responsiveness by PRL.

Dimerization, ubiquitylation and endocytosis go together in growth hormone receptor function

Internalization of membrane proteins has been studied for more than three decades without solving all the underlying mechanisms. Our knowledge of the clathrin-coated endocytosis is sufficient to understand the basic principles. However, more detailed insight is required to recognize why different proteins enter clathrin-coated pits with different rates and affinities. In addition to clathrin coat components, several adapter systems and even more accessory proteins have been described to preselect membrane proteins before they can enter cells. Recent experimental data have identified the ubiquitin-proteasome system as a regulatory system both in endocytic and lysosomal membrane traffic. This system is well-known for its basic regulatory function in protein degradation, and controls a magnitude of key events. In this review, we will discuss the complexity and implications of this mechanism for membrane trafficking with emphasis on the growth hormone receptor.

Proteasome regulates the delivery of LDL receptor-related protein into the degradation pathway

The low-density lipoprotein receptor (LDLR)-related protein (LRP) is a multiligand endocytic receptor that has broad cellular and physiological functions. Previous studies have shown that both tyrosine-based and di-leucine motifs within the LRP cytoplasmic tail are responsible for mediating its rapid endocytosis. Little is known, however, about the mechanism by which LRP is targeted for degradation. By examining both endogenous full-length and a minireceptor form of LRP, we found that proteasomal inhibitors, MG132 and lactacystin, prolong the cellular half-life of LRP. The presence of proteasomal inhibitors also significantly increased the level of LRP at the cell surface, suggesting that the delivery of LRP to the degradation pathway was blocked at a compartment from which recycling of the receptor to the cell surface still occurred. Immunoelectron microscopy analyses demonstrated a proteasomal inhibitor-dependent reduction in LRP minireceptor within both limiting membrane and internal vesicles of the multivesicular bodies, which are compartments that lead to receptor degradation. In contrast to the growth hormone receptor, we found that the initial endocytosis of LRP minireceptor does not require a functional ubiquitin-proteasome system. Finally, using truncated cytoplasmic mutants of LRP minireceptors, we found that a region of 19 amino acids within the LRP tail is required for proteasomal regulation. Taken together our results provide strong evidence that the cellular turnover of a cargo receptor, i.e., LRP, is regulated by the proteasomal system, suggesting a broader function of the proteasome in regulating the trafficking of receptors into the degradation pathway.

Ligand-independent growth hormone receptor dimerization occurs in the endoplasmic reticulum and is required for ubiquitin system-dependent endocytosis

The regulatory effect of growth hormone (GH) on its target cells is mediated via the GH receptor (GHR). GH binding to the GHR results in the formation of a GH-(GHR)(2) complex and the initiation of signal transduction cascades via the activation of the tyrosine kinase JAK2. Subsequent endocytosis and transport to the lysosome of the ligand-receptor complex is regulated via the ubiquitin system and requires the presence of an intact ubiquitin-dependent endocytosis (UbE) motif in the cytosolic tail of the GHR. Recently, the model of ligand-induced receptor dimerization has been challenged. In this study, ligand-independent GHR dimerization is demonstrated in the endoplasmic reticulum and at the cell surface by coimmunoprecipitation of an epitope-tagged truncated GHR with wild-type GHR. In addition, evidence is provided that the extracellular domain of the GHR is not required to maintain this interaction. Internalization of a chimeric receptor, which fails to dimerize, is independent of an intact UbE-motif. Therefore, we postulate that dimerization of GHR molecules is required for ubiquitin system-dependent endocytosis.

Bilayered clathrin coats on endosomal vacuoles are involved in protein sorting toward lysosomes

In many cells endosomal vacuoles show clathrin coats of which the function is unknown. Herein, we show that this coat is predominantly present on early endosomes and has a characteristic bilayered appearance in the electron microscope. By immunoelectron microscopy we show that the coat contains clathrin heavy as well as light chain, but lacks the adaptor complexes AP1, AP2, and AP3, by which it differs from clathrin coats on endocytic vesicles and recycling endosomes. The coat is insensitive to short incubations with brefeldin A, but disappears in the presence of the phosphatidylinositol 3-kinase inhibitor wortmannin. No association of endosomal coated areas with tracks of tubulin or actin was found. By quantitative immunoelectron microscopy, we found that the lysosomal-targeted receptors for growth hormone (GHR) and epidermal growth factor are concentrated in the coated membrane areas, whereas the recycling transferrin receptor is not. In addition, we found that the proteasomal inhibitor MG 132 induces a redistribution of a truncated GHR (GHR-369) toward recycling vesicles, which coincided with a redistribution of endosomal vacuole-associated GHR-369 to the noncoated areas of the limiting membrane. Together, these data suggest a role for the bilayered clathrin coat on vacuolar endosomes in targeting of proteins to lysosomes.

The ubiquitin-proteasome pathway regulates the availability of the GH receptor

GH promotes not only longitudinal growth in children but is active throughout life in protein, fat, and carbohydrate metabolism. The multiple actions of GH start when GH binds to the cell surface-expressed GH receptor. Effectiveness of the hormone depends both on its presence in the circulation and the availability of receptors at the cell surface of target cells. In this study, we examined the role of the ubiquitin-proteasome pathway in regulating GH receptor availability. We show that receptor turnover is rapid, and almost 3-fold prolonged in the internalization-deficient mutant GH receptor (F327A). Using a monovalent GH antagonist, B2036, we could quantify the internalization of the nonactivated receptor. By comparing internalization of the receptor with shedding of the GH-binding protein, we show that in Chinese hamster lung cell lines, internalization followed by lysosomal degradation is the major pathway for receptor degradation and that the ubiquitin-proteasome pathway controls this process. Inhibition of endocytosis resulted in a 200% increase in receptor availability at the cell surface at steady state.

Wnt signals are transmitted through N-terminally dephosphorylated beta-catenin

beta-catenin mediates Wnt signaling by acting as the essential co-activator for TCF transcription factors. Wnt signaling increases the half-life and therefore the absolute level of beta-catenin in responding cells. The current model states that these changes in beta-catenin stability set the threshold for Wnt signaling. However, we find that pharmacological inhibition of proteasome activity by ALLN leads to accumulation of cytosolic beta-catenin but not to increased TCF-mediated transcription. In addition, in temperature-sensitive ubiquitylation mutant CHO cells inhibition of ubiquitylation increases beta-catenin levels, but does not induce transcriptional activation of TCF reporter genes. Using an antibody specific for beta-catenin dephosphorylated at residues Ser37 and Thr41, we show that Wnt signals specifically increase the levels of dephosphorylated beta-catenin, whereas ALLN does not. We conclude that changes in the phosphorylation status of the N-terminus of beta-catenin that occur upon Wnt signaling independently affect the signaling properties and half-life of beta-catenin. Hence, Wnt signals are transduced via N-terminally dephosphorylated beta-catenin.

Beta-arrestin and Mdm2, unsuspected partners in signaling from the cell surface

Mdm2 is a ubiquitin-protein ligase known to ubiquitinate p53, promoting its degradation by the ubiquitin-proteasome system. Shenoy and co-workers showed that Mdm2 can act as a key factor in the sequestration of the cell surface beta(2)-adrenergic receptor (beta-AR) through interactions with beta-arrestin. Strous and Schantl discuss how Mdm2 may be a switch connecting extracellular signals mediated through G protein-coupled receptors (GPCRs) to p53 and its functions in apoptosis and cell cycle progression.

The ubiquitin-dependent endocytosis motif is required for efficient incorporation of growth hormone receptor in clathrin-coated pits, but not clathrin-coated lattices

Endocytosis of the growth hormone receptor (GHR) requires an active ubiquitin-conjugation system. In addition, it depends on a 10 amino acid residues motif in the GHR-cytoplasmic tail, the ubiquitin dependent-endocytosis or UbE-motif. To gain insight into the role of ubiquitination in the early steps of endocytosis, we performed an ultrastructural analysis of GH-uptake in Chinese hamster cells expressing wild-type or mutant GHRs. In wild-type GHR cells, GH was found to be exclusively taken up via clathrin-coated pits. In early endosomes it was efficiently sorted from recycling transferrin and targeted to the degradative pathway. Mutation of all lysine residues of a truncated GHR (GHR-399K–) precludes ubiquitination of the receptor, but internalization of GHR-399K– still depends on an active ubiquitin system. We found that GHR-399K– incorporates GH into clathrin-coated vesicles with the same efficiency as wild-type GHR. By contrast, a mutation in the UbE-motif (GHR-F327A) largely abolished incorporation of GH into clathrin-coated vesicles. Notably, access of GH to clathrin-coated lattices was not affected in GHR-F327A cells. These data corroborate and extend previous data that the UbE-motif but not ubiquitination of the receptor itself recruits GHR into clathrin-coated vesicles. Moreover, they suggest that incorporation of GHR into clathrin-coated lattices is differentially regulated from incorporation into clathrin-coated pits.

Growth hormone receptor ubiquitination, endocytosis, and degradation are independent of signal transduction via Janus kinase 2

The ubiquitin-proteasome system is required in growth hormone receptor (GHR) endocytosis. For cytokine receptors, which lack intrinsic tyrosine kinase activity, signal transduction is initiated by the activation of a member of the Janus kinase (JAK) family. Previously, we have shown that GHR and JAK2 tyrosine (de) phosphorylation are regulated via the ubiquitin system. In this study, we examined the role of JAK2-mediated signal transduction in GHR internalization and down-regulation. Mutation of the attachment site for JAK2, box-1, in the GHR cytoplasmic tail resulted in the complete absence of GHR and JAK2 phosphorylation. This modification did not alter the rate and extent of receptor-bound growth hormone internalization as compared with a functional GHR, nor did it change its turnover and transport to the plasma membrane. In addition, the receptor was still normally ubiquitinated and remained dependent on both an intact ubiquitin system and proteasomal action for its internalization. Thus, GHR ubiquitination, endocytosis, and degradation occur independently of GHR signal transduction via JAK2. We conclude that whereas endocytosis and degradation require the ubiquitin system, they are independent of GHR signal transduction.

The ubiquitin-proteasome pathway regulates lysosomal degradation of the growth hormone receptor and its ligand

The growth hormone (GH) receptor (GHR) is a mammalian plasma membrane protein whose internalization is mediated by the ubiquitin-proteasome pathway. GH internalization and degradation are inhibited when cells are treated with proteasome inhibitors. Here we show that a GHR truncated at residue 369 can enter the cells in the presence of a proteasome inhibitor, but that the subsequent lysosomal degradation of GH is blocked. Lysosomal inhibitors prolong the half-life of both receptor and ligand. Experiments with antibodies against different receptor tail sections show that degradation of the GHR cytosolic domain precedes degradation of the extracellular GH-binding domain. A possible role for the ubiquitin-proteasome pathway in the degradation of the receptor and ligand is discussed.

Proteasome inhibitors block a late step in lysosomal transport of selected membrane but not soluble proteins

The ubiquitin-proteasome pathway acts as a regulator of the endocytosis of selected membrane proteins. Recent evidence suggests that it may also function in the intracellular trafficking of membrane proteins. In this study, several models were used to address the role of the ubiquitin-proteasome pathway in sorting of internalized proteins to the lysosome. We found that lysosomal degradation of ligands, which remain bound to their receptors within the endocytic pathway, is blocked in the presence of specific proteasome inhibitors. In contrast, a ligand that dissociates from its receptor upon endosome acidification is degraded under the same conditions. Quantitative electron microscopy showed that neither the uptake nor the overall distribution of the endocytic marker bovine serum albumin-gold is substantially altered in the presence of a proteasome inhibitor. The data suggest that the ubiquitin-proteasome pathway is involved in an endosomal sorting step of selected membrane proteins to lysosomes, thereby providing a mechanism for regulated degradation.

The signal transduction of the growth hormone receptor is regulated by the ubiquitin/proteasome system and continues after endocytosis

The growth hormone receptor (GHR) intracellular domain contains all of the information required for signal transduction as well as for endocytosis. Previously, we showed that the proteasome mediates the clathrin-mediated endocytosis of the GHR. Here, we present evidence that the proteasomal inhibitor MG132 prolongs the GH-induced activity of both GHR and JAK2, presumably through stabilization of GHR and JAK2 tyrosine phosphorylation. If proteasomal inhibitor was combined with ligand in an endocytosis-deficient GHR mutant, the same phenomenon occurred indicating that proteasomal action on tyrosine dephosphorylation is independent of endocytosis. Experiments with a GHR-truncated tail mutant (GHR-(1-369)) led to a prolonged JAK2 phosphorylation caused by the loss of a phosphatase-binding site. This raised the question of what happens to the signal transduction of the GHR after its internalization. Co-immunoprecipitation of GH.GHR complexes before and after endocytosis showed that JAK2 as well as other activated proteins are bound to the GHR not only at the cell surface but also intracellularly, suggesting that the GHR signal transduction continues in endosomes. Additionally, these results provide evidence that GHR is present in endosomes both in its full-length and truncated form, indicating that the receptor is down-regulated by the proteasome.

Growth hormone receptor ubiquitination coincides with recruitment to clathrin-coated membrane domains

Endocytosis of the growth hormone receptor (GHR) depends on a functional ubiquitin conjugation system. A 10-amino acid residue motif within the GHR cytosolic tail (the ubiquitin-dependent endocytosis motif) is involved in both GHR ubiquitination and endocytosis. As shown previously, ubiquitination of the receptor itself is not required. In this paper ubiquitination of the GHR was used as a tool to address the question of at which stage the ubiquitin conjugation system acts in the process of GHR endocytosis. If potassium depletion was used to interfere with early stages of coated pit formation, both GHR endocytosis and ubiquitination were inhibited. Treatment of cells with methyl-beta-cyclodextrin inhibited endocytosis at the stage of coated vesicle formation. Growth hormone addition to methyl-beta-cyclodextrin-treated cells resulted in an accumulation of ubiquitinated GHR at the cell surface. Using immunoelectron microscopy, the GHR was localized in flattened clathrin-coated membranes. In addition, when clathrin-mediated endocytosis was inhibited in HeLa cells expressing a temperature-sensitive dynamin mutant, ubiquitinated GHR accumulated at the cell surface. Together, these data show that the GHR is ubiquitinated at the plasma membrane, before endocytosis occurs, and indicate that the resident time of the GHR at the cell surface is regulated by the ubiquitin conjugation system together with the endocytic machinery.

Identification of a major cyclic AMP-dependent protein kinase A phosphorylation site within the cytoplasmic tail of the low-density lipoprotein receptor-related protein: implication for receptor-mediated endocytosis

The low-density lipoprotein (LDL) receptor-related protein (LRP) is a multiligand endocytic receptor that belongs to the LDL receptor family. Recently, studies have revealed new roles of LDL receptor family members as transducers of extracellular signals. Our previous studies have demonstrated LRP phosphorylation within its cytoplasmic tail, but the nature of LRP phosphorylation and its potential function was unknown. In the present study using both in vivo and in vitro analysis, we found that LRP phosphorylation is mediated by the cAMP-dependent protein kinase A (PKA). Using site-directed mutagenesis and LRP minireceptor constructs, we further identified the predominant LRP phosphorylation site at serine 76 of its cytoplasmic tail. Finally, we demonstrated that mutations of serine 76, which abolish LRP phosphorylation by PKA, result in a decrease in the initial endocytosis rate of LRP and a lower efficiency in delivery of ligand for degradation. Thus, the role of PKA phosphorylation of LRP in receptor-mediated endocytosis may provide a mechanism by which the endocytic function of LRP can be regulated by external signals.

The YXXL motif, but not the two NPXY motifs, serves as the dominant endocytosis signal for low density lipoprotein receptor-related protein

All members of the low density lipoprotein (LDL) receptor family contain at least one copy of the NPXY sequence within their cytoplasmic tails. For the LDL receptor, it has been demonstrated that the NPXY motif serves as a signal for rapid endocytosis through coated pits. Thus, it is generally believed that the NPXY sequences function as endocytosis signals for all the LDL receptor family members. The primary aim of this study is to define the endocytosis signal(s) within the cytoplasmic tail of LDL receptor-related protein (LRP). By using LRP minireceptors, which mimic the function and trafficking of full-length endogenous LRP, we demonstrate that the YXXL motif, but not the two NPXY motifs, serves as the dominant signal for LRP endocytosis. We also found that the distal di-leucine motif within the LRP tail contributes to its endocytosis, and its function is independent of the YXXL motif. Although the proximal NPXY motif and the proximal di-leucine motif each play a limited role in LRP endocytosis in the context of the full-length tail, these motifs were functional within the truncated receptor tail. In addition, we show that LRP minireceptor mutants defective in endocytosis signal(s) accumulate at the cell surface and are less efficient in delivery of ligand for degradation.

Endocytosis and degradation of the growth hormone receptor are proteasome-dependent

The ubiquitin conjugation system is involved in ligand-induced endocytosis of the growth hormone receptor (GHR) via a cytosolic 10-amino acid ubiquitin-dependent endocytosis motif. Herein, we demonstrate that the proteasome is also involved in growth hormone receptor down-regulation. Ligand-induced degradation was blocked in the presence of specific proteasomal inhibitors. In addition, growth hormone (GH) internalization was inhibited, whereas the transferrin receptor cycle remained unaffected. A truncated GHR entered the cells independent of proteasome action. In addition, we show that GH internalization is independent of the presence of lysine residues in the cytosolic domain of the receptor, whereas its internalization can still be inhibited by proteasomal inhibitors. Thus, GHR internalization requires proteasome action in addition to an active ubiquitin conjugation system, but ubiquitination of the GHR itself seems not to be required.

The ubiquitin-proteasome system and endocytosis

Internalization of membrane proteins has been studied for more than three decades without solving all the underlying mechanisms. Our knowledge of clathrin-mediated endocytosis is certainly sufficient to understand the basic principles. However, more detailed insight is required to recognize why different proteins enter clathrin-coated pits with different rates and affinities. In addition to clathrin coat components, at least two adaptor systems and even more accessory proteins have been described to preselect membrane proteins before they can enter cells. Recent experimental data have identified the ubiquitin-proteasome system as a regulatory system for endocytosis. This system is well-known for its basic regulatory function in protein degradation, and controls a magnitude of key events. The ubiquitin-proteasome system is now identified as a regulator of the endocytosis of selected membrane proteins. In this review, we will discuss the complexity and implications of this mechanism for receptor-mediated endocytosis.

Identification of a novel ubiquitin conjugation motif, required for ligand-induced internalization of the growth hormone receptor

In addition to its role in selective protein degradation, the conjugation of ubiquitin to proteins has also been implicated in the internalization of plasma membrane proteins, including the alpha-factor receptor Ste2p, uracil permease Fur4p, epithelial sodium channel ENaC and the growth hormone receptor (GHR). Binding of GH to its receptor induces receptor dimerization, resulting in the activation of signal transduction pathways and an increase of GHR ubiquitination. Previously, we have shown that the ubiquitin conjugation system mediates GH-induced GHR internalization. Here, we present evidence that a specific domain of the GHR regulates receptor endocytosis via the ubiquitin conjugation system. This ubiquitin-dependent endocytosis (UbE) motif consists of the amino acid sequence DSWVEFIELD and is homologous to sequences in other proteins, several of which are known to be ubiquitinated. In addition, we show that GH internalization by a truncated GHR is independent of the presence of lysine residues in the cytosolic domain of this receptor, while internalization still depends on an intact ubiquitin conjugation system. Thus, GHR internalization requires the recruitment of the ubiquitin conjugation system to the GHR UbE motif rather than the conjugation of ubiquitin to the GHR itself.

Di-leucine-mediated internalization of ligand by a truncated growth hormone receptor is independent of the ubiquitin conjugation system

The growth hormone receptor (GHR) is a member of the cytokine receptor family. Its function is to mediate cellular responses upon binding of growth hormone. Ligand binding induces dimerization and activation of the GHR. One mechanism by which the GHR is rapidly inactivated involves the ubiquitin conjugation system, a system implicated in the degradation of cytosolic and nuclear proteins. We have shown previously that the ubiquitin-conjugating system mediates internalization of the GHR. Here, we present evidence that in addition to the ubiquitin-dependent endocytosis signal, the cytosolic tail of the GHR contains a di-leucine motif. Upon truncation of the GHR at amino acid residue 349, this di-leucine motif is activated and mediates ubiquitin-independent internalization of the receptor. Di-leucine-mediated GHR internalization requires functional clathrin-coated pits and results in GHR transport to the lysosome. Although the full-length GHR internalizes independent of the di-leucine motif, this motif may function in internalization of GHR isoforms.

Molecular cloning of human gastric mucin MUC5AC reveals conserved cysteine-rich D-domains and a putative leucine zipper motif

To further clone the human gastric mucin MUC5AC cDNA, we screened a human gastric cDNA library with previously identified MUC5AC sequences. We obtained 32 independent clones encoding newly identified sequences comprising the entire N-terminal sequence of MUC5AC, up to 3024 bp upstream of the previously identified MUC5AC sequences. The N-terminus of MUC5AC shows high homology (43% identity) with the N-terminus of MUC2 and contains three domains homologous to the D-domains found in the pro-von Willebrand factor. Furthermore, the N-terminus of MUC5AC contains a putative leucine zipper motif not found in any other mucin identified so far. Moreover, a large central repetitive sequence was identified encoding approximately 2500 amino acids (7.5 kb). We were able to establish that the MUC5AC cDNA together with the previously identified 6.1 kb of MUC5AC cDNA sequence is about 16.6 kb, encoding 5525 amino acids. A model of the domain structure of MUC5AC is presented.