Homeodomain-interacting protein kinase-2 phosphorylates p53 at Ser 46 and mediates apoptosis

Phosphorylation of p53 at Ser 46 was shown to regulate p53 apoptotic activity. Here we demonstrate that homeodomain-interacting protein kinase-2 (HIPK2), a member of a novel family of nuclear serine/threonine kinases, binds to and activates p53 by directly phosphorylating it at Ser 46. HIPK2 localizes with p53 and PML-3 into the nuclear bodies and is activated after irradiation with ultraviolet. Antisense inhibition of HIPK2 expression reduces the ultraviolet-induced apoptosis. Furthermore, HIPK2 and p53 cooperate in the activation of p53-dependent transcription and apoptotic pathways. These data define a new functional interaction between p53 and HIPK2 that results in the targeted subcellular localization of p53 and initiation of apoptosis.

Regulation of p53 activity by its interaction with homeodomain-interacting protein kinase-2

Transcriptional activity of p53, a central regulatory switch in a network controlling cell proliferation and apoptosis, is modulated by protein stability and post-translational modifications including phosphorylation and acetylation. Here we demonstrate that the human serine/threonine kinase homeodomain-interacting protein kinase-2 (HIPK2) colocalizes and interacts with p53 and CREB-binding protein (CBP) within promyelocytic leukaemia (PML) nuclear bodies. HIPK2 is activated by ultraviolet (UV) radiation and selectively phosphorylates p53 at Ser 46, thus facilitating the CBP-mediated acetylation of p53 at Lys 382, and promoting p53-dependent gene expression. Accordingly, the kinase function of HIPK2 mediates the increased expression of p53 target genes, which results in growth arrest and the enhancement of UV-induced apoptosis. Interference with HIPK2 expression by antisense oligonucleotides impairs UV-induced apoptosis. Our results imply that HIPK2 is a novel regulator of p53 effector functions involved in cell growth, proliferation and apoptosis.

Homeodomain interacting protein kinase 2 promotes apoptosis by downregulating the transcriptional corepressor CtBP

Genetic knockout of the transcriptional corepressor CtBP in mouse embryo fibroblasts upregulates several genes involved in apoptosis. We predicted, therefore, that a propensity toward apoptosis might be regulated through changes in cellular CtBP. To identify pathways involved in this regulation, we screened a mouse embryo cDNA library with an E1A-CtBP complex and identified the homeodomain interacting protein kinase 2 (HIPK2), which had previously been linked to UV-directed apoptosis through its ability to phosphorylate p53. Expression of HIPK2 or exposure to UV irradiation reduced CtBP levels via a proteosome-mediated pathway. The UV effect was prevented by coexpression of kinase-inactive HIPK2 or reduction in HIPK2 levels via siRNA. Mutation of the residue phosphorylated by HIPK2 prevented UV- and HIPK2-directed CtBP clearance. Finally, reduction in CtBP levels, either by genetic knockout or siRNA, promoted apoptosis in p53-deficient cells. These findings provide a pathway for UV-induced apoptosis in cells lacking p53.

The specificity of the CRM1-Rev nuclear export signal interaction is mediated by RanGTP

Nuclear export of intron-containing human immunodeficiency virus type 1 (HIV-1) RNA is mediated by the viral Rev protein that contains both an RNA binding domain specific for the viral Rev response element (RRE) and a nuclear export signal (NES). The cellular CRM1 (Exportin1) protein functions as a nuclear export receptor for proteins carrying a Rev-like NES in a process that also requires the GTP bound form of the Ran GTPase. Using purified recombinant factors, we show by co-precipitation, gel mobility shift and protein footprinting assays that full-length Rev protein interacts directly with CRM1 in vitro independently of both the integrity of the characteristic leucine residues of the NES and the presence of the cytotoxin leptomycin B (LMB). Addition of RanGTP induces the formation of an RRE-Rev-CRM1-RanGTP complex that is sensitive to LMB, NES mutations, and Ran being charged with GTP. Within this complex, CRM1 is readily cross-linked to Cys89 near the NES of Rev. By protein footprinting, we demonstrate that the NES of Rev and two regions in CRM1 become inaccessible to endoproteinases upon binding suggesting that these regions are involved in protein-protein interactions. Our data are consistent with a model in which CRM1 is the nuclear export receptor for the Rev-RRE ribonucleoprotein complex and that RanGTP binds to a preformed Rev-CRM1 complex and specifies a functional interaction with the NES.

A DNA damage-regulated BRCT-containing protein, TopBP1, is required for cell survival

BRCA1 carboxyl-terminal (BRCT) motifs are present in a number of proteins involved in DNA repair and/or DNA damage-signaling pathways. Human DNA topoisomerase II binding protein 1 (TopBP1) contains eight BRCT motifs and shares sequence similarity with the fission yeast Rad4/Cut5 protein and the budding yeast DPB11 protein, both of which are required for DNA damage and/or replication checkpoint controls. We report here that TopBP1 is phosphorylated in response to DNA double-strand breaks and replication blocks. TopBP1 forms nuclear foci and localizes to the sites of DNA damage or the arrested replication forks. In response to DNA strand breaks, TopBP1 phosphorylation depends on the ataxia telangiectasia mutated protein (ATM) in vivo. However, ATM-dependent phosphorylation of TopBP1 does not appear to be required for focus formation following DNA damage. Instead, focus formation relies on one of the BRCT motifs, BRCT5, in TopBP1. Antisense Morpholino oligomers against TopBP1 greatly reduced TopBP1 expression in vivo. Similar to that of ataxia telangiectasia-related protein (ATR), Chk1, or Hus1, downregulation of TopBP1 leads to reduced cell survival, probably due to increased apoptosis. Taken together, the data presented here suggest that, like its putative counterparts in yeast species, TopBP1 may be involved in DNA damage and replication checkpoint controls.

Differential role of caspase-8 and BID activation during radiation- and CD95-induced apoptosis

Activation of the CD95 death receptor as well as ionizing radiation induces apoptotic cell death in human lymphoma cells. The activation of caspases is a hallmark of apoptosis induction irrespective of the apoptotic trigger. In contrast to death receptor signaling, the exact mechanisms of radiation-induced caspase activation are not well understood. We provide evidence that both, radiation and CD95 stimulation, induce the rapid activation of caspase-8 and BID followed by apoptosis in Jurkat T-cells. To analyse the relative position of caspase-8 within the apoptotic cascade we studied caspase activation and apoptosis in Jurkat cells overexpressing Bcl-2 or Bcl-xL. Caspase-8 activation, pro-apoptotic BID cleavage and apoptosis in response to radiation were abrogated in these cells, while the responses to CD95 stimulation were only partially attenuated by overexpression of Bcl-2 family members. In parallel, the breakdown of the mitochondrial transmembrane potential (DeltaPsim) in response to radiation was inhibited by overexpression of Bcl-2/Bcl-xL Jurkat cells genetically deficient for caspase-8 were found to be completely resistant towards CD95. However, radiation-induced apoptotic responses in caspase-8-negative cells displayed only a modest reduction. We conclude that ionizing radiation activates caspase-8 and BID downstream of mitochondrial damage suggesting that, in contrast to CD95, both events function as executioners rather than initiators of the apoptotic process.

Multiple photoreceptors mediate the light-induced reduction of GUS-COP1 from Arabidopsis hypocotyl nuclei

The subcellular localization of COP1, a key photomorphogenic repressor, is regulated by light in Arabidopsis seedlings. Photoreceptor loss-of-function mutants and dominant gain-of-function overexpression transgenes were both used to analyze the influences of the three photoreceptors, phyA, phyB, and CRY1, on the light-regulated subcellular localization of COP1. Through a semiquantitative analysis of the nuclear abundance of GUS-COP1 in the various genetic backgrounds, the specific roles of the individual photoreceptors have been established. The data suggest that multiple photoreceptors influence the light-regulated subcellular localization of COP1 in white light. Under specific wavelengths of light, phyA, phyB, and CRY1 each play critical roles in mediating far-red, red, and blue light signals, respectively. Our data also support an interdependency between CRY1 and the phytochromes in mediating the light-regulated subcellular localization of COP1 and thus seedling development.

Activation of JNK1, RSK2, and MSK1 is involved in serine 112 phosphorylation of Bad by ultraviolet B radiation

The Bcl-2 family member Bad is a pro-apoptotic protein, and phosphorylation of Bad by cytokines and growth factors promotes cell survival in many cell types. Induction of apoptosis by UV radiation is well documented. However, little is known about UV activation of cell survival pathways. Here, we demonstrate that UVB induces Bad phosphorylation at serine 112 in JNK1, RSK2, and MSK1-dependent pathways. Inhibition of mitogen-activated protein (MAP) kinases including ERKs, JNKs, and p38 kinase by the use of their respective dominant negative mutant or a specific inhibitor for MEK1 or p38 kinase, PD98059 or SB202190, resulted in abrogation of UVB-induced phosphorylation of Bad at serine 112. Incubation of active MAP kinase members with Bad protein showed serine 112 phosphorylation of Bad by JNK1 only. However, activated RSK2 and MSK1, downstream kinases of ERKs and p38 kinase, respectively, also phosphorylated Bad at serine 112 in vitro. Cells from a Coffin-Lowry syndrome patient (deficient in RSK2) or expressing an N-terminal or C-terminal kinase-dead mutant of MSK1 were defective for UVB-induced serine 112 phosphorylation of Bad. Furthermore, MAP kinase pathway-dependent serine 112 phosphorylation was shown to be required for dissociation of Bad from Bcl-X(L). These data illustrated that UVB-induced phosphorylation of Bad at serine 112 was mediated through MAP kinase signaling pathways in which JNK1, RSK2, and MSK1 served as direct mediators.

Radiolytic footprinting. Beta rays, gamma photons, and fast neutrons probe DNA-protein interactions

Ionizing radiations induce numerous damages in DNA, especially strand breaks. The hydroxyl radical OH., produced by the radiolysis of water, is mainly responsible for this effect. The fact that strand breakage occurs at all nucleotides and that bound proteins may locally radioprotect DNA at the binding site lead us to develop a radiolytic footprinting method to study DNA-protein interactions. Three different radiations were used: beta rays, gamma photons, and fast neutrons. In order to validate this technique, three well-known interaction systems were tested: the lac repressor-lac operator of Escherichia coli, the cyclic AMP receptor protein (CRP) of E. coli and its specific site in the lac regulation region, and the core nucleosome. Radiolytic footprinting gives results similar to those obtained by more classical probes: DNase I, complexes of orthophenanthroline (OP) and copper, complexes of ethylenediaminetetraacetate ion (EDTA) and iron, and UV light. For the same system (lac repressor), irradiation with either gamma photons or fast neutrons gives identical results.

Phosphorylation of 4E-BP1 is mediated by the p38/MSK1 pathway in response to UVB irradiation

In resting cells, eIF4E-binding protein 1 (4E-BP1) binds to the eukaryotic initiation factor-4E (eIF-4E), preventing formation of a functional eIF-4F complex essential for cap-dependent initiation of translation. Phosphorylation of 4E-BP1 dissociates it from eIF-4E, relieving the translation block. Studies suggested that insulin- or growth factor-induced 4E-BP1 phosphorylation is mediated by phosphatidylinositol 3-kinase (PI3-kinase) and its downstream protein kinase, Akt. In the present study we demonstrated that UVB induced 4E-BP1 phosphorylation at multiple sites, Thr-36, Thr-45, Ser-64, and Thr-69, leading to dissociation of 4E-BP1 from eIF-4E. UVB-induced phosphorylation of 4E-BP1 was blocked by p38 kinase inhibitors, PD169316 and SB202190, and MSK1 inhibitor, H89, but not by mitogen-activated protein kinase kinase inhibitors, PD98059 or U0126. The PI3-kinase inhibitor, wortmannin, did not block UVB-induced 4E-BP1 phosphorylation, but blocked both UVB- and insulin-induced activation of PI3-kinase and phosphorylation of Akt. 4E-BP1 phosphorylation was blocked in JB6 Cl 41 cells expressing a dominant negative p38 kinase or dominant negative MSK1, but not in cells expressing dominant negative ERK2, JNK1, or PI3-kinase p85 subunit. Our results suggest that UVB induces phosphorylation of 4E-BP1, leading to the functional dissociation of 4E-BP1 from eIF-4E. The p38/MSK1 pathway, but not PI3-kinase or Akt, is required for mediating the UVB-induced 4E-BP1 phosphorylation.

Light-regulated, tissue-specific immunophilins in a higher plant

In addition to their application in organ transplantation, immunosuppressive drugs are valuable tools for studying signal transduction in eukaryotic cells. Using affinity chromatography, we have purified immunosuppressive drug receptors (immunophilins) from fava bean. Proteins belonging to both major classes of the immunophilin family identified from animal sources [FK506- and rapamycin-binding proteins (FKBPs) and cyclophilins] were present in this higher plant. FKBP13, the most abundant FKBP family member in leaf tissues, was not detected in root tissues, whereas other FKBPs were present in both tissues. While the abundance of cyclophilin A in leaves was similar to that in roots, cyclophilin B/C was expressed at a much higher level in leaf tissues than in root tissues. Subcellular localization of immunophilins in mesophyll cells showed that chloroplasts contained FKBP13 and cyclophilin B/C but not other members, which explains the preferential expression of these two proteins in leaves over roots. The abundance of chloroplast-localized immunophilins, FKBP13 and cyclophilin B/C, was regulated by light. Although etiolated leaves produced detectable levels of cyclophilin B/C, they did not express FKBP13. Illumination of etiolated plants dramatically increased the expression of both FKBP13 and cyclophilin B/C. The light-induced expression of FKBP13 is closely correlated with the accumulation of chlorophyll in the leaf tissue. Our findings suggest that FKBP13 and cyclophilin B/C may play a specific role in chloroplasts.

CP43', the isiA gene product, functions as an excitation energy dissipator in the cyanobacterium Synechococcus sp. PCC 7942

Under conditions of iron deficiency certain cyanobacteria induce a chlorophyll (Chl)-binding protein, CP43', which is encoded by the isiA gene. We have previously suggested that CP43' functions as a nonradiative dissipator of light energy. To further substantiate its functional role an isiA overexpression construct was introduced into the genome of a cyanobacterium Synechococcus sp. PCC 7942 (giving isiAoe cells). The presence of functional CP43' in isiAoe cells was confirmed by Western blot as well as by the presence of a characteristic blueshift of the red Chl a absorption peak and a notable increase in the 77 K fluorescence peak at 685 nm. Compared to wild-type cells isiAoe cells, with induced CP43', had both smaller functional antenna size and decreased yields of room temperature Chl fluorescence at various light irradiances. These observations strongly suggest that isiAoe cells, with induced CP43', have an increased capacity for dissipating light energy as heat. In agreement with this hypothesis isiAoe cells were also more resistant to photoinhibition of photosynthesis than wild-type cells. Based on these results we have further strengthened the hypothesis that CP43' functions as a nonradiative dissipator of light energy, thus protecting photosystem II from excessive excitation under iron-deficient conditions.

MEK/ERK pathway protects ionizing radiation-induced loss of mitochondrial membrane potential and cell death in lymphocytic leukemia cells

MEK/ERK-mediated signals have recently been found to inhibit Fas-mediated cell death through inhibition of caspase-8 activity. It remains unknown whether MEK/ERK-mediated signals affect ionizing radiation (IR)-induced cell death. Here we demonstrate that MEK/ERK-mediated signals selectively inhibit IR-induced loss of mitochondrial membrane potential (DeltaPsi(m)) and subsequent cell death. In Jurkat cells, TPA strongly activated ERK and inhibited the IR-induced caspase-8/Bid cleavage and the loss of DeltaPsi(m). The inhibitory effect of TPA was mostly abrogated by pretreatment of a specific MEK inhibitor PD98059, indicating that the effect depends upon MEK/ERK-mediated signals. Moreover, BAF-B03 transfectants expressing IL-2 receptor (IL-2R) beta(c) chain lacking the acidic region, which is responsible for MEK/ERK-mediated signals, revealed higher sensitivity to IR than the transfectants expressing wild-type IL-2R. Interestingly, the signals could neither protect the DeltaPsi(m) loss nor cell death in UV-irradiated cells. These data imply that the anti-apoptotic effect of MEK/ERK-mediated signals appears to selectively inhibit the IR-induced cell death through protection of the DeltaPsi(m) loss. Our data enlighten an anti-apoptotic function of MEK/ERK pathway against IR-induced apoptosis, thereby implying its contribution to radioresistance.

Purification of the Golgi adenosine 3'-phosphate 5'-phosphosulfate transporter, a homodimer within the membrane

Sulfation of proteoglycans, secretory and membrane proteins, and glycolipids occurs in the lumen of the Golgi apparatus. Adenosine 3'-phosphate 5'-phosphosulfate (PAPS), the sulfate donor in these reactions, must be transported from the cytosol, its site of synthesis, into the lumen of the Golgi apparatus. We have identified and purified to apparent homogeneity the rat liver Golgi membrane PAPS transporter by a combination of conventional and affinity chromatography as well as photoaffinity radiolabeling with adenosine 3',5'-bisphosphate, a competitive inhibitor of PAPS transport. The transporter, a 75-kDa protein, was purified 70,000-fold over homogenate (6% yield) and transported PAPS into phosphatidylcholine liposomes selectively and in a saturable manner (apparent Km of 1.7 microM). Radiation target-inactivation analyses of the transport activity in rat liver Golgi vesicles, together with the above described biochemical approaches, demonstrate that the PAPS transporter within the Golgi membrane is a homodimer.

Ligand-specific and non-specific in vivo modulation of human epidermal cellular retinoic acid binding protein (CRABP)

Retinoic acid (RA) is bound intracellularly by a specific, low molecular weight protein (CRABP), that is unrelated to its nuclear receptor and whose function and regulation are still unknown. In the present study we were able to obtain an in vivo modulation of CRABP by different stimuli in one of the major target organs of RA: the human skin. We found increased CRABP after daily application during 4 days of natural or synthetic retinoids (RA, acitretin, isotretinoin, Ro137410, retinol), that have either a high affinity to CRABP or can be transformed into RA. Only Ro150778 with no affinity and no reported transformation had no effect. No macro- or microscopical changes could be observed with any of the tested compounds. Induction of inflammatory and hyperproliferative changes in the skin by topical dithranol treatment, UVB irradiation or scotch tape stripping also induced a significant increase of CRABP 3 days after exposure. Topical diflucortolone showed not only a tendancy to decrease intrinsic CRABP levels, but significantly reduced the retinoid stimulated rise of CRABP. Thus we conclude that the increase of CRABP in a fully differentiated adult tissue seems to be a biological phenomenon following processes of inflammation and proliferation with a lag of several days, while retinoids seem to be able to induce such a rise independently of, or before, the appearance of such processes. Corticosteroids seem to be inhibitors of this reaction. We discuss the hypothesis that CRABP might function as an intracellular 'buffer' in the case of RA overload.

Photoaffinity labeling of the nucleotide-binding site of the uncoupling protein from hamster brown adipose tissue

The nucleotide binding center of the uncoupling protein from brown adipose tissue (UCP) was probed by photoaffinity labeling with 8-azido-ATP. The isolated dimeric UCP in non-ionic detergent was used. 8-azido-ATP binds to UCP with a Kd = 3 microM, i.e. with an only threefold lower affinity than ATP and a maximum number of binding sites of about 12 mumol/g protein corresponding to about 1 mol/mol dimer UCP. UCP is rapidly degraded by ultraviolet radiation, and therefore only near ultraviolet and visible light can be used for photoaffinity labeling. The total covalent incorporation is shown to be dependent on the concentration of azido-ATP and on competing phospholipids. The specific, i.e. ATP-sensitive incorporation only to the binding site depends on the presence of cysteine. With CNBr cleavage the 8-azido-[gamma-32P]ATP insertion within the primary structure was located by identifying ATP-sensitive labeled peptides in SDS/PAGE. A major specific 8-azido-ATP incorporation was found by autoradiography in the smallest CNBr fragments. Identification of the radioactive peptides was difficult since 8-azido-ATP insertion causes a distinct shift in the gels from the stained peptides. Identification was possible by specific disulfide formation at the C-terminal within the UCP dimer which only removed the CB7 (CB, CNBr fragment) portion of the low-molecular-mass peptides but did not move the radioactive band. This excludes the C-terminal CB7 and identifies the labeled peptide as CB6. Also, limited tryptic cleavage of intact UCP at Lys293 did not remove the radioactivity. Cleavage of tryptophanes support localization of 8-azido-ATP between residues 173-280 which includes CB6. Solid-phase sequencing of the labeled CB6 both after serine lactone and carboxyl coupling suggest incorporation into Thr260. These results indicate that the adenine-binding site is within the third domain of the tripartite UCP structure at a putative hydrophilic channel which can be assessed both from the cytosol and matrix of mitochondria.

Vanadate catalyzes photocleavage of adenylate kinase at proline-17 in the phosphate-binding loop

Irradiation of adenylate kinase (AK) from chicken muscle with 300-400-nm light in the presence of 0.25 mM vanadate ion first inactivated the enzyme and then cleaved the polypeptide chain near the NH2 terminus. The addition of the multisubstrate analogue, P1,P5-bis(5'-adenosyl) pentaphosphate, prevented both effects. ATP, but not AMP, blocked both inactivation and cleavage in a saturable manner, suggesting that both effects were due to modification at the ATP-binding site. The polypeptide products of the photocleavage were isolated by HPLC and characterized by amino acid composition, peptide sequencing, and mass spectral analyses. The predominant (greater than 90%) small peptide fragment contained the first 16 amino acids from the amino terminus of the enzyme. The amino terminus of this peptide contained an acetylated serine, and the "carboxy" terminus was modified by a cyclized gamma-aminobutyric acid which originated from photooxidation and decarboxylation of proline-17 by vanadate. Edman sequencing indicated that the majority of the large peptide fragment (Mr approximately 19,500) was amino-terminal blocked, but a small portion was sequenceable starting at either glycine-18 (7%) or serine-19 (2%). These studies indicate that in the ATP-AK complex proline-17 is close to the phosphate chain of ATP but not AMP, consistent with the latest evaluation of nucleotide-binding sites on mitochondrial matrix AK by X-ray crystallography [Diederichs, K., & Schulz, G.E. (1991) J. Mol. Biol. 217, 541-549]. Furthermore, this is the first report that an amino acid other than serine can be involved in vanadate-promoted photocleavage reactions.

Facilitation of Stress-induced Phosphorylation of β-Amyloid Precursor Protein Family Members by X11-like/Mint2 Protein

Beta-amyloid precursor protein (APP) is the precursor of beta-amyloid (Abeta), which is implicated in Alzheimer's disease pathogenesis. APP complements amyloid precursor-like protein 2 (APLP2), and together they play essential physiological roles. Phosphorylation at the Thr(668) residue of APP (with respect to the numbering conversion for the APP 695 isoform) and the Thr(736) residue of APLP2 (with respect to the numbering conversion for the APLP2 763 isoform) in their cytoplasmic domains acts as a molecular switch for their protein-protein interaction and is implicated in neural function(s) and/or Alzheimer's disease pathogenesis. Here we demonstrate that both APP and APLP2 can be phosphorylated by JNK at the Thr(668) and Thr(736) residues, respectively, in response to cellular stress. X11-like (X11L, also referred to as X11beta and Mint2), which is a member of the mammalian LIN-10 protein family and a possible regulator of Abeta production, elevated APP and APLP2 phosphorylation probably by facilitating JNK-mediated phosphorylation, whereas other members of the family, X11 and X11L2, did not. These observations revealed an involvement of X11L in the phosphorylation of APP family proteins in cellular stress and suggest that X11L protein may be important in the physiology of APP family proteins as well as in the regulation of Abeta production.

Structural state of the Na+/D-glucose cotransporter in calf kidney brush-border membranes. Target size analysis of Na+-dependent phlorizin binding and Na+-dependent D-glucose transport

Target sizes of the renal sodium-D-glucose cotransport system in brush-border membranes of calf kidney cortex were estimated by radiation inactivation. In brush-border vesicles irradiated at -50 degrees C with 1.5 MeV electron beams, sodium-dependent phlorizin binding, and Na+-dependent D-glucose tracer exchange decreased exponentially with increasing doses of radiation (0.4-4.4 Mrad). Inactivation of phlorizin binding was due to a reduction in the number of high-affinity phlorizin binding sites but not in their affinity. The molecular weight of the Na+-dependent phlorizin binding unit was estimated to be 230 000 +/- 38 000. From the tracer exchange experiments a molecular weight of 345 000 +/- 24 500 was calculated for the D-glucose transport unit. The validity of these target size measurements was established by concomitant measurements of two brush-border enzymes, alkaline phosphatase and gamma-glutamyltransferase, whose target sizes were found to be 68 570 +/- 2670 and 73 500 +/- 2270, respectively. These findings provide further evidence for the assumption that the sodium-D-glucose cotransport system is a multimeric structure, in which distinct complexes are responsible for phlorizin binding and D-glucose translocation.

Labelling of the human erythrocyte glucose transporter with 3H-labelled cytochalasin B occurs via protein photoactivation

Irradiation of human erythrocyte membranes with 3H-labelled cytochalasin B results in specific photolabelling of the glucose transporter. The action spectrum of photolabelling has a maximum at approx. 280 nm, whereas the absorption spectrum of cytochalasin B is maximal at 210 nm. By irradiating with narrow-band-width light centered at 280 nm for 2 h, 8% of the transporters become covalently labelled and 47% of the remaining cytochalasin B-binding sites are obliterated. We conclude that photolabelling driven by narrow-bandwidth irradiation proceeds via photoactivation of an aromatic amino acid residue on the transporter molecule, and when compared to wide-bandwidth irradiation, permits more efficient incorporation of the label without causing additional photodamage to the remaining transporters.

Identification of nuclear cap specific proteins in HeLa cells

Two polypeptides of apparent molecular mass of 20 and 115 kilodaltons in nuclear fractions from HeLa cells were shown to recognize and be crosslinked to the cap structure of eukaryotic mRNAs in a cap-dependent fashion. Crosslinking of the 20 and 115 kDa polypeptides was sensitive to inhibition by low concentrations of the cap analogue m7GDP and resistant to inhibition by high KCl concentrations. In addition, crosslinking of these polypeptides to the cap structure occurred in nuclear extracts prepared from poliovirus-infected cells, under conditions where cytoplasmic cap binding proteins were incapable of interacting with the mRNA cap structure. The possible function of nuclear cap binding proteins is discussed.

Identification by UV cross-linking of oligo(U)-binding proteins in mitochondria of the insect trypanosomatid Crithidia fasciculata

RNA editing in trypanosomes is the process of insertion and deletion of U residues at specific sites of mitochondrial transcripts mediated by short guide RNAs (gRNAs) that have a 3' oligo(U) extension. Here we describe the identification by UV cross-linking of proteins present in mitochondrial extracts from Crithidia fasciculata with a high affinity for gRNAs, and the characterization of the binding specificity. A 65-kDa protein binds to gRNAs provided they are equipped with a U tail, to post-transcriptionally labelled mitoribosomal 9S and 12S RNAs that also possess a 3' terminal stretch of U residues, and to free oligo(U) sequences with a minimal length of 23-29 nucleotides. It does not bind to a number of control RNAs, one of which has an internal U stretch of 13 residues. Poly(U), but not poly(C) or total yeast RNA, efficiently competes for binding to gRNA. Proteins of 88 kDa and 30 kDa also bind to gRNAs with a U tail, to mitochondrial ribosomal RNAs and to oligo(U). These proteins, however, require longer oligo(U) for binding (> 39 nucleotides) and they also have an affinity for other U-rich RNAs and poly(C). For comparison, part of the analysis was also carried out with a mitochondrial extract from Trypanosoma brucei. In this organism, gRNA-binding proteins of 83 kDa and 64 kDa were found with the same preference for 3'-terminal oligomeric U stretches as the C. fasciculata 65-kDa protein, whereas the binding specificity of a 26-kDa protein resembled that of the C. fasciculata 88-kDa and 30-kDa proteins. The possible involvement of the proteins in the editing process is discussed.

Isolation and characterization of a retinal-binding protein from the squid retina

A retinal-binding protein (RALBP) was isolated from the squid retina, and purified by anion-exchange and size-exclusion chromatography. The molecular weight was determined to be 51,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by gel filtration. The purified sample showed absorption maxima at about 330 and 400 nm in addition to a protein band, indicating the occurrence of retinol and retinal, respectively. The relative heights of these two peaks varied from preparation to preparation, depending on retinoid ligands. Irradiation of RALBP caused no marked change in absorption, but the amount of 11-cis-retinal decreased to form a photosteady state mixture with all-trans- and 13-cis-retinals. RALBP was fairly stable even in the presence of hydroxylamine (100 mM), but was affected by sodium borohydride (30 mM) or borane dimethylamine (400 mM), with the retinal reduced to retinol. When incubated with metaretinochrome-carrying membranes in the dark, RALBP specifically took up 11-cis-retinal and lost all-trans-retinol. Upon further incubation of this RALBP with opsin-containing membranes, rhodopsin was progressively formed in the dark. Squid RALBP may act as a shuttle in transferring the 11-cis-retinal from metaretinochrome to opsin in the visual cells.

Biologic factors and response to radiotherapy in carcinoma of the cervix

Ionizing radiation has been used to treat cancers for a century. However, radioresistance remains a major problem in the clinic. Recent advances in the understanding of the molecular events that occur following ionizing radiation leading to DNA damage and repair, apoptosis, and cell cycle arrests suggest new ways in which the radiation response might be manipulated. Seventy-eight cases of carcinoma of the cervix of the same stage (II A and B) were analyzed retrospectively. All patients were treated with radiotherapy (RT) with a dose varying from 35 Gy to 50 Gy with 200 cGy per fraction. Subsequent to the completion of radiotherapy, all patients underwent surgery 4-6 weeks later. On histological examination of the surgical specimens, 51% of the cases (40) showed a complete response to therapy with no viable tumor cells. 49% of cases (38) had residual tumors ranging from a small focus to lesions extending through more than half the thickness of the cervical wall. p53 (mutant), bcl-2, p21 and bax proteins were studied on the paraffin sections of the biopsies (pretreatment) of those patients who failed to respond to RT and compared to similar studies on biopsies of patients who had a complete response to RT. In addition, the minichromosome maintenance (MCM) 2 proliferative marker was also done on all cases. Expression of all proteins was done using immunohistochemsitry. In the radioresistant cases, 15% (six cases) showed positivity for bcl-2 and p21, respectively, and 34% (13 cases) showed mutant p53. None of the radiosensitive tumors were positive for the above proteins. 75% of the radiosensitive tumors (30 cases) were positive for the bax antibody, whereas 81% of the radioresistant tumors (31 cases) were negative for bax. The MCM2 proliferative marker was positive in > 80% of cells in 81.5% of radioresistant tumors (31 cases) as compared to < 40% of cells that were positive in 70% of radiosensitive tumors (28 cases). The P-value for the biological markers was calculated using the chi-squared test, and was highly significant (P < 0.01) for all the parameters tested. However, there was no statistical significance by univariate analysis when the dose of radiation was analyzed with respect to the markers and the histological response. There was also no correlation between the radiation response and timing of surgery. The above data strongly suggest that bax, along with proliferative markers, could play a role in determining which tumors are likely to respond to radiation therapy. The presence of bcl-2, p21 and p53 could also be related to radioresistance of the tumors.