Regulation of copper-dependent endocytosis and vacuolar degradation of the yeast copper transporter, Ctr1p, by the Rsp5 ubiquitin ligase

The Saccharomyces cerevisiae high-affinity copper transporter, Ctr1p, mediates cellular uptake of Cu(I). We report that when copper (50 microm CuSO(4)) is added to the growth medium of copper-starved cells, Ctr1p is rapidly internalized by endocytosis, delivered to the lumen of the lysosome-like vacuole and slowly degraded by vacuolar proteases. Through analysis of the trafficking and degradation of Ctr1p mutants, two lysine residues in the C-terminal cytoplasmic tail of Ctr1p, Lys340 and Lys345, were found to be critical for copper-dependent endocytosis and degradation. In response to copper addition, Ctr1p was found to be ubiquitylated and a mutation in the Rsp5 ubiquitin ligase largely abolished ubiquitylation, endocytosis and degradation. In a strain lacking the Rsp5p accessory factors Bul1p and Bul2p, endocytosis and degradation of Ctr1p-green fluorescent protein were substantially diminished. Surprisingly, a Ctr1p mutant that lacks Lys340 and Lys345 was still ubiquitylated in a copper-dependent manner, indicating that ubiquitylation of Ctr1p on other sites is insufficient to drive copper-dependent endocytosis and degradation. This study demonstrates that copper regulates turnover of Ctr1p by stimulating Rsp5p-dependent endocytosis and degradation of Ctr1p in the vacuole.     

Multiple cysteine residues are necessary for sorting and transport activity of the arsenite permease Acr3p from Saccharomyces cerevisiae

The yeast transporter Acr3p is a low affinity As(III)/H⁺ and Sb(III)/H⁺ antiporter located in the plasma membrane. It has been shown for bacterial Acr3 proteins that just a single cysteine residue, which is located in the middle of the fourth transmembrane region and conserved in all members of the Acr3 family, is essential for As(III) transport activity. Here, we report a systematic mutational analysis of all nine cysteine residues present in the Saccharomyces cerevisiae Acr3p. We found that mutagenesis of highly conserved Cys151 resulted in a complete loss of metalloid transport function. In addition, lack of Cys90 and Cys169, which are conserved in eukaryotic members of Acr3 family, impaired Acr3p trafficking to the plasma membrane and greatly reduced As(III) efflux, respectively. Mutagenesis of five other cysteines in Acr3p resulted in moderate reduction of As(III) transport capacities and sorting perturbations. Our data suggest that interaction of As(III) with multiple thiol groups in the yeast Acr3p may facilitate As(III) translocation across the plasma membrane.     

Ear1p and Ssh4p are new adaptors of the ubiquitin ligase Rsp5p for cargo ubiquitylation and sorting at multivesicular bodies

The ubiquitylation of membrane proteins destined for the vacuole/lysosome is essential for their recognition by the endosomal sorting machinery and their internalization into vesicles of multivesicular bodies (MVBs). In yeast, this process requires Rsp5p, an essential ubiquitin ligase of the Nedd4 family. We describe here two redundant proteins, Ear1p and Ssh4p, required for the vacuolar targeting of several cargoes originating from the Golgi or the plasma membrane. Ear1p is an endosomal protein that interacts with Rsp5p through its PPxY motifs, and it is required for the ubiquitylation of selected cargoes before their MVB sorting. In-frame fusion of cargo to ubiquitin overcomes the need for Ear1p/Ssh4p, confirming a role for these proteins in cargo ubiquitylation. Interestingly, Ear1p is itself ubiquitylated by Rsp5p and targeted to the vacuole. Finally, Ear1p overexpression leads to Rsp5p accumulation at endosomes, interfering with some of its functions in trafficking. Therefore, Ear1p/Ssh4p recruit Rsp5p and assist it in its function at MVBs by directing the ubiquitylation of specific cargoes.     

Targeting of Sna3p to the endosomal pathway depends on its interaction with Rsp5p and multivesicular body sorting on its ubiquitylation

Rsp5p is an ubiquitin (Ub)-protein ligase of the Nedd4 family that carries WW domains involved in interaction with PPXY-containing proteins. It plays a key role at several stages of intracellular trafficking, such as Ub-mediated internalization of endocytic cargoes and Ub-mediated sorting of membrane proteins to internal vesicles of multivesicular bodies (MVBs), a process that is crucial for their subsequent targeting to the vacuolar lumen. Sna3p is a membrane protein previously described as an Ub-independent MVB cargo, but proteomic studies have since shown it to be an ubiquitylated protein. Sna3p carries a PPXY motif. We observed that this motif mediates its interaction with Rsp5p WW domains. Mutation of either the Sna3p PPXY motif or the Rsp5p WW3 domain or reduction in the amounts of Rsp5 results in the mistargeting of Sna3p to multiple mobile vesicles and prevents its sorting to the endosomal pathway. This sorting defect appears to occur prior to the defect displayed in rsp5 mutants by other MVB cargoes, which are correctly sorted to the endosomal pathway but missorted to the vacuolar membrane instead of the vacuolar lumen. Sna3p is polyubiquitylated on one target lysine, and a mutant Sna3p lacking its target lysine displays defective MVB sorting. Sna3p undergoes Rsp5-dependent polyubiquitylation, with K63-linked Ub chains.     

Involvement of the Acr3 and DctA anti‐porters in arsenite oxidation in Agrobacterium tumefaciens 5A

Microbial arsenite (AsIII) oxidation forms a critical piece of the arsenic cycle in nature, though our understanding of how and why microorganisms oxidize AsIII remains rudimentary. Our model organism Agrobacterium tumefaciens 5A contains two distinct ars operons (ars1 and ars2) that are similar in their coding region content. The ars1 operon is located nearby the aio operon that is essential for AsIII oxidation. The AsIII/H⁺ anti‐porters encoded by acr3‐1 and acr3‐2 are required for maximal AsIII and antimonite (SbIII) resistance, but acr3‐1 (negatively regulated by ArsR‐1) appears more active in this regard and also required for AsIII oxidation and expression of aioBA. A malate‐phosphate anti‐porter DctA is regulated by RpoN and AsIII, and is required for normal growth with malate as a sole carbon source. Qualitatively, a ΔdctA mutant was normal for AsIII oxidation and AsIII/SbIII resistance at metalloid concentrations inhibitory to the Δacr3‐1 mutant; however, aioBA induction kinetics was significantly phase‐shift delayed. Acr3 involvement in AsIII/SbIII resistance is reasonably well understood, but the role of Acr3 and DctA anti‐porters in AsIII oxidation and its regulation is unexpected, and suggests that controlled AsIII trafficking across the cytoplasmic membrane is important to a process understood to occur in the periplasm.     

The yeast Npi1/Rsp5 ubiquitin ligase lacking its N-terminal C2 domain is competent for ubiquitination but not for subsequent endocytosis of the gap1 permease

The yeast ubiquitin ligase Npi1/Rsp5 and its mammalian homologue Nedd4 are involved in ubiquitination of various cell surface proteins, these being subsequently internalized by endocytosis and degraded in the vacuole/lysosome. Both enzymes consist of an N-terminal C2 domain, three to four successive WW(P) domains, and a C-terminal catalytic domain (HECT) containing a highly conserved cysteine residue involved in ubiquitin thioester formation. In this study, we show that the conserved cysteine of the HECT domain is required for yeast cell viability and for ubiquitination and subsequent endocytosis of the Gap1 permease. In contrast, the C2 domain of Npi1/Rsp5 is not essential to cell viability. Its deletion impairs internalization of Gap1, without detectably affecting ubiquitination of the permease. This suggests that Npi1/Rsp5 participates, via its C2 domain, in endocytosis of ubiquitinated permeases.     

Ca(2+)-dependent inhibition of Na+/H+ exchanger 3 (NHE3) requires an NHE3-E3KARP-alpha-actinin-4 complex for oligomerization and endocytosis

Two PDZ domain-containing proteins, NHERF and E3KARP are necessary for cAMP-dependent inhibition of Na(+)/H(+) exchanger 3 (NHE3). In this study, we demonstrate a specific role of E3KARP, which is not duplicated by NHERF, in Ca(2+)-dependent inhibition of NHE3 activity. NHE3 activity is inhibited by elevation of intracellular Ca(2+) ([Ca(2+)](i)) in PS120 fibroblasts stably expressing E3KARP but not those expressing NHERF. In addition, this Ca(2+)-dependent inhibition requires Ca(2+)-dependent association between alpha-actinin-4 and E3KARP. NHE3 is indirectly connected to alpha-actinin-4 in a protein complex through Ca(2+)-dependent interaction between alpha-actinin-4 and E3KARP, which occurs through the actin-binding domain plus spectrin repeat domain of alpha-actinin-4. Elevation of [Ca(2+)](i) results in oligomerization and endocytosis of NHE3 as well as in inhibition of NHE3 activity. Overexpression of alpha-actinin-4 potentiates the inhibitory effect of ionomycin on NHE3 activity by accelerating the oligomerization and endocytosis of NHE3. In contrast, overexpression of the actin-binding domain plus spectrin repeat domain acts as a dominant-negative mutant and prevents the inhibitory effect of ionomycin on NHE3 activity as well as the oligomerization and internalization of NHE3. From these results, we propose that elevated Ca(2+) inhibits NHE3 activity through oligomerization and endocytosis of NHE3, which occurs via formation of an NHE3-E3KARP-alpha-actinin-4 complex.     

Establishment of an interleukin-5-dependent subclone from an interleukin-3-dependent murine hemopoietic progenitor cell line, LyD9, and its malignant transformation by autocrine secretion of interleukin-5.

An interleukin-5 (IL-5)-dependent subclone, K-5, was established from an IL-3-dependent murine hemopoietic progenitor cell line by co-culturing with bone marrow stroma cells. K-5 cells were induced to differentiate into myeloid lineage cells by co-culturing with cloned PA6 stroma cells. By co-culturing with another cloned stroma cell (ST-2s10), K-5 cells gave rise to a factor-independent transformant cell line LT-5 which proliferated in an autocrine manner by secretion of IL-5 and produced tumors in nude mice. Molecular cloning of the IL-5 gene of LT-5 cells and the nucleotide sequencing of its 5' flanking region indicate that a transposition of an intracisternal A-particle (IAP) element to the 5' flanking region of the IL-5 gene is responsible for the constitutive expression of IL-5 mRNA of an aberrant size in LT-5 cells.     

NPI1, an essential yeast gene involved in induced degradation of Gap1 and Fur4 permeases, encodes the Rsp5 ubiquitin—protein ligase

When yeast cells growing on a poor nitrogen source are supplied with NH₄⁺ ions, several nitrogen permeases including the general amino acid permease (Gap1p) are rapidly and completely inactivated. This report shows that inactivation by NH₄⁺ of the Gap1 permease is accompanied by its degradation. A functional NPI1 gene product is required for both inactivation and degradation of Gap1p. Molecular analysis of the NPI1 gene showed that it is identical to RSP5 . The RSP5 product is a ubiquitin—protein ligase (E3 enzyme) whose physiological function was, however, unknown. Its C-terminal region is very similar to that of other members of the E6-AP-like family of ubiquitin-protein ligases. Its N-terminal region contains a single C₂ domain that may be a Ca²⁺-dependent phospholipid interaction motif, followed by several copies of a recently identified domain called WW(P). The Npi1/Rsp5 protein has a homologue both in humans and in mice, the latter being involved in brain development. Stress-induced degradation of the uracil permease (Fur4p), a process in which ubiquitin is probably involved, was also found to require a functional NPI1/RSP5 product. Chromosomal deletion of NPI1/RSP5 showed that this gene is essential for cell viability. In the viable np1/rsp5 strain, expression of NPI1/RSP5 is reduced as a result of insertion of a Ty1 element in its 5' region. Our results show that the Npi1/Rsp5 ubiquitin-protein ligase participates in induced degradation of at least two permeases, Gap1p and Fur4p, and probably also other proteins.     

Cell entry of avian reovirus follows a caveolin-1-mediated and dynamin-2-dependent endocytic pathway that requires activation of p38 mitogen-activated protein kinase (MAPK) and Src signaling pathways as well as microtubules and small GTPase Rab5 protein

Very little is known about the mechanism of cell entry of avian reovirus (ARV). The aim of this study was to explore the mechanism of ARV entry and subsequent infection. Cholesterol mainly affected the early steps of the ARV life cycle, because the presence of cholesterol before and during viral adsorption greatly blocked ARV infectivity. Although we have demonstrated that ARV facilitating p38 MAPK is beneficial for virus replication, its mechanism remains unknown. Here, we show that ARV-induced phosphorylation of caveolin-1 (Tyr(14)), dynamin-2 expression, and Rac1 activation through activation of p38 MAPK and Src in the early stage of the virus life cycle is beneficial for virus entry and productive infection. The strong inhibition by dynasore, a specific inhibitor of dynamin-2, and depletion of endogenous caveolin-1 or dynamin-2 by siRNAs as well as the caveolin-1 colocalization study implicate caveolin-1-mediated and dynamin-2-dependent endocytosis as a significant avenue of ARV entry. By means of pharmacological inhibitors, dominant negative mutants, and siRNA of various cellular proteins and signaling molecules, phosphorylation of caveolin-1, dynamin-2 expression, and Rac1 activation were suppressed, suggesting that by orchestrating p38 MAPK, Src, and Rac1 signaling cascade in the target cells, ARV creates an appropriate intracellular environment facilitating virus entry and productive infection. Furthermore, disruption of microtubules, Rab5, or endosome acidification all inhibited ARV infection, suggesting that microtubules and small GTPase Rab5, which regulate transport to early endosome, are crucial for survival of ARV and that exposure of the virus to acidic pH is required for productive infection.     

Ghitm is an ortholog of the Bombyx mori prothoracic gland-derived receptor (Pgdr) that is ubiquitously expressed in mammalian cells and requires an N-terminal signal sequence for expression

In a previous paper, we reported the cloning of a cDNA encoding a putative receptor, Pgdr, from the prothoracic gland of the silkworm, Bombyx mori. Few studies concerning the orthologous cDNA of Pgdr in mammals, a growth hormone-inducible transmembrane protein (Ghitm) that encodes a putative receptor, have been performed. Analysis of the distribution of Ghitm expression revealed ubiquitous expression in mouse embryo and adult tissues, as well as mammalian cell lines. The pattern of Ghitm expression suggested that once Ghitm mRNA was expressed in the putative brain region of mouse embryo, Ghitm-expressing cells spread ubiquitously throughout all tissues during embryonic development. In addition, Western blot analyses demonstrated that cleavage of the N-terminal portion in GHITM appears to regulate the expression level, suggesting that cleavage is essential for the proper expression of GHITM.     

Glutathione utilization by Candida albicans requires a functional glutathione degradation (DUG) pathway and OPT7, an unusual member of the oligopeptide transporter family

Candida albicans lacks the ability to survive within its mammalian host in the absence of endogenous glutathione biosynthesis. To examine the ability of this yeast to utilize exogenous glutathione, we exploited the organic sulfur auxotrophy of C. albicans met15Δ strains. We observed that glutathione is utilized efficiently by the alternative pathway of glutathione degradation (DUG pathway). The major oligopeptide transporters OPT1-OPT5 of C. albicans that were most similar to the known yeast glutathione transporters were not found to contribute to glutathione transport to any significant extent. A genomic library approach to identify the glutathione transporter of C. albicans yielded OPT7 as the primary glutathione transporter. Biochemical studies on OPT7 using radiolabeled GSH uptake revealed a K(m) of 205 μm, indicating that it was a high affinity glutathione transporter. OPT7 is unusual in several aspects. It is the most remote member to known yeast glutathione transporters, lacks the two highly conserved cysteines in the family that are known to be crucial in trafficking, and also has the ability to take up tripeptides. The transporter was regulated by sulfur sources in the medium. OPT7 orthologues were prevalent among many pathogenic yeasts and fungi and formed a distinct cluster quite remote from the Saccharomyces cerevisiae HGT1 glutathione transporter cluster. In vivo experiments using a systemic model of candidiasis failed to detect expression of OPT7 in vivo, and strains disrupted either in the degradation (dug3Δ) or transport (opt7Δ) of glutathione failed to show a defect in virulence.     

5-Aza-deoxycytidine induces selective degradation of DNA methyltransferase 1 by a proteasomal pathway that requires the KEN box, bromo-adjacent homology domain, and nuclear localization signal

5-Azacytidine- and 5-aza-deoxycytidine (5-aza-CdR)-mediated reactivation of tumor suppressor genes silenced by promoter methylation has provided an alternate approach in cancer therapy. Despite the importance of epigenetic therapy, the mechanism of action of DNA-hypomethylating agents in vivo has not been completely elucidated. Here we report that among three functional DNA methyltransferases (DNMT1, DNMT3A, and DNMT3B), the maintenance methyltransferase, DNMT1, was rapidly degraded by the proteasomal pathway upon treatment of cells with these drugs. The 5-aza-CdR-induced degradation, which occurs in the nucleus, could be blocked by proteasomal inhibitors and required a functional ubiquitin-activating enzyme. The drug-induced degradation occurred even in the absence of DNA replication. Treatment of cells with other nucleoside analogs modified at C-5, 5-fluorodeoxyuridine and 5-fluorocytidine, did not induce the degradation of DNMT1. Mutation of cysteine at the catalytic site of Dnmt1 (involved in the formation of a covalent intermediate with cytidine in DNA) to serine (CS) did not impede 5-aza-CdR-induced degradation. Neither the wild type nor the catalytic site mutant of Dnmt3a or Dnmt3b was sensitive to 5-aza-CdR-mediated degradation. These results indicate that covalent bond formation between the enzyme and 5-aza-CdR-incorporated DNA is not essential for enzyme degradation. Mutation of the conserved KEN box, a targeting signal for proteasomal degradation, to AAA increased the basal level of Dnmt1 and blocked its degradation by 5-aza-CdR. Deletion of the catalytic domain increased the expression of Dnmt1 but did not confer resistance to 5-aza-CdR-induced degradation. Both the nuclear localization signal and the bromo-adjacent homology domain were essential for nuclear localization and for the 5-aza-CdR-mediated degradation of Dnmt1. Polyubiquitination of Dnmt1 in vivo and its stabilization upon treatment of cells with a proteasomal inhibitor indicate that the level of Dnmt1 is controlled by ubiquitin-dependent proteasomal degradation. Overexpression of the substrate recognition component, Cdh1 but not Cdc20, of APC (anaphase-promoting complex)/cyclosome ubiquitin ligase reduced the level of Dnmt1 in both untreated and 5-aza-CdR-treated cells. In contrast, the depletion of Cdh1 with small interfering RNA increased the basal level of DNMT1 that blocked 5-aza-CdR-induced degradation. Dnmt1 interacted with Cdh1 and colocalized in the nucleus at discrete foci. Both Dnmt1 and Cdh1 were phosphorylated in vivo, but only Cdh1 was significantly dephosphorylated upon 5-aza-CdR treatment, suggesting its involvement in initiating the proteasomal degradation of DNMT1. These results demonstrate a unique mechanism for the selective degradation of DNMT1, the maintenance DNA methyltransferase, by well-known DNA-hypomethylating agents.