Biomedical subjects

P Jaunin

Publications and source records attributed to P Jaunin.

10 recordsLinked to original sources

PubMed1997

Role of glycosylation and disulfide bond formation in the beta subunit in the folding and functional expression of Na,K-ATPase.

Initial folding is a prerequisite for subunit assembly in oligomeric proteins. In this study, we have compared the role of co-translational modifications in the acquisition of an assembly-competent conformation of the beta subunit, the assembly of which is required for the structural and functional maturation of the catalytic Na,K-ATPase alpha subunit. Cysteine or asparagine residues implicated in disulfide bond formation or N-glycosylation, respectively, in the Xenopus beta1 subunit were eliminated by site-directed mutagenesis, and the assembly efficiency of the mutants and the functional expression of Na+,K+ pumps were studied after expression in Xenopus oocytes. Our results show that lack of each one of the two most C-terminal disulfide bonds indeed permits short term but completely abolishes long term assembly of the beta subunit. On the other hand, lack of the most N-terminal disulfide bonds allows the expression of a small number of functional Na+,K+ pumps at the cell surface. Elimination of all three but not of one or two glycosylation sites produces beta subunits that remain stably expressed in the endoplasmic reticulum, in association with binding protein but not as irreversible aggregates. The assembly efficiency of nonglycosylated beta subunits is decreased but a reduced number of functional Na+,K+ pumps is expressed at the cell surface. The lack of sugars does not influence the apparent K+ or ouabain affinity of the Na+,K+ pumps. Thus, these data show that disulfide bond formation and N-glycosylation may play important but qualitatively distinct roles in the initial folding of oligomeric protein subunits. Moreover, the results suggest that an endoplasmic reticulum degradation pathway exists, which is glycosylation-dependent.

Animals↗

PubMed1996

Oligomerization and maturation of Na,K-ATPase: functional interaction of the cytoplasmic NH2 terminus of the beta subunit with the alpha subunit.

Subunit assembly plays an essential role in the maturation of oligomeric proteins. In this study, we have characterized the main structural and functional consequences of the assembly of alpha and beta subunits of Na,K-ATPase. Xenopus oocytes injected with alpha and/or beta cRNA were treated with brefeldin A, which permitted the accumulation of individual subunits or alpha-beta complexes in the ER. Only alpha subunits that are associated with beta subunits become resistant to trypsin digestion and cellular degradation. Similarly, assembly with beta subunits is necessary and probably sufficient for the catalytic alpha subunit to acquire its main functional properties at the level of the ER, namely the ability to adopt different ligand-dependent conformations and to hydrolyze ATP in an Na(+)- and K(+)-dependent, ouabain-inhibitable fashion. Not only the alpha but also the beta subunit undergoes a structural change after assembly, which results in a global increase in its protease resistance. Furthermore, extensive and controlled proteolysis assays on wild-type and NH2-terminally modified beta subunits revealed a K(+)-dependent interaction of the cytoplasmic NH2 terminus of the beta subunit with the alpha subunit, which is likely to be involved in the modulation of the K(+)-activation of the Na,K-pump transport activity. Thus, we conclude that the ER assembly process not only establishes the basic structural interactions between individual subunits, which are required for the maturation of oligomeric proteins, but also distinct, functional interactions, which are involved in the regulation of functional properties of mature proteins.

Amino Acid Sequence↗

PubMed1994

Modulation of the Na,K-pump function by beta subunit isoforms.

To study the role of the Na,K-ATPase beta subunit in the ion transport activity, we have coexpressed the Bufo alpha 1 subunit (alpha 1) with three different isotypes of beta subunits, the Bufo Na,K-ATPase beta 1 (beta 1NaK) or beta 3 (beta 3NaK) subunit or the beta subunit of the rabbit gastric H,K-ATPase (beta HK), by cRNA injection in Xenopus oocyte. We studied the K+ activation kinetics by measuring the Na,K-pump current induced by external K+ under voltage clamp conditions. The endogenous oocyte Na,K-ATPase was selectively inhibited, taking advantage of the large difference in ouabain sensitivity between Xenopus and Bufo Na,K pumps. The K+ half-activation constant (K1/2) was higher in the alpha 1 beta 3NaK than in the alpha 1 beta 1NaK groups in the presence of external Na+, but there was no significant difference in the absence of external Na+. Association of alpha 1 and beta HK subunits produced active Na,K pumps with a much lower apparent affinity for K+ both in the presence and in the absence of external Na+. The voltage dependence of the K1/2 for external K+ was similar with the three beta subunits. Our results indicate that the beta subunit has a significant influence on the ion transport activity of the Na,K pump. The small structural differences between the beta 1NaK and beta 3NaK subunits results in a difference of the apparent affinity for K+ that is measurable only in the presence of external Na+, and thus appears not to be directly related to the K+ binding site. In contrast, association of an alpha 1 subunit with a beta HK subunit results in a Na,K pump in which the K+ binding or translocating mechanisms are altered since the apparent affinity for external K+ is affected even in the absence of external Na+.

Animals↗

PubMed1994

Rapid detection and identification of Candida albicans and Torulopsis (Candida) glabrata in clinical specimens by species-specific nested PCR amplification of a cytochrome P-450 lanosterol-alpha-demethylase (L1A1) gene fragment.

PCR of a Candida albicans cytochrome P-450 lanosterol-alpha-demethylase (P450-L1A1) gene segment is a rapid and sensitive method of detection in clinical specimens. This enzyme is a target for azole antifungal action. In order to directly detect and identify the clinically most important species of Candida, we cloned and sequenced 1.3-kbp fragments of the cytochrome P450-L1A1 genes from Torulopsis (Candida) glabrata and from Candida krusei. These segments were compared with the published sequences from C. albicans and Candida tropicalis. Amplimers for gene sequences highly conserved throughout the fungal kingdom were first used; positive PCR results were obtained for C. albicans, T. glabrata, C. krusei, Candida parapsilosis, C. tropicalis, Cryptococcus neoformans, and Trichosporon beigelii DNA extracts. Primers were then selected for a highly variable region of the gene, allowing the species-specific detection from purified DNA of C. albicans, T. glabrata, C. krusei, and C. tropicalis. The assay sensitivity as tested for C. albicans in seeded clinical specimens such as blood, peritoneal fluid, or urine was 10 to 20 cells per 0.1 ml. Compared with results obtained by culture, the sensitivity, specificity, and efficiency of the species-specific nested PCR tested with 80 clinical specimens were 71, 95, and 83% for C. albicans and 100, 97, and 98% for T. glabrata, respectively.

Amino Acid Sequence↗

PubMed1993

Hydrophobic C-terminal amino acids in the beta-subunit are involved in assembly with the alpha-subunit of Na,K-ATPase.

To define the structural basis of oligomerization for the alpha- and beta-subunits of Na,K-ATPase, we have attempted to identify the amino acids in the C-terminus of the beta-subunit that are involved in subunit assembly. We predicted that the last 10 amino acids form a beta-strand-like structure exposing on one side a hydrophilic and on the other side a continuous hydrophobic domain. The relative importance of the two domains in assembly was probed by introducing point mutations in either domain of Xenopus beta 3-subunits and by testing the ability of these mutants to stabilize newly synthesized alpha-subunits expressed in Xenopus oocytes and to form functional alpha-beta complexes at the plasma membrane. All single and double mutants with changes at R268 and/or K272 to either uncharged or negatively charged amino acids associated with coexpressed alpha-subunits and increased the number of ouabain binding sites and Rb uptake into oocytes. On the other hand, mutations affecting the hydrophobic amino acids influenced the assembly efficiency with alpha-subunits to a variable extent. The single mutants V269N and I275N did not influence and the mutant V273N slightly affected the assembly process. On the other hand, the cellular accumulation of alpha-subunits and the expression of functional Na,K pumps was considerably reduced with the mutant F271N and totally abolished with the double mutant V269N/F271N. Finally, replacement of V269 and F271 or V273 and I275 with the less hydrophobic alanine also significantly decreased subunit assembly, which was no longer detectable after replacement of all four amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1993

Role of the transmembrane and extracytoplasmic domain of beta subunits in subunit assembly, intracellular transport, and functional expression of Na,K-pumps.

The ubiquitous Na,K- and the gastric H,K-pumps are heterodimeric plasma membrane proteins composed of an alpha and a beta subunit. The H,K-ATPase beta subunit (beta HK) can partially act as a surrogate for the Na,K-ATPase beta subunit (beta NK) in the formation of functional Na,K-pumps (Horisberger et al., 1991. J. Biol. Chem. 257:10338-10343). We have examined the role of the transmembrane and/or the ectodomain of beta NK in (a) its ER retention in the absence of concomitant synthesis of Na,K-ATPase alpha subunits (alpha NK) and (b) the functional expression of Na,K-pumps at the cell surface and their activation by external K+. We have constructed chimeric proteins between Xenopus beta NK and rabbit beta HK by exchanging their NH2-terminal plus transmembrane domain with their COOH-terminal ectodomain (beta NK/HK, beta HK/NK). We have expressed these constructs with or without coexpression of alpha NK in the Xenopus oocyte. In the absence of alpha NK, Xenopus beta NK and all chimera that contained the ectodomain of beta NK were retained in the ER while beta HK and all chimera with the ectodomain of beta HK could leave the ER suggesting that ER retention of unassembled Xenopus beta NK is mediated by a retention signal in the ectodomain. When coexpressed with alpha NK, only beta NK and beta NK/HK chimera assembled efficiently with alpha NK leading to similar high expression of functional Na,K-pumps at the cell surface that exhibited, however, a different apparent K+ affinity. beta HK or chimera with the transmembrane domain of beta HK assembled less efficiently with alpha NK leading to lower expression of functional Na,K-pumps with a different apparent K+ affinity. The data indicate that the transmembrane domain of beta NK is important for efficient assembly with alpha NK and that both the transmembrane and the ectodomain of beta subunits play a role in modulating the transport activity of Na,K-pumps.

Amino Acid Sequence↗

PubMed1993

Mutation of a conserved proline residue in the beta-subunit ectodomain prevents Na(+)-K(+)-ATPase oligomerization.

A highly conserved sequence motif (4 tyrosines and 1 proline: YYPYY) of the Na(+)-K(+)-adenosinetriphosphatase (ATPase) beta 1-subunit ectodomain has been mutagenized to study its possible role in alpha/beta-assembly and sodium pump function. Single as well as double tyrosine mutants (tyrosine to phenylalanine: Y to F) of Xenopus laevis beta 1-subunits are able to associate with alpha 1-subunits and form functional Na-K pumps at the plasma membrane that are indistinguishable from wild-type alpha 1, beta 1-Na-K pumps (as assessed by measurements of ouabain binding, 86Rb flux, Na-K pump current, and activation by external potassium). In contrast, a single proline mutation (proline to glycine: P244G) reduced by > 90% the proper assembly and function of Na(+)-K(+)-ATPase, despite a normal rate of synthesis and core glycosylation. Our data indicate that proline-244 plays a critical role in the proper folding of the beta-subunit and its ability to associate efficiently with the alpha 1-subunit in the endoplasmic reticulum.

Animals↗

PubMed1992

Processing, intracellular transport, and functional expression of endogenous and exogenous alpha-beta 3 Na,K-ATPase complexes in Xenopus oocytes.

The minimal functional Na,K-ATPase unit is composed of a catalytic alpha-subunit and a glycosylated beta-subunit. So far three putative beta-isoforms have been described, but only beta 1-isoforms have been identified clearly as part of a purified active enzyme complex. In this study we provide evidence that a putative beta 3-isoform might be the functional component of Xenopus oocyte Na,K-ATPase. beta 3-isoforms are expressed in the oocyte plasma membrane together with alpha-subunits, but beta 3-isoforms are synthesized to a lesser extent than alpha-subunits. The unassembled oocyte alpha-subunits accumulate in an immature trypsin-sensitive form most likely in the endoplasmic reticulum (ER). Injection of both beta 1- and beta 3-cRNA into oocytes abolishes the transport constraint of the oocyte alpha-subunit, renders it trypsin-resistant, and finally leads to an increased number of functional pumps at the plasma membrane. In addition, beta 3-isoforms as beta 1-isoforms depend on the concomitant synthesis of alpha-subunits to be able to leave the ER and to become fully glycosylated. Finally, alpha-beta 1 and alpha-beta 3 complexes expressed at the plasma membrane appear to have similar transport properties as assessed by ouabain binding, rubidium uptake, and electrophysiological measurements in oocytes coexpressing exogenous alpha 1- and beta 1- or beta 3-isoforms. Thus our data indicate that beta 3-isoforms have functional qualities similar to beta 1-isoforms. They can assemble and impose a structural reorganization to newly synthesized alpha-subunits which permits the exit from the ER and the expression of functional Na,K-pumps at the plasma membrane.

Animals↗

PubMed1991

The H,K-ATPase beta-subunit can act as a surrogate for the beta-subunit of Na,K-pumps.

Na,K-ATPase and H,K-ATPase are the only members of the P-type ATPases in which a glycosylated beta-subunit is part of the purified active enzyme. In this study, we have followed the synthesis and the posttranslational processing of the beta-subunit of H,K-ATPase (beta HK) in Xenopus oocytes injected with beta HK cRNA and have tested whether it can act as a surrogate for the beta-subunit of Na,K-ATPase (beta NaK) to support the functional expression of Na,K-pumps. In Xenopus oocytes, beta HK is processed from an Endo H-sensitive 51-kDa coreglycosylated form to an Endo H-resistant 71-kDa fully glycosylated form. Similar to beta NaK, beta HK can stabilize and increase the trypsin resistance of alpha-subunits of Na,K-ATPase (alpha NaK). Finally, expression of beta HK together with alpha NaK leads to an increased number of ouabain binding sites at the plasma membrane accompanied by an increased Rb+ uptake and Na,K-pump current. Our data suggest that beta HK, similar to beta NaK, can assemble to alpha NaK, support the structural maturation and the intracellular transport of catalytic alpha NaK, and ultimately form active alpha NaK-beta HK complexes with Na,K-pump transport properties.

Adenosine Triphosphatases↗

PubMed1991

Coexpression of alpha 1 with putative beta 3 subunits results in functional Na+/K+ pumps in Xenopus oocytes.

The active Na+/K+ pump is composed of an alpha and a beta subunit. Until now, three putative isoforms of the beta subunit have been identified that share sequence similarity. We have expressed the beta 1 and beta 3 isoforms of Xenopus laevis Na+/K(+)-ATPase in Xenopus oocytes to compare functional properties of the Na+/K+ pump, including either of these two isoforms. Na+/K+ pump current, estimated as K(+)-induced outward current in voltage-clamped oocytes, was doubled by coexpression of alpha 1 subunits with either isoform of the beta subunit compared to expression of alpha 1 subunits alone. The kinetics of activation by external K+ and the voltage dependence of the electrogenic activity of the Na+/K+ pump were similar with both beta isoforms, indicating that both beta 1 and beta 3 isoforms can support expression at the oocyte surface of an active Na+/K+ pump with similar functional properties.

Animals↗