Determination of the sidedness of the carboxy-terminus of the Na+/K(+)ATPase alpha-subunit using lactoperoxidase iodination

State of oncomarker protein B23/nucleophosmin in HeLa cells

Western blot after SDS-PAGE for protein separation showed two immunoreactive bands corresponding to monomers (38-40 kDa) and oligomers (210-230 kDa) of nucleophosmin in HeLa cell lysates. Decreasing the buffer ionic strength during the incubation of cells and nuclei destabilized these oligomers. We also showed the existence of two B23/nucleophosmin pools in nuclei of HeLa cells with different sensitivity to hypotonic buffer treatment: one extractable from the nucleus and the other non-extractable and tightly bound to the nucleus. A detailed structural analysis of the extractable B23 pool was carried out: two closely related nucleophosmin isoforms (B23.1 and B23.2) were identified as a result of analysis of C-terminal amino acid sequences using carboxypeptidase hydrolysis; the N-termini of both isoforms are blocked by an acetyl group. As a result of sequencing of the deacetylated proteins, it has been established that the N-terminal amino acid sequence of nucleophosmin in these preparations is truncated by nine amino acid residues and the acetylated residue is Ser. The truncated monomer of nucleophosmin (represented only by the extractable part of the protein) on addition of magnesium ions to low ionic strength buffer or increase in buffer ionic strength was shown to form oligomers with molecular weights (210-230 kDa) similar to those revealed in the total cell lysate. It should be noted that the set of oligomers in this case differs from the one in total cell lysate. Our strategy of characterization of B23 forms for HeLa cells can be applied for other tumor cells.

[Antibodies to synthetic peptides for the detection of survivin in tumor tissues]

Survivin, an endogenous protein, is a promising marker for the diagnosis of cancer. The aim of the present work was to obtain antibodies specific to survivin and capable of detecting this protein in tumor tissues. Four peptides corresponding to fragments (1-22), (54-74), (80-88)-(153-165), and (118-144) of the survivin-2B sequence were selected and synthesized. Rabbits were immunized with the synthetic peptides. It has been shown that all peptides in a free state, without conjugation with a high-molecular-weight carrier, stimulate the production of antibodies capable of binding with recombinant survivin. Antipeptide antibodies were isolated from sera and their performance in the immunohistochemical detection of survivin in human tumor tissues was studied. It was shown that only antibodies to the (80-88)-(153-165) peptide bind to the survivin present in breast and bladder tumors. The ability of antibodies to this peptide to detect survivin in tumor tissue lysates was demonstrated by immunoblotting. The part of the sequence targeted by the antibodies against the (80-88)-(153-165) peptide was localized using truncated peptide fragments.

[Antibodies to synthetic fragments of nucleophosmin for the specific detection of its monomeric and oligomeric forms]

Immunoactive fragments corresponding to the N-terminal (19-36) and C-terminal (283-294) regions of the NPM1.1 isoform of nucleophosmin and their shortened fragments were chosen and synthesized. Rabbits were immunized with free full-size peptides and their protein conjugates. Antibodies produced against the 19-36 and 283-294 peptides were purified by affinity chromatography on bromocyanogen-activated sepharose that was preliminary conjugated with the synthetic peptides. An analysis of immunoblots of lysates of the HeLa and Ramos cells demonstrated that the antibodies produced against the 19-36 peptide detected the monomeric form of nucleophosmin, whereas the antibodies against the 283-294 peptide predominantly revealed its oligomeric form. It was established by immunocytochemical analysis that the antibodies induced by the 19-36 peptide stained the nucleoplasm and perinuclear space of the cytoplasm of the HeLa and Ramos cells, but did not stain the nucleoli, while the antibodies against the 283-294 peptide stained only the nucleoli of the same cells. On the basis of these results, one could propose that the monomeric and oligomeric forms of nucleophosmin were located in the nucleoplasm and nucleoli of the examined cells, respectively. Thus, antibodies which can predominantly detect monomeric and oligomeric forms of nucleophosmin were produced for the first time. An analysis of the monomeric-oligomeric state and the location of the nucleophosmin in tumor cells could be performed using these antibodies.

[Production of monoclonal antibodies to the prion protein and their characterization]

Antibodies to the prion protein (PrP), particularly, monoclonal antibodies, are necessary tools in the diagnostics and study of prion diseases and potential means of their immunotherapy. For the production of monoclonal antibodies, BALB/c mice were immunized by a recombinant bovine PrP. Three stable hybridomas producing antibodies of IgM class were prepared. The antibodies were bound to PrP in a solid-phase enzyme immunoassay and immunoblotting. The epitope mapping accomplished with the use of synthetic peptides showed that an epitope located in region 25-36 of PrP corresponds to one antibody, and epitopes located in region 222-229, to the other two. The antibodies to fragment 222-229 purified by affinity chromatography recognized with a high specificity conglomerates of a pathogenic prion in the brain tissue of cows suffering from spongiform encephalopathy. Thus, in nontransgenic mice, PrP-specific monoclonal antibodies were produced, useful in studies and diagnostics of prion diseases.

State of nucleolar proteins B23/nucleophosmin and UBF in HeLa cells during apoptosis induced by tumor necrosis factor

The structural state of two major nucleolar proteins, UBF and B23/nucleophosmin (both monomeric and oligomeric forms), was for the first time established in HeLa cells treated with apoptosis inducers: tumor necrosis factor (TNF-alpha), emetine, and their combination. The treatment of the cells with either TNF-alpha or emetine did not induce apoptosis and affect the state of UBF and nucleophosmin (both monomers and oligomers). Apoptosis was rather pronounced only if HeLa cells were treated with a mixture of TNF-alpha and emetine. States of the UBF and B23 proteins were analyzed in samples containing 25, 45, and 100% of cells with apoptotic nuclei. It was shown by immunoblotting that TNF-alpha-induced apoptosis of HeLa cells was associated with proteolysis of UBF and production of a 76-kD fragment, the content of which increased in correlation with the fraction of apoptotically changed cells. The N- and C-terminal amino acid sequences of UBF and its 76-kD fragment were characterized, and the site of the apoptosis-induced specific proteolysis was identified. As differentiated from UBF, protein B23 did not undergo proteolytic degradation during the TNF-alpha-induced apoptosis of HeLa cells and its content was unchanged even in the cell fraction with fragmentation of virtually all nuclei. However, the ratio between the monomeric and oligomeric states of B23 protein was changed in apoptotic cells, and apoptosis-specific forms of nucleophosmin were detected.

Structural analysis of the products of chymotryptic cleavage of the E1 form of Na,K-ATPase alpha-subunit: identification of the N-terminal fragments containing the transmembrane H1-H2 domain

Chymotryptic cleavage of the Na,K-ATPase in NaCl medium abolishes ATPase activity and alters other functional parameters. The structure of this modified enzyme is uncertain since only one product of selective proteolysis, the 83-kDa fragment of the alpha-subunit (Ala267-C-terminus) has been identified previously. Here, we applied additional tryptic digestion followed by oxidative cross-linking to identify the products originating from the N-terminal part of the alpha-subunit. These fragments start at Ala72 or Thr74 and contain the transmembrane H1-H2 domain. Formation of cross-linked product between alpha-fragments containing H1-H2 and H7-H10 demonstrate that the structural integrity of the membrane moiety is preserved. We also determined that secondary cleavage of the 83-kDa fragment leads to the formation of C-terminal 48-kDa alpha-fragments with multiple N-termini at Ile582, Ser583, Met584 and Ile585.

Transmembrane topology of alpha- and beta-subunits of Na+,K+-ATPase derived from beta-galactosidase fusion proteins expressed in yeast

Various models of the transmembrane topology of the Na+,K+-ATPase predict either 8 or 10 membrane spans for the alpha-subunit and one to three membrane spans for the beta-subunit. Structure/function analysis, however, requires precise knowledge about the folding of enzymes. Therefore, the intention of this work was to establish a transmembrane topology model for the subunits of Na+,K+-ATPase. The bacterial enzyme beta-galactosidase was fused to the C termini of truncated alpha- and beta-subunits of Na+,K+-ATPase. Fusions were generated at Arg60 (LTTAR60), Glu116 (AATEE116), Ala247 (VEGTA247), Leu311 (YTWEL311), Ala444 (VAGDA444), Ala789 (IFIIA789), Met809 (LGTDM809), Asp884 (RVTWD884), Ile946 (MKNKI946), and Arg972 (GVALR972) of the sheep alpha1-subunit and at Pro236 (LGGYP236) of the dog beta-subunit. The fusion constructs were expressed in yeast cells for studies on the localization of the fused reporter enzyme. Activity measurements of the reporter enzyme revealed that only intracellular fusion sites lead to active beta-galactosidase. Indirect immunofluorescence microscopy with cells expressing alpha1/beta-galactosidase and beta/beta-galactosidase hybrid proteins demonstrated that inactive beta-galactosidase is associated with the yeast plasma membrane and can be detected from the extracellular side. The data obtained suggest that Pro236 of the beta-subunit is located on the extracellular surface, corresponding to a model with one transmembrane segment, and that the alpha-subunit of the Na+,K+-ATPase consists of 10 membrane-associated spans. They also suggest that a stretch of the alpha1-subunit between membrane spans M7 and M8 might be hidden within the membrane, surrounded by the other hydrophobic spans, in analogy to the P-loop of Na+ or K+ channels and to the "hourglass" structure of water channels.

Epitope topology of Na,K-ATPase alpha subunit analyzed in basolateral cell membranes of rat kidney tubules

For topological analysis of integral membrane protein in situ, we used a novel immunoelectron microscopic technique, SDS-digested freeze-fracture replica labeling (SDS-FRL), and oligopeptide-specific antibodies to clarify the sidedness of Na,K-ATPase alpha subunit epitopes in basolateral cell membranes of kidney tubules. Unfixed tissue slices from rat kidney outer medulla were frozen with liquid helium, freeze-fractured, and replicated. After digestion with SDS to solubilize unfractured membranes and cytoplasm, the platinum/carbon replicas, along with attached cytoplasmic and exoplasmic membrane halves, were processed for immunocytochemistry. Immunogold labeling using antibodies against the N-terminus (Gly1-His13), Leu815-Gln828 and the C-terminus (Ile1002-Tyr1006) was superimposed on the images of the electron microscope protoplasmic fracture face of the basolateral plasma membranes, thus demonstrating cytoplasmic locations of these epitopes. On the contrary, SDS-FRL showed specific binding of Asn889-Gln903 to cross-fractured basolateral plasma membranes suggesting that this epitope is located on the extracellular side of the membrane.