Biomedical subjects

Manuel Miranda

Publications and source records attributed to Manuel Miranda.

7 recordsLinked to original sources

PubMed2006

Three ubiquitin conjugation sites in the amino terminus of the dopamine transporter mediate protein kinase C-dependent endocytosis of the transporter.

Dopamine levels in the brain are controlled by the plasma membrane dopamine transporter (DAT). The amount of DAT at the cell surface is determined by the relative rates of its internalization and recycling. Activation of protein kinase C (PKC) leads to acceleration of DAT endocytosis. We have recently demonstrated that PKC activation also results in ubiquitylation of DAT. To directly address the role of DAT ubiquitylation, lysine residues in DAT were mutated. Mutations of each lysine individually did not affect ubiquitylation and endocytosis of DAT. By contrast, ubiquitylation of mutants carrying multiple lysine substitutions was reduced in cells treated with phorbol ester to the levels detected in nonstimulated cells. Altogether, mutagenesis data suggested that Lys19, Lys27, and Lys35 clustered in the DAT amino-terminus are the major ubiquitin-conjugation sites. The data are consistent with the model whereby at any given time only one of the lysines in DAT is conjugated with a short ubiquitin chain. Importantly, cell surface biotinylation, immunofluorescence and down-regulation experiments revealed that PKC-dependent internalization of multilysine mutants was essentially abolished. These data provide the first evidence that the ubiquitin moieties conjugated to DAT may serve as a molecular interface of the transporter interaction with the endocytic machinery.

Amino Acid Substitution↗

PubMed2006

RNA interference screen reveals an essential role of Nedd4-2 in dopamine transporter ubiquitination and endocytosis.

The function of the dopamine transporter (DAT) to terminate dopamine neurotransmission is regulated by endocytic trafficking of DAT. To elucidate the mechanisms of DAT endocytosis, we generated a fully functional mutant of the human DAT in which a hemagglutinin epitope (HA) was incorporated into the second extracellular loop. The endocytosis assay, based on the uptake of an HA antibody, was designed to study constitutive- and protein kinase C (PKC)-dependent internalization of HA-DAT expressed in non-neuronal cells and rat dopaminergic neurons. Large-scale RNA interference analysis of PKC-dependent endocytosis of HA-DAT revealed the essential and specific role of an E3 ubiquitin ligase, Nedd4-2 (neural precursor cell expressed, developmentally downregulated 4-2), as well as the involvement of adaptor proteins present in clathrin-coated pits, such as epsin, Eps15 (epidermal growth factor pathway substrate clone 15), and Eps15R (Eps15-related protein). Depletion of Nedd4-2 resulted in a dramatic reduction of PKC-dependent ubiquitination of DAT. Endogenous Nedd4-2, epsin, and Eps15 were coimmunoprecipitated with heterologously expressed human HA-DAT and endogenous DAT isolated from rat striatum. A new mechanistic model of DAT endocytosis is proposed whereby the PKC-induced ubiquitination of DAT mediated by Nedd4-2 leads to interaction of DAT with adaptor proteins in coated pits and acceleration of DAT endocytosis.

Animals↗

PubMed2005

Enhanced ubiquitylation and accelerated degradation of the dopamine transporter mediated by protein kinase C.

Dopamine transporter (DAT) localization in dopaminergic neurons plays an important role in regulating dopamine signaling. However, the mechanisms of DAT trafficking that control DAT localization are still poorly understood. To gain insight into these mechanisms, human DAT was purified in large amounts using a two-step affinity chromatography procedure from untreated HeLa cells or cells treated with phorbol 12-myristate 13-acetate (PMA). Mass spectrometric analysis of purified DAT complexes revealed the presence of several proteins, among which ubiquitin was particularly abundant in the PMA-treated sample. Western blotting of highly purified DAT protein confirmed constitutive ubiquitylation of DAT and a dramatic increase in DAT ubiquitylation in cells treated with PMA. This increase was blocked by pretreatment with the protein kinase C (PKC) inhibitor bis-indolylmaleimide. DAT ubiquitylation by ectopically expressed ubiquitin was demonstrated in cells transiently transfected with yellow fluorescent protein-tagged ubiquitin. In addition, fluorescence resonance energy transfer was detected between cyan fluorescent protein-tagged DAT and yellow fluorescent protein-tagged ubiquitin, indicative of DAT-ubiquitin conjugation. Interestingly, the largest fluorescence resonance energy transfer signals were observed in endosomes. Ubiquitylated DAT was detected in the plasma membrane using cell surface biotinylation as well as in intracellular compartments, suggesting that ubiquitylation begins at the plasma membrane and is maintained in endosomes. In both porcine aortic endothelial and HeLa cells, where PKC-dependent DAT ubiquitylation was observed, PKC activation resulted in rapid degradation of DAT (t12 = 1-2 h). Altogether, these data suggest that PKC-induced DAT ubiquitylation may target DAT to lysosomal degradation.

Aggrecans↗

PubMed2004

Multiple molecular determinants in the carboxyl terminus regulate dopamine transporter export from endoplasmic reticulum.

The plasma membrane dopamine transporter (DAT) has an essential role in terminating dopaminergic neurotransmission by reuptake of dopamine into the presynaptic neurons. Therefore, the amount of DAT at the cell surface is a critical determinant of DAT function. In this study, we examined the role of the carboxyl terminus of DAT in trafficking of the transporter through the biosynthetic pathway to the plasma membrane. Live cell fluorescence microscopy and cell surface biotinylation were used to study the effects of systematic deletions and alanine substitutions in the carboxyl terminus on DAT localization. It was found that alanine substitutions of Lys-590 and Asp-600 significantly delayed the delivery of DAT to the plasma membrane because of retention of DAT in the endoplasmic reticulum (ER). Most surprising, mutation of Gly-585 to alanine completely blocked the exit of DAT from the ER and surface expression of the transporter. The effect of these three mutations on ER export of DAT was demonstrated in porcine aortic endothelial cells and the immortalized neuronal cell line 1RB3AN27. In primary cultures of rat embryonic midbrain neurons, DAT G585A, K590A, and D600A mutants were restricted to the cell soma and did not traffic to the dendrites or axonal processes. These data are consistent with the model whereby the local conformation and/or intramolecular interactions of the sequences of the DAT carboxyl terminus proximal to the last transmembrane domain are essential for the ER export of the transporter.

Alanine↗

PubMed2003

Use of a fluorescent maleimide to probe structure-function relationships in stalk segments 4 and 5 of the yeast plasma-membrane H+-ATPase.

In the yeast plasma-membrane H(+)-ATPase and other P-type ATPases, conformational changes are transmitted between cytoplasmic and membrane-embedded domains via a stalk region composed of cytoplasmic extensions of membrane segments 2, 3, 4, and 5. The present study has used a fluorescent maleimide (Alexa-488) to probe Cys residues introduced into stalk segments 4 and 5 of the yeast enzyme. In the case of S5, Cys substitutions along one face led to a constitutive, 5- to 10-fold activation of the ATPase in the absence of glucose. Based on homology with SERCA Ca(2+)-ATPase, this face is likely to be buried in the interior of the protein, close to the P domain. Three Cys residues on the opposite face of S5 (A668C, S672C, and D676C) were accessible to Alexa-488 under all conditions tested. In addition, three other Cys residues at or near the boundary between the two faces reacted with Alexa-488 only (V665C, L678C) or preferentially (Y689C) in plasma membranes from glucose-metabolizing cells; this result provides the first direct evidence for a change in conformation of S5 during glucose activation. For stalk segment 4, site-directed mutagenesis gave no sign of a role in glucose-dependent regulation. Rather, substitutions at 13 consecutive positions along S4 caused kinetic changes consistent with a shift in equilibrium from E2 to E1. Four Cys residues along this stretch of S4 (Q357C, K362C, S364C, and S368C) reacted with Alexa-488, indicating that they are exposed to the aqueous medium as predicted in the SERCA-based structural model.

Amino Acid Sequence↗

PubMed2002

Stalk segment 5 of the yeast plasma membrane H(+)-ATPase. Labeling with a fluorescent maleimide reveals a conformational change during glucose activation.

Glucose is well known to cause a rapid, reversible activation of the yeast plasma membrane H(+)-ATPase, very likely mediated by phosphorylation of two or more Ser/Thr residues near the C terminus. Recent mutagenesis studies have shown that glucose-dependent activation can be mimicked constitutively by amino acid substitutions in stalk segment 5 (S5), an alpha-helical stretch connecting the catalytic part of the ATPase with transmembrane segment 5 (Miranda, M., Allen, K. E., Pardo, J. P., and Slayman, C. W. (2001) J. Biol. Chem. 276, 22485-22490). In the present work, the fluorescent maleimide Alexa-488 has served as a probe for glucose-dependent changes in the conformation of S5. Experiments were carried out in a "3C" version of the ATPase, from which six of nine native cysteines had been removed by site-directed mutagenesis to eliminate background labeling by Alexa-488. In this construct, three of twelve cysteines introduced at various positions along S5 (A668C, S672C, and D676C) reacted with the Alexa dye in a glucose-independent manner, as shown by fluorescent labeling of the 100 kDa Pma1 polypeptide and by isolation and identification of the corresponding tryptic peptides. Especially significant was the fact that three additional cysteines reacted with Alexa-488 more rapidly (Y689C) or only (V665C and L678C) in plasma membranes from glucose-metabolizing cells. The results support a model in which the S5 alpha-helix undergoes a significant change in conformation to expose positions 665, 678, and 689 during glucose-dependent activation of the ATPase.

Amino Acid Sequence↗

PubMed2002

The KlTrk1 gene encodes a low affinity transporter of the K+ uptake system in the budding yeast Kluyveromyces lactis.

Potassium uptake in Saccharomyces cerevisiae is mediated by at least two proteins, known as Trk1p and Trk2p. Direct involvement in cation movements has been demonstrated for Trk1p, which is the high affinity transporter. S. cerevisiae cells also show low affinity potassium uptake, perhaps mediated by Trk2p. Mutants lacking Trk1p, lose high affinity system, but when grown with moderate potassium concentrations, Trk2p seems to replace it. Mutants lacking both proteins are viable but require at least 10 mM K(+) in the medium to sustain growth. Here we report the cloning and characterization of a gene from Kluyveromyces lactis encoding a homologue of these two proteins. KlTrkp is a 1070 amino acid peptide that shows, overall, higher homology with Trk2p than with Trk1p, and its disruption gives rise to cells with deficient potassium transport and with an increased K(+) requirement for normal growth. Determination of kinetic parameters in the K. lactis wild-type and Kltrk1Delta strains, as well as in Sctrk1Delta Sctrk2Delta S. cerevisiae cells expressing KlTrk1, indicated that this is a low affinity component of a major potassium uptake system in K. lactis.

Amino Acid Sequence↗