hnRNPs H, H' and F behave differently with respect to posttranslational cleavage and subcellular localization

hnRNPs H, H' and F belong to a subfamily of the hnRNPs sharing a high degree of sequence identity. Eukaryotic expression and specific C-terminal antibodies were used to demonstrate great variation in the intracellular fate of the proteins. hnRNPs H and H' become posttranslational cleaved into C-terminal 35 kDa proteins (H(C), H'(C)) and possibly into N-terminal 22 kDa proteins. No detectable cleavage was observed for hnRNP F. hnRNP H/H' is almost exclusively localized to the nucleus of many cell types while hnRNP F varies from a predominant nuclear localization in some cells to a predominant cytoplasmic localization in other cells. The different fates may reflect differences in functional roles that so far only have included nuclear functions. The presence of significant quantities of hnRNP F in the cytoplasm of many cells indicates that it also may have a functional role here.

Psoriasis upregulated phorbolin-1 shares structural but not functional similarity to the mRNA-editing protein apobec-1

Earlier studies of psoriatic and normal primary keratinocytes treated with phorbol 12-myristate-1-acetate identified two low-molecular-weight proteins, termed phorbolin-1 (20 kDa; pI 6.6) and phorbolin-2 (17.6 kDa; pI 6.5). As a first step towards elucidating the role of these proteins in psoriasis, we report here the molecular cloning and chromosomal mapping of phorbolin-1 and a related cDNA that codes for a protein exhibiting a similar amino acid sequence. The phorbolins were mapped to position 22q13 immediately centromeric to the c-sis proto-oncogene. Transient expression of the phorbolin-1 cDNA in COS cells and by in vitro transcription/translation, yielded polypeptides that comigrated with phorbolins-1 and -2. Comparative sequence analysis revealed 22% overall identity and a similarity of 44% of the phorbolins to apobec-1, the catalytic subunit of the mammalian apolipoprotein B mRNA editing enzyme; however, recombinant-expressed phorbolin-1 exhibited no cytidine deaminase activity, using either a monomeric nucleoside or apolipoprotein B cRNA as substrate, and failed to bind an AU-rich RNA template. Whereas the precise function of the phorbolins remains to be elucidated, the current data suggest that it is unlikely to include a role in the post-transcriptional modification of RNA in a manner analogous to that described for apobec-1.

Human calumenin localizes to the secretory pathway and is secreted to the medium

Calumenin belongs to a family of multiple EF-hand proteins that include reticulocalbin, ERC-55, and Cab45. Reticulocalbin and ERC-55 localize to the ER due to a C-terminal HDEL retrieval signal. Cab45 contains a HEEF C-terminal sequence and is localized to the Golgi apparatus. The murine homologue of calumenin is reported to be present in the ER due to a C-terminal HDEF retrieval signal. The human homologue differs from the murine at 7 amino acid positions but the HDEF signal is conserved. However, in the cultured human cell lines, HaCaT keratinocytes, normal and transformed MRC-5 fibroblasts, as well as in transfected COS-1 cells, human calumenin could be demonstrated in the ER as well as in the Golgi complex. Especially in MRC-5 cells, a certain heterogeneity was observed, with some of the cells having calumenin localized solely to the ER while in other cells calumenin could be demonstrated in the ER as well as in the Golgi complex. Immunoelectron microscopy of placental syncytiotrophoblast cells showed that a substantial fraction of calumenin is localized in close association with the ER membrane. In addition, the protein may be recovered from the medium of cultured cells in an endoglycosidase H-resistant form, suggesting that the glycosylated protein has been further modified in the Golgi apparatus and secreted to the medium.

Molecular basis of indomethacin-human serum albumin interaction

Studies on the strength and extent of binding of the non-steroidal anti-inflammatory drug indomethacin to human serum albumin (HSA) have provided conflicting results. In the present work, the serum-binding of indomethacin was studied in 55 mM sodium phosphate buffer (pH 7.0) at 28 degrees C, by using a fluorescence quench titration technique. The interaction of indomethacin with human serum albumin has been studied as a function of temperature, ionic strength and pH. The results suggest that electrostatic interaction plays a major role in the binding. The possible role of lysine residues in this interaction was studied by modifying exposed and buried lysine residues of HSA with potassium cyanate and studying indomethacin binding with the modified HSA. The data suggest that the interaction takes place via a salt bridge formation between the carboxylate group of indomethacin and a buried lysine residue of HSA. A technique involving fluorescence enhancement of bilirubin upon its interaction with HSA was used to study its displacement by indomethacin. The displacement, although apparently competitive in nature, was not strong suggesting that the primary sites of interaction of bilirubin and indomethacin are different.

Evidence for an essential role of megalin in transepithelial transport of retinol

Transepithelial transport of retinol is linked to retinol-binding protein (RBP), which is taken up and also synthesized in a number of epithelia. By immunocytochemistry of human, rat, and mouse renal proximal tubules, a strong staining in apical endocytic vacuoles, lysosomes, endoplasmic reticulum, Golgi, and basal vesicles was observed, in accordance with luminal endocytic uptake as well as a constitutive synthesis and basal secretion of RBP. Analysis of mice with target disruption of the gene for the major endocytic receptor of proximal tubules, megalin, revealed no RBP in proximal tubules of these mice. Western blotting and HPLC of the urine of the megalin-deficient mice instead revealed a highly increased urinary excretion of RBP and retinol, demonstrating that glomerular filtered RBP-retinol of megalin-deficient mice escapes uptake by proximal tubules. A direct megalin-mediated uptake of purified RBP-retinol was indicated by surface plasmon resonance analysis and uptake in immortalized rat yolk sac cells. Uptake was partially inhibited by a polyclonal megalin antibody and the receptor-associated protein. The present data show that the absence of RBP-binding megalin causes a significantly increased loss of RBP and retinol in the urine, demonstrating a crucial role of megalin in vitamin A homeostasis.

An endocytic pathway essential for renal uptake and activation of the steroid 25-(OH) vitamin D3

Steroid hormones may enter cells by diffusion through the plasma membrane. However, we demonstrate here that some steroid hormones are taken up by receptor-mediated endocytosis of steroid-carrier complexes. We show that 25-(OH) vitamin D3 in complex with its plasma carrier, the vitamin D-binding protein, is filtered through the glomerulus and reabsorbed in the proximal tubules by the endocytic receptor megalin. Endocytosis is required to preserve 25-(OH) vitamin D3 and to deliver to the cells the precursor for generation of 1,25-(OH)2 vitamin D3, a regulator of the calcium metabolism. Megalin-/- mice are unable to retrieve the steroid from the glomerular filtrate and develop vitamin D deficiency and bone disease.

Structural basis for the recognition of carbohydrates by human galectin-7

Knowledge about carbohydrate recognition domains of galectins, formerly known as S-type animal lectins, is important in understanding their role(s) in cell-cell interactions. Here we report the crystal structure of human galectin-7 (hGal-7), in free form and in the presence of galactose, galactosamine, lactose, and N-acetyl-lactosamine at high resolution. This is the first structure of a galectin determined in both free and carbohydrate-bound forms. The structure shows a fold similar to that of the prototype galectins -1 and -2, but has greater similarity to a related galectin molecule, Gal-10. Even though the carbohydrate-binding residues are conserved, there are significant changes in this pocket due to shortening of a loop structure. The monomeric hGal-7 molecule exists as a dimer in the crystals, but adopts a packing arrangement considerably different from that of Gal-1 and Gal-2, which has implications for carbohydrate recognition.

Molecular cloning of a cDNA encoding human calumenin, expression in Escherichia coli and analysis of its Ca2+-binding activity

By microsequencing and cDNA cloning we have identified the transformation-sensitive protein No. IEF SSP 9302 as the human homologue of calumenin. The nucleotide sequence predicts a 315 amino acid protein with high identity to murine and rat calumenin. The deduced protein contains a 19 amino acid N-terminal signal sequence, 7 EF-hand domains and, at the C-terminus, a HDEF sequence which has been reported to function as retrieval signal to the ER. The calumenin transcript is ubiquitously expressed in human tissue, at high levels in heart, placenta and skeletal muscle, at lower levels in lung, kidney and pancreas and at very low levels in brain and liver. Calumenin belongs to a family of multiple EF-hand proteins that include the ER localized proteins reticulocalbin and ERC-55 and the Golgi localized Cab45. Since its Ca2+ binding may be important for the function of the protein we have used microdialysis experiments in order to analyse for the affinity and the capacity of recombinant human (rh) calumenin. All 7 EF-hands of the protein are functional and bind Ca2+, each with an affinity of 1.6x103 M-1. The relatively low affinity for the EF-hands may suggest a role for the protein in Ca2+-dependent processes in the ER.

Expression of recombinant psoriasis-associated fatty acid binding protein in Escherichia coli: gel electrophoretic characterization, analysis of binding properties and comparison with human serum albumin

The psoriasis-associated fatty acid binding protein (PA-FABP, also known as FABP5) is a novel keratinocyte protein that is highly up-regulated in psoriatic plaques (P. Madsen, H. H. Rasmussen, H. Leffers, B. Honoré and J. E. Celis, J. Invest. Dermatol. 1992, 99, 299-305). Here we have expressed PA-FABP in Escherichia coli as a fusion protein containing an NH2-terminal hexa-His tag followed by a factor Xa cleavage site. The recombinant protein was expressed at a level of about 30% of the soluble proteins and was purified to homogeneity using a simple two-step protocol consisting of affinity chromatography on Ni2+-nitrilotriacetic acid agarose followed by gel filtration. The recombinant protein was then digested with factor Xa and characterized by two-dimensional gel electrophoresis. The ability of PA-FABP to bind saturated fatty acids ranging from 6 to 16 carbons was determined directly by dialysis and compared to human serum albumin (HSA). The results showed that PA-FABP binds multiple molecules of the fatty acids hexanoate (C6:0), octanoate (C8:0), decanoate (C10:0) and laurate (C12:0), all with a K1 of about 10(4) M(-l), and myristate (C14:0) with a K1 of 4.4 X 10(5) M(-l). Palmitate (C16:0) also bound strongly with multiple molecules. Due to the very low solubility of palmitate its affinity to PA-FABP was measured relatively to HSA and found to be 8.1 times lower. At ligand/protein ratios below 1, all fatty acids bound to PA-FABP with about one to three orders of magnitude lower affinity than to HSA. The difference in the fatty acid binding properties of the two proteins may reflect differences in their three-dimensional structures, which in the case of PA-FABP consists mainly of beta-sheets while HSA contains predominantly alpha-helices.

Mannose 6-phosphate/insulin-like growth factor-II receptor targets the urokinase receptor to lysosomes via a novel binding interaction

The urokinase-type plasminogen activator receptor (uPAR) plays an important role on the cell surface in mediating extracellular degradative processes and formation of active TGF-beta, and in nonproteolytic events such as cell adhesion, migration, and transmembrane signaling. We have searched for mechanisms that determine the cellular location of uPAR and may participate in its disposal. When using purified receptor preparations, we find that uPAR binds to the cation-independent, mannose 6-phosphate/insulin-like growth factor-II (IGF-II) receptor (CIMPR) with an affinity in the low micromolar range, but not to the 46-kD, cation-dependent, mannose 6-phosphate receptor (CDMPR). The binding is not perturbed by uPA and appears to involve domains DII + DIII of the uPAR protein moiety, but not the glycosylphosphatidylinositol anchor. The binding occurs at site(s) on the CIMPR different from those engaged in binding of mannose 6-phosphate epitopes or IGF-II. To evaluate the significance of the binding, immunofluorescence and immunoelectron microscopy studies were performed in transfected cells, and the results show that wild-type CIMPR, but not CIMPR lacking an intact sorting signal, modulates the subcellular distribution of uPAR and is capable of directing it to lysosomes. We conclude that a site within CIMPR, distinct from its previously known ligand binding sites, binds uPAR and modulates its subcellular distribution.

Proteome profiling of bladder squamous cell carcinomas: identification of markers that define their degree of differentiation

One hundred fifty fresh bladder tumors were analyzed blindly by two-dimensional PAGE in combination with proteome identification techniques (microsequencing and mass spectrometry) and immunofluorescence of cryostat sections. Of these, six showed protein expression patterns corresponding to squamous cell carcinomas (SCCs). All tumors were already invasive at the time of presentation, and in most cases, the histopathological grade could not be determined with certainty. The more differentiated of the tumors included SCC 589-1, a lesion showing extensive keratinization, and 536-1, a pure SCC that resembled normal skin growing invasively into the muscle. Both tumors expressed keratins 5, 6, 10, 14, 16, 17, and 20, as well as the differentiation-associated proteins psoriasin, psoriasis-associated fatty acid-binding protein (PA-FABP), and galectin 7. SCC 589-1, however, exhibited higher levels of keratin 10, PA-FABP, and galectin 7 and, in addition, expressed keratins 13, 15, and 19, which were not detected in the pure SCC. Involucrin, glutathione S-transferase pi, stratifin (14-3-3 sigma), and the SCC antigen 1, on the other hand, were less abundant in SCC 589-1. In comparison, less-differentiated tumors did not express keratin 10 and were characterized by a decreased expression of keratin 14, psoriasin, PA-FABP, galectin 7, and stratifin (14-3-3 sigma). Indeed, two of these lesions (553-1 and 651-1) could be readily lined up in order of decreasing degree of differentiation based on the expression of these markers. The degree of differentiation of the other two SCCs could not be assessed with certainty because they may represent special cases (SCC 646-1, solid tumor; SCC 485-1, special differentiation pattern). All six SCCs externalized psoriasin to the urine, supporting the contention that this protein, alone or in combination with other polypeptides, may represent a useful marker for the early detection of these lesions.

Palmitate and stearate binding to human serum albumin. Determination of relative binding constants

Multiple binding equilibria of two apparently insoluble ligands, palmitate and stearate, to defatted human serum albumin were studied in a 66 mM sodium phosphate buffer (pH 7.4) at 37 degrees C, by determination of dialytic exchange rates of ligands among identical equilibrium solutions. The experimental data were analysed by a computerised curve fitting procedure using equilibrium equations for multiple binding of ligands, containing relative binding constants, valid whether the ligands are truly insoluble or are slightly soluble and irrespective of aggregation in aqueous solution. A best-fit set of relative binding constants was found, and subsequently 30 sets of acceptable constants for each set of data in order to evaluate the variation. The data were first fitted by the relative Scatchard's equation, then by the relative, stoichiometric equation. Scatchard's equation is deduced on the presumption that cooperativity is absent while the stoichiometric equation is valid even when cooperativity is present. It was found with palmitate as well as with stearate that the two equations fitted the data equally well, and it was concluded that the observations were compatible with absence of cooperativity. The relative Scatchard binding constants were converted to relative, stoichiometric constants and it was found that the variations of the latter were slight.

Human 2-D PAGE databases for proteome analysis in health and disease: http://biobase.dk/cgi-bin/celis

Human 2-D PAGE Databases established at the Danish Centre for Human Genome Research are now available on the World Wide Web (http://biobase.dk/cgi-bin/celis). The databanks, which offer a comprehensive approach to the analysis of the human proteome both in health and disease, contain data on known and unknown proteins recorded in various IEF and NEPHGE 2-D PAGE reference maps (non-cultured keratinocytes, non-cultured transitional cell carcinomas, MRC-5 fibroblasts and urine). One can display names and information on specific protein spots by clicking on the image of the gel representing the 2-D gel map in which one is interested. In addition, the database can be searched by protein name, keywords or organelle or cellular component. The entry files contain links to other databases such as Medline, Swiss-Prot, PIR, PDB, CySPID, OMIM, Methabolic pathways, etc. The on-line information is updated regularly.

Expression and divalent cation binding properties of the novel chemotactic inflammatory protein psoriasin

Psoriasin is a novel chemotactic inflammatory protein that possesses weak similarity to the S100 family members of Ca(2+)-binding proteins, and that is highly up-regulated in hyperproliferative psoriatic keratinocytes. Here we have used the psoriasin cDNA to express recombinant human (rh) psoriasin in Escherichia coli as a fusion protein containing a hexa His tag and a factor Xa cleavage site in the NH2-terminus. The protein was purified by affinity chromatography on Ni(2+)-nitrilotriacetic acid agarose, digested with factor Xa, further purified by ion-exchange chromatography and characterized by two-dimensional (2-D) gel electrophoresis and NH2-terminal sequencing. The ability of rh psoriasin to bind Ca2+, Zn2+, and Mg2+ was determined by dialysis experiments. We found that rh psoriasin may bind at least seven molecules of Ca2+ in KCl and several molecules in NaCl, with an affinity for the first bound molecule of 1.3-1.6 x 10(4) M-1. This indicates that psoriasin may cooperatively bind several molecules of Ca2+ when present in the extracellular space, or putatively, if localized in subcellular compartments where the concentration of Ca2+ is relatively high. At least eight molecules of Zn2+ were bound in KCl and four in NaCl, with an affinity just below 1 x 10(4) M-1 for the first molecule. Thus psoriasin does not bind significant amounts of Zn2+ at physiological concentrations. Mg2+ and Ca2+ are bound anti-cooperatively and binding of each of the ions (Ca2+, Zn2+, or Mg2+), is accompanied by conformational changes that move tyrosine residues to more hydrophobic areas.

The receptor-associated protein (RAP) binds calmodulin and is phosphorylated by calmodulin-dependent kinase II

The receptor-associated protein, RAP, is an intracellular protein that may function as a chaperone for the LDL-receptor family receptors. Here we report calmodulin as the first identified RAP binding protein outside of the LDL-receptor family members. We demonstrate that RAP binds calmodulin in a Ca2+- and pH-dependent manner characteristic of calmodulin-dependent enzymes, and present evidence that RAP is a substrate for calmodulin-dependent enzymes. Thus, CaM-kinase II and calcineurin readily phosphorylate and dephosphorylate, respectively, serine residues in RAP, and in the individual RAP domains D2 (amino acids 113-218) and D3 (amino acids 219-323) which both contain sites for CaM-kinase II-mediated phosphorylation and for calmodulin binding. In addition, we provide evidence that RAP is phosphorylated by other kinases such as casein kinase II. Studies of 32[ortho]P-labelled cell cultures demonstrate that RAP is phosphorylated in vivo. Our results suggest that RAP may have hitherto unknown functions implicating phosphorylation and calmodulin-mediated modulation.

Influence of fatty acids on the binding of warfarin and phenprocoumon to human serum albumin with relation to anticoagulant therapy

Warfarin and phenprocoumon binding to human serum albumin was studied by equilibrium dialysis. The first stoichiometric binding constant was 1.89 x 10(5) M-1 for warfarin and 2.40 x 10(5) M-1 for phenprocoumon. The affinity of warfarin was markedly increased on addition of up to 3 mol mol-1 albumin of palmitic, stearic, oleic or linoleic acids with energetic couplings for co-binding of one molecule of each of the fatty acids and one molecule of warfarin of 0.9, 1.1, 0.7 and 0.6 kJ mol-1, respectively. The affinity of phenprocoumon was only increased slightly on addition of palmitate with an energetic coupling of 0.3 kJ mol-1. Six consecutive serum samples were obtained from each of 14 patients undergoing surgery. The serum affinity of the drugs varied considerably corresponding to free drug concentrations between 0.7 and 2.7% for warfarin and between 0.8 and 4.9% for phenprocoumon. The affinity of warfarin but not of phenprocoumon was correlated to the increasing plasma fatty acid concentration. Anticoagulant therapy with phenprocoumon may thus be less sensitive than warfarin to changes in the fatty acid concentration of plasma.

Psoriasin: a novel chemotactic protein

Inflammatory skin disorders such as psoriasis show a preferential epidermal infiltration of neutrophils and T lymphocytes. This observation raises a question as to which factors determine the appearance and composition of leukocyte tissue infiltrations. Previously, we described a low molecular mass calcium-binding protein (psoriasin, molecular mass 11,457 Da, pI 6.77) belonging to the S1OO family that is highly upregulated in psoriatic keratinocytes and whose expression patterns implied a role in the inflammatory response. Here we report that human psoriasin is a potent and selective chemotactic inflammatory protein for CD4+ T lymphocytes and neutrophils at concentrations of about 10(-11) M. Psoriasin is not structurally related to the alpha or the beta chemokine subfamilies or to lymphotactin, a member of a newly described class of chemokines. Thus, we have observed a chemotactic protein outside the chemokine subfamilies that could be an important new inflammatory mediator.

Bladder squamous cell carcinomas express psoriasin and externalize it to the urine

PURPOSE

To report a single biomarker, psoriasin (Mr 11.0 kd, pI 6.2), a calcium binding protein which is expressed largely by stratified squamous epithelia and is externalized to the urine of bladder squamous cell carcinoma (SCC) bearing patients.

MATERIALS AND METHODS

Protein expression profiles of SCCs obtained immediately after surgery were analyzed by two-dimensional gel electrophoresis and Coomassie blue staining. Protein identity was determined by microsequencing and immunoblotting. Protein expression in cryosections was studied by immunofluorescence.

RESULTS

Four patients with SCC were identified from 100 samples of patients with suspected transitional cell carcinoma (TCC). The protein profiles of the 4 SCCs (56-1, grade III, T4; 181-1, grade I, T3; 219-1, grade III, T3 and 239-1, grade not determined, T2-4) resembled that of keratinocytes, suggesting that these cells express an early developmental pattern of gene expression. Besides expressing markers characteristic of keratinizing stratified squamous epithelia, the SCCs exhibited psoriasin, a protein externalized to the medium by keratinocytes. Immunohistochemistry of 3 of the SCCs with psoriasin antibodies showed that the positive cells were confined chiefly to the "squamous pearls." The presence of psoriasin in the urine of the 4 SCC patients was demonstrated by two-dimensional gel immunoblotting. Similar analysis of 43 urines from patients with bladder tumors other than SCC revealed 7 positives, some of which may reflect squamous differentiation. Analysis of the urine of 13 control individuals (12 males matched by age and a 42-year-old female) revealed 2 positives. Immunoblotting of the SCC patients' serum proteins with psoriasin antibodies failed to reveal the protein.

CONCLUSION

The results point towards psoriasin, alone or as part of a biomarker profile, as a potential marker for the noninvasive follow-up of patients with SCC.

Heterogeneous nuclear ribonucleoproteins H, H', and F are members of a ubiquitously expressed subfamily of related but distinct proteins encoded by genes mapping to different chromosomes

Molecular cDNA cloning, two-dimensional gel immunoblotting, and amino acid microsequencing identified three sequence-unique and distinct proteins that constitute a subfamily of ubiquitously expressed heterogeneous nuclear ribonucleoproteins corresponding to hnRNPs H, H', and F. These proteins share epitopes and sequence identity with two other proteins, isoelectric focusing sample spot numbers 2222 (37.6 kDa; pI 6.5) and 2326 (39.5 kDa; pI 6.6), indicating that the subfamily may contain additional members. The identity between hnRNPs H and H' is 96%, between H and F 78%, and between H' and F 75%, respectively. The three proteins contain three repeats, which we denote quasi-RRMs (qRRMs) since they have a remote similarity to the RNA recognition motif (RRM). The three qRRMs of hnRNP H, with a few additional NH2-terminal amino acids, were constructed by polymerase chain reaction amplification and used for ribohomopolymer binding studies. Each qRRM repeat bound poly(rG), while only the NH2-terminal qRRM bound poly(rC) and poly(rU). None of the repeats bound detectable amounts of poly(rA). The expression levels of hnRNPs H and F were differentially regulated in pairs of normal and transformed fibroblasts and keratinocytes. In normal human keratinocytes, the expression level of H was unaffected by treatment with several substances tested including two second messengers and seven cytokines. Likewise the expression level of F was independent of these substances, although it was strikingly down-regulated by long term treatment with 4 beta-phorbol 12-myristate 13-acetate, indicating that the protein kinase C signaling pathway regulates its expression. No effect of 4 beta-phorbol 12-myristate 13-acetate was observed on the expression of hnRNP H. The genes coding for hnRNPs H, H', and F were chromosome-mapped to 5q35.3 (HNRPH1), 6q25.3-q26, and/or Xq22 (HNRPH2) and 10q11.21-q11.22 (HNRPF), respectively.

The human keratinocyte two-dimensional gel protein database (update 1995): mapping components of signal transduction pathways

The master two-dimensional (2-D) gel database of human keratinocytes currently lists 3154 cellular proteins (2224 isoelectric focusing, IEF; and 930 nonequilibrium pH gradient electrophoresis, NEPHGE), many of which correspond to post-translational modifications. 1082 polypeptides have been identified (protein name, organelle components, etc.) using a procedure or a combination of procedures that include (i) comigration with known human proteins, (ii) 2-D gel immunoblotting using specific antibodies, (iii) microsequencing of Coomassie Brilliant Blue stained proteins, (iv) mass spectrometry, (v)vaccinia virus expression of full length cDNAs, and (vi) in vitro transcription/translation of full-length cDNAs. This year, special emphasis has been given to the identification of signal transduction components by using 2-D gel immunoblotting of crude keratinocyte lysates in combination with enhanced chemoluminescence (ECL) detection. Identified proteins are listed both in alphabetical order and with increasing SSP number, together with their M(r), pI, cellular localization and credit to the investigator(s) that aided in the identification. Ultimately, the aim of the comprehensive database is to gather--through a systematic study of ekeratinocytes--qualitative and quantitative information on proteins and their genes that may allow us to identify abnormal patterns of gene expression and to pinpoint signaling pathways and components affected in various skin diseases, cancer included.

Calcium ion binding to clinically relevant chemical modifications of human serum albumin

Calcium binding to glycated, penicilloylated, acetylated, and normal defatted human serum albumin as well as to mercapt- and nonmercaptalbumin was studied by equilibrium dialysis of radioactive Ca2+. Binding was quantified by five Scatchard constants [ni = 1, (i = 1-4) and n5 = 10]. Glycation resulted in increased k1- and k2-values and unchanged k3-k5-values, whereas penicilloylation increased all five association constants. The increments were greater the more pronounced the modification, and the enhancements caused by penicilloylation were, for the same degree of modification, greater than those produced by glycation. In contrast, acetylation by acetylsalicylate did not affect calcium binding. Likewise, binding to mercapt- and nonmercaptalbumin was the same, a finding showing that the thiol group of cysteine 34 is not important for calcium binding. D-Glucose and penicillin G are known to react with lysine residues of albumin, and the enhancement of binding resulting from glycation or penicilloylation is probably brought about by unspecific electrostatic effects, possibly supplemented by conformational changes of the protein molecule. The relative importance of the three domains of human serum albumin for calcium binding is discussed.

Calcium ion binding to clinically relevant chemical modifications of human serum albumin

Calcium binding to glycated, penicilloylated, acetylated, and normal defatted human serum albumin as well as to mercapt- and nonmercaptalbumin was studied by equilibrium dialysis of radioactive Ca2+. Binding was quantified by five Scatchard constants [ni = 1, (i = 1-4) and n5 = 10]. Glycation resulted in increased k1- and k2-values and unchanged k3-k5-values, whereas penicilloylation increased all five association constants. The increments were greater the more pronounced the modification, and the enhancements caused by penicilloylation were, for the same degree of modification, greater than those produced by glycation. In contrast, acetylation by acetylsalicylate did not affect calcium binding. Likewise, binding to mercapt- and nonmercaptalbumin was the same, a finding showing that the thiol group of cysteine 34 is not important for calcium binding. D-Glucose and penicillin G are known to react with lysine residues of albumin, and the enhancement of binding resulting from glycation or penicilloylation is probably brought about by unspecific electrostatic effects, possibly supplemented by conformational changes of the protein molecule. The relative importance of the three domains of human serum albumin for calcium binding is discussed.

Evidence that epithelial glycoprotein 330/megalin mediates uptake of polybasic drugs

Glycoprotein 330 (gp330) is an endocytic receptor expressed in the renal proximal tubules and some other absorptive epithelia, e.g., in the inner ear. The present study shows that the antifibrinolytic polypeptide, aprotinin, and the nephro- and ototoxic antibiotics, aminoglycosides, and polymyxin B compete for binding of 125I-urokinase-plasminogen activator inhibitor type-1 complexes to purified rabbit gp330. Half maximal inhibition was measured at 4 microM for aprotinin, 50 microM for gentamicin, and 0.5 microM for polymyxin B. Drug binding to gp330 was validated by equilibrium dialysis of [3H] gentamicin-gp330 incubations and binding/uptake studies in rat proximal tubules and gp330-expressing L2 carcinoma cells. Analyses of mutant aprotinins expressed in Saccharomyces cerevisiae revealed that basic residues are essential for the binding to gp330 and renal uptake. The polybasic drugs also antagonized ligand binding to the human alpha 2-macroglobulin receptor. However, the rapid glomerular filtration of the drugs suggests kidney gp330 to be the quantitatively most important target. In conclusion, a novel role of gp330 as a drug receptor is demonstrated. The new insight into the mechanism of epithelial uptake of polybasic drugs might provide a basis for future design of drugs with reduced toxicity.