The effect of N-linked glycosylation on the substrate preferences of UDP glucuronosyltransferases

The presence of potential N-linked glycosylation sites (Asn-X-Ser/Thr) in two forms of UDP glucuronosyltransferase, designated UDPGTr-2 and UDPGTr-4, has been deduced from cDNA sequence data. These forms were glycosylated when synthesized from expression vectors transfected into COS cells and were converted to faster migrating species on SDS polyacrylamide gels when treated with endoglycosidase H. The role of glycosylation was investigated by determining the substrate specificities and stabilities of the glycosylated enzymes and their unglycosylated variants which were synthesized in the presence of tunicamycin. Analysis of the activities towards 13 different aglycones showed that the glycosyl moiety was not essential for catalytic activity and had no effect on the substrate preference of each form. The stabilities of the proteins were not adversely affected by the absence of this posttranslational modification. A possible effect of N-linked oligosaccharides on the catalytic properties of these two forms of UDP glucuronosyltransferase is discussed.     

Rat liver UDP-glucuronosyltransferase. cDNA sequence and expression of a form glucuronidating 3-hydroxyandrogens

A cDNA clone, pUDPGTr-4, encoding a form of rat UDP-glucuronosyltransferase has been isolated from a SV40 expression library. Sequence analysis revealed that the cDNA is 1970 base pairs in length and encodes a protein of 530 amino acids, which has amino- and carboxyl-terminal sequences characteristic of signal peptide and transmembrane segments, respectively. There is one potential asparagine-linked glycosylation site. Transfection of UDPGTr-4 cDNA into COS cells resulted in the glucuronidation of etiocholanolone, androsterone, and lithocholic acid in a transient expression assay. Several other common substrates of UDP-glucuronosyltransferase were not conjugated by the UDPGTr-4 enzyme. UDPGTr-4 cDNA is identical in sequence over a common 1.7 kilobase-region of overlap to UDPGTr-1, a cDNA previously isolated in this laboratory (Mackenzie, P. I., Gonzalez, F. J., and Owens, I. S. (1984) J. Biol. Chem. 259, 12153-12160). UDPGTr-4 cDNA, however, contains a shorter 3'-untranslated region. Northern analysis showed that the poly(A) RNA counterparts of UDPGTr-4 and UDPGTr-1 cDNAs are approximately 2.3 and 3.0 kilobases in length, respectively. The steady-state level of UDPGTr-4 poly(A) RNA in the liver is 20-fold higher than that of UDPGTr-1 poly(A) RNA. These data indicate that the UDPGTr-4 enzyme is a 3-hydroxyandrogen UDP-glucuronosyltransferase which is encoded by two distinct species of mRNA transcribed from the same gene.     

Purification of a form of mouse liver UDP glucuronosyltransferase which glucuronidates androgens

A form of UDP glucuronosyltransferase active in the glucuronidation of the androgens, testosterone, androsterone and dihydrotestosterone has been purified to apparent homogeneity as judged by sodium dodecylsulfate polyacrylamide gel electrophoresis from the livers of phenobarbital-treated C57BL/6N mice. This UDP glucuronosyltransferase is inactive towards estrone as substrate. Data from chromatofocusing and purification experiments suggest that testosterone and androsterone are glucuronidated primarily by this enzyme form and to a lesser extent by an enzyme form which has a slightly higher isoelectric point. However, this major form is only responsible for about half the capacity to glucuronidate dihydrotestosterone.     

A calcium-dependent 35-kilodalton substrate for epidermal growth factor receptor/kinase isolated from normal tissue

We have previously reported the isolation of a 35-kDa protein from A-431 cells that, in the presence of Ca2+, can serve as a substrate for the epidermal growth factor (EGF) receptor/tyrosine kinase (Fava, R.A., and Cohen, S. (1984) J. Biol. Chem. 259, 2636-2645). We now report the detection of an antigenically related 35-kDa protein in a number, but not all, of rat, pig, and human tissues. These antigenically related proteins also can serve as substrates for the EGF receptor/kinase in the presence of Ca2+. All of these proteins share the property of reversible, Ca2+-dependent binding to the particulate fraction (presumably membranes) of cell homogenates. We have isolated the 35-kDa substrate from porcine lung and have demonstrated that it is a Ca2+-binding protein. The amino-terminal sequence and the site of tyrosine phosphorylation therein have been determined. The positions of the acidic amino acid residues amino-terminal to the tyrosine phosphorylation site bear a distinct resemblance to the sequence in the homologous region of a number of other substrates for tyrosine kinases. Based on available data, the 35-kDa protein clearly differs from the protein I complex derived from intestinal mucosa and thought to be related to the proteins isolated herein (Gerke, V., and Weber, K. (1985) J. Biol. Chem. 260, 1688-1695). Finally, we report a striking sequence homology between the porcine 35-kDa described herein and human lipocortin, a phospholipase A2 inhibitor.     

Biogenesis of glycophorin A in K562 human erythroleukemia cells

A monoclonal antibody (mAb-233) directed against an epitope in the nonglycosylated carboxyl-terminal region of human erythrocyte glycophorin A (GPA) was used in combination with metabolic labeling, the modification of N- and O-linked oligosaccharide processing by tunicamycin and monensin, and digestions with neuraminidase and O-glycanase to elucidate the pathway of GPA biogenesis in K562 human erythroleukemia cells. Cell-surface GPA is derived from two obligatory precursors in a stepwise manner. The initial GPA precursor has a Mr of 27,000 and appears to contain one N-linked high mannose oligosaccharide chain. In tunicamycin-treated cells, the initial precursor is similar in size (Mr = 24,000) to deglycosylated GPA from human erythrocytes. The 27-kDa initial precursor is rapidly converted to a transient 31-kDa intermediate by the addition of N-acetylgalactosamine residues to serine/threonine hydroxyl groups. Subsequent maturation involves the conversion of the high mannose chain to a complex-type oligosaccharide and the concomitant addition of galactose and sialic acid to internal N-acetylgalactosamine residues to extend the O-linked chains. These results define a single, stepwise processing pathway for the generation of all cell-surface GPA molecules and document for the first time the occurrence of both a unique initial precursor that contains a high mannose N-linked oligosaccharide chain but no O-linked sugars and a transient intermediate that appears to contain the same N-linked group and N-acetylgalactosamine at multiple serine/threonine residues. The properties of the intracellular GPA precursors and the relatively simple nature of the processing pathway reported herein contrast markedly with the characteristics of three intermediates and the complexity of two independent pathways in previously postulated schemes for GPA biogenesis (Gahmberg, C. G., Jokinen, M., Karhi, K. K., Kampe, O., Peterson, P. A., and Andersson, L. C. (1983) Methods Enzymol. 96, 281-298; Jokinen, M., Andersson, L. C., and Gahmberg, C. G. (1985) J. Biol. Chem. 260, 11314-11321).     

Recombinant 70-kDa protein from the amino-terminal region of rat fibronectin inhibits binding of fibronectin to cells and bacteria

Binding of fibronectin to substrate-attached cells and to Staphylococcus aureus is mediated by the amino-terminal 70-kDa portion of fibronectin. The 70-kDa amino-terminus is composed of nine type I and two type II internal homology units, each containing two intrachain disulfide bonds. The exact structural features of the 70-kDa amino-terminus that are necessary for binding to cells and bacteria are not known. We characterized a recombinant 70-kDa protein from the amino-terminus of rat fibronectin using a baculovirus expression system. Recombinant 70-kDa (r70kDa) protein was easily purified in high amounts from the conditioned medium by affinity chromatography on gelatin-agarose. Secretion was much less when N-linked glycosylation was blocked by tunicamycin. Like the native fragment, the r70kDa protein contains intrachain disulfide bonds. In addition, the r70kDa protein was indistinguishable from the nonrecombinant 70-kDa fragment in its ability to compete for binding sites on fibroblasts and S. aureus. Thus, the r70kDa protein retains the important functional characteristics of the native fragment. This expression system is well adapted to studying the structural features important for the interaction of 70-kDa protein with cells.     

DNA and nucleotide-induced conformational changes in the Escherichia coli Rep and helicase II (UvrD) proteins

The domain structures of the Escherichia coli Rep and Helicase II proteins and their ligand-dependent conformational changes have been examined by monitoring the sensitivity of these helicases to proteolysis by trypsin and chymotrypsin. Limited treatment of unliganded Rep protein (73 kDa) with trypsin results in cleavage at a single site in its carboxyl-terminal region, producing a 68-kDa polypeptide which is stabilized in the presence of ATP, ADP, or adenosine 5'-O-thiotriphosphate) (ATP gamma S). The purified 68-kDa Rep tryptic polypeptide retains single-stranded (ss) DNA binding, DNA unwinding (helicase), and full ATPase activities. When bound to ssDNA, the Rep protein can be cleaved by trypsin at an additional site in its carboxyl-terminal region, producing a 58-kDa polypeptide that also retains ssDNA binding and ATPase activities. This 58-kDa Rep tryptic polypeptide can also be produced by further tryptic treatment of the 68-kDa Rep tryptic polypeptide when the latter is bound to ssDNA. This 58-kDa polypeptide displays a lower affinity for ssDNA indicating that the 10-kDa carboxyl-terminal peptide facilitates Rep protein binding to ssDNA. The 58-kDa Rep tryptic polypeptide is also stabilized in the presence of nucleotides. Based on these and previous studies that showed that the 68-kDa Rep tryptic polypeptide cannot support DNA replication in a system that is dependent upon the phi X174 cis-A protein (Arai, N. & Kornberg, A. (1981) J. Biol. Chem. 256, 5294-5298), we conclude that the carboxyl-terminal end (approximately 5 kDa) of the Rep protein is not required for its helicase or ATPase activities. However, we suggest that this region of the Rep protein is important for its interactions with the phi X174 cis-A protein. Limited treatment of unliganded Helicase II protein (82 kDa) with chymotrypsin results in cleavage after Tyr254, producing a 29-kDa amino-terminal polypeptide and a 53-kDa carboxyl-terminal polypeptide, which remain associated under nondenaturing conditions. This chymotrypsin cleavage reduces the ssDNA binding activity and eliminates the ssDNA-dependent ATPase and helicase activities of the Helicase II protein. The binding of ATP, ADP, ATP gamma S, and/or DNA to Helicase II protein results in protection of this site (Tyr254) from cleavage by chymotrypsin. Limited treatment of Helicase II protein with trypsin results in cleavage near its carboxyl-terminal end producing two polypeptides with apparent Mr = 72,000, in a manner similar to that observed with the Rep protein; these polypeptides are also stabilized by binding ATP or single-stranded DNA.(ABSTRACT TRUNCATED AT 400 WORDS)     

Identification of monoclonal antibody 4A-binding site on the transducin alpha subunit. Immunoblotting of submaxillary Arg-C protease fragments of transducin.

Monoclonal antibody 4A (mAb 4A) against the T alpha subunit of transducin has been widely used to study the structure and function of signal transducing GTP-binding proteins involved in the regulation of visual excitation, hormonal regulation of adenylyl cyclase and ionic channels. Results of mapping the epitope-binding site of mAb 4A on T alpha have been controversial. Hamm and co-workers (Deretic, D., and Hamm, H. E. (1987) J. Biol. Chem. 262, 10839-10847) reported that mAb 4A interacts with T alpha at the carboxyl-terminal peptide, whereas Fung and co-workers (Navon, S. E., and Fung, B. K.-k. (1988) J. Biol. Chem. 263, 489-498) showed that mAb 4A binds mainly at the amino-terminal peptide. In this report, we examine the epitope-binding site of mAb 4A by Western immunoblotting of the proteolytic fragments of T alpha generated by submaxillary Arg-C protease digestion. Submaxillary Arg-C protease cleaved T alpha at two sites, Arg-204 and Arg-310, generating two major fragments of apparent size 35 (T alpha'SM-35) and 23 kDa (T alpha'SM-23). Both fragments contain the amino-terminal peptide of T alpha but lack the carboxyl-terminal peptide. Western immunoblotting showed that mAb 4A cross-reacted with both peptides. Treatment of T alpha'SM-35 and T alpha'SM-25 with L-1-(tosylamido)-2-phenyethyl chloromethyl ketone-trypsin removed the amino-terminal 2-kDa peptide with concomitant loss of mAb 4A reactivity. This observation unequivocally confirms the result of Fung and co-workers that the epitope for mAb 4A is located on the amino-terminal 2-kDa peptide of T alpha. This conclusion should provide a more accurate interpretation of results in the literature as well as of future studies in which mAb 4A is used.     

Influence of the N-linked oligosaccharides on the biosynthesis, intracellular routing, and function of the human asialoglycoprotein receptor

The human asialoglycoprotein receptor (ASGP-R) is a membrane glycoprotein of 46,000 Da which possesses two N-linked oligosaccharide chains (Schwartz, A. L., and Rup, D. (1983) J. Biol. Chem. 258, 11249-11255). In order to examine the role of N-linked oligosaccharides in the biosynthesis, intracellular routing, and function of the ASGP-R, we have used Hep G2 cells, which have a large number of ASGP-R, and two inhibitors of glycosylation, swainsonine and tunicamycin. In the presence of swainsonine, newly synthesized ASGP-R is a 43,000-Da species which is endoglycosidase H-sensitive, appears on the Hep G2 cell surface, and specifically binds 125I-asialoorosomucoid (ASOR). In the presence of tunicamycin newly synthesized ASGP-R is a 34,000-Da nonglycosylated species which appears on the Hep G2 cell surface where it specifically binds 125I-ASOR. There is no major effect on subsequent uptake and degradation of 125I-ASOR in cells whose ASGP-R was synthesized in the presence of tunicamycin. The turnover of ASGP-R synthesized in the presence of either swainsonine or tunicamycin is not significantly altered from that found for the normal 46,000-Da species. Thus, it appears that the two N-linked oligosaccharide chains of the human ASGP-R do not play a major role in the intracellular routing, turnover, or function of ASGP-R.     

Ezrin contains cytoskeleton and membrane binding domains accounting for its proposed role as a membrane-cytoskeletal linker

Ezrin, a widespread protein present in actin-containing cell-surface structures, is a substrate of some protein tyrosine kinases. Based on its primary and secondary structure similarities with talin and band 4.1 it has been suggested that this protein could play a role in linking the cytoskeleton to the plasma membrane (Gould, K.L., A. Bretscher, F.S. Esch, and T. Hunter. 1989. EMBO (Eur. Mol. Biol. Organ.), J. 8:4133-4142; Turunen, O., R. Winqvist, R. Pakkanen, K.-H. Grzeschik, T. Wahlstrom, and A. Vaheri. 1989. J. Biol. Chem. 264:16727- 16732). To test this hypothesis, we transiently expressed the complete human ezrin cDNA, or truncated cDNAs encoding the amino- and carboxy- terminal domains of the protein, in CV-1 cells. Protein epitope tagging was used to unambiguously determine the subcellular distribution of the protein encoded by the transfected cDNA. We show that this protein is concentrated underneath the dorsal plasma membrane in all actin- containing structures and is partially detergent insoluble. The amino- terminal domain displays the same localization but is readily extractable by nonionic detergent. The carboxy-terminal domain colocalizes with microvillar actin filaments as well as with stress fibers and remains associated with actin filaments after detergent extraction, and with disorganized actin structures after cytochalasin D treatment. Our results clearly demonstrate that ezrin interacts with membrane-associated components via its amino-terminal domain, and with the cytoskeleton via its carboxy-terminal domain. The amino-terminal domain could include the main determinant that restricts the entire protein to the cortical cytoskeleton in contact with the dorsal plasma membrane and its specialized microdomains such as microvilli, microspikes and lamellipodia.     

Structural analysis of rod GTP-binding protein, Gt. Limited proteolytic digestion pattern of Gt with four proteases defines monoclonal antibody epitope.

The epitope of monoclonal antibody (mAb 4A), which recognizes the alpha subunit of the rod G protein, Gt, has been suggested to be both at the carboxyl terminus (Deretic, D., and Hamm, H.E. (1987) J. Biol. Chem. 262, 10839-10847) and the amino terminus (Navon, S.E., and Fung, B.K.-K. (1988) J. Biol. Chem. 263, 489-496) of the molecule. To characterize further the mAb 4A binding site on alpha t and to resolve the discrepancy between these results limited proteolytic digestion of Gt or alpha t using four proteases with different substrate specificities has been performed. Endoproteinase Arg-C, which cleaves the peptide bond at the carboxylic side of arginine residues, cleaved the majority of alpha t into two fragments of 34 and 5 kDa. The alpha t 34-kDa fragment in the holoprotein, but not alpha t-guanosine 5'-O-(3-thiotriphosphate), was converted further to a 23-kDa fragment. A small fraction of alpha t-GDP was cleaved into 23- and 15-kDa fragments. Endoproteinase Lys-C, which selectively cleaves at lysine residues, progressively removed 17 and then 8 residues from the amino terminus, forming 38- and 36-kDa fragments. Staphylococcus aureus V8 protease is known to remove 21 amino acid residues from the amino-terminal region of alpha t, with the formation of a 38-kDa fragment. L-1-Tosylamido-2-phenylethyl chloromethyl ketone-treated trypsin cleaved alpha t progressively into fragments of known amino acid sequences (38, then 32 and 5, then 21 and 12 kDa) and a transient 34 kDa fragment. The binding of mAb 4A to proteolytic fragments was analyzed by Western blot and immunoprecipitation. The major fragments recognized by mAb 4A on Western blots were the 34- and 23-kDa fragments obtained by endoproteinase Arg-C and tryptic digestion. Under conditions that allowed sequencing of the 15- and 5-kDa fragments neither the 34- nor the 23-kDa fragments could be sequenced by Edman degradation, indicating that they contained a blocked amino terminus. The smallest fragment that retained mAb 4A binding was the 23-kDa fragment containing Met1 to Arg204. Thus the main portion of the mAb 4A antigenic site was located within this fragment, indicating that the carboxyl-terminal residues from Lys205 to Phe350 were not required for recognition by the antibody. Additionally, the antibody did not bind the 38- and 36-kDa or other fragments containing the carboxyl terminus, showing that the amino-terminal residues from Met1 to Lys17 were essential for antibody binding to alpha t.     

Purification and immunochemical characterization of a low-pI form of UDP glucuronosyltransferase from mouse liver

A liver UDP glucuronosyltransferase (GT) enzyme from either phenobarbital- or 3-methylcholanthrene-treated C57BL/6N mice was isolated by phenyl-Sepharose, DEAE-ion exchange, and UDP hexanolamine chromatographic steps. This enzyme had a broad substrate specificity and was mainly responsible for the microsomal capacity to glucuronidate testosterone, 1-naphthol, and morphine. This UDP glucuronosyltransferase ( GTM1 ) appeared to be at least 95% homogeneous and had a subunit molecular weight of 51,000 using sodium dodecyl sulfate-polyacrylamide gel and two-dimensional gel electrophoreses. Antibodies prepared against the purified protein developed a single immunoprecipitin line by double-diffusion analysis with purified antigen and with solubilized microsomes from both control and drug-induced C57BL/6N and DBA/2N mice. A precipitin line was also observed with microsomal proteins which isoelectrofocused at approximately pH 6.7, but not with those which isoelectrofocused at approximately pH 8.5. GTM1 was, therefore, designated at low-pI form. Immunopurified antibody preferentially inhibited and immunoprecipitated GT activities toward testosterone, 1-naphthol, and morphine. To a lesser extent, activities toward phenolphthalein, 3-hydroxybenzo[a]pyrene, and estrone were inhibited while activities toward 4-nitrophenol and 4-methylumbelliferone were not affected. All activities, however, were immunoadsorbed in the presence of protein A-Sepharose. This observation can be explained by the following results. Immunoprecipitates from labeled microsomes contained primarily a 51,000-Da protein. When the immune complexes were adsorbed with protein A-Sepharose, a 54,000-Da protein as well as the expected 51,000-Da GTM1 was detected. This 54,000-Da protein was associated with the glucuronidation of 3-hydroxybenzo[a]pyrene and 4-nitrophenol, and was designated GTM2 .     

Evidence for a nonsecretory, acidic degradation pathway for amyloid precursor protein in 293 cells. Identification of a novel, 22-kDa, beta-peptide-containing intermediate.

We have analyzed the metabolic pathway of maturation of APP751 in stably transfected 293 cells, in the presence of either of the cysteine protease inhibitors leupeptin or E-64. Metabolic labeling, followed by immunoprecipitation at various times in the chase with a rabbit polyclonal antibody (anti-BX6) specific to the carboxyl-terminal end of amyloid precursor protein (APP), revealed the accumulation of a novel approximately 22-kDa carboxyl-terminal fragment (22-CTF) in the inhibitor-treated cells. This fragment, which was not detectable in untreated cells, was immunoprecipitated by four separate antibodies to the carboxyl-terminal region of APP as well as by polyclonal and monoclonal antibodies specific to the first 16 amino acids of the beta-peptide domain. Antibodies to the amino-terminal end of APP do not, however, recognize the fragment. Co-treatment of the inhibitor-treated cells with either of the lysosomotropic agents chloroquine or ammonium chloride completely blocked the generation of this fragment but did not significantly affect APP maturation or secretion. All, however, slowed the intracellular turnover of the cell-associated, approximately 9-kDa carboxyl-terminal fragment (c-CTF) produced during constitutive secretion. Densitometric analyses of these results suggest that this non-secretory pathway of APP degradation, mediated by cysteine proteases in an intracellular acidic compartment, accounts for approximately 70% of total APP metabolism and that a key processing intermediate in this pathway is a 22-kDa, beta-peptide-containing APP carboxyl-terminal fragment. It is possible that inefficient degradation of such an intermediate leads to the formation of aggregating beta-peptide.     

Structure and expression of the rat inositol 1,4,5-trisphosphate receptor

The complete primary structure of the inositol 1,4,5-trisphosphate receptor from rat brain was elucidated using a series of overlapping cDNA clones. Two different sets of clones that either contain or lack a 45-nucleotide sequence in the amino-terminal third of the protein were isolated, suggesting a differential splicing event that results in the biosynthesis of either a 2734- or 2749-amino acid receptor protein. Hydrophobicity analysis demonstrates the presence of a cluster of hydrophobic sequences in the carboxyl-terminal third of the protein that probably comprise eight transmembrane regions and that may form the calcium channel intrinsic to the receptor. The receptor was universally expressed at low levels in all tissues and cultured cells tested. Transfection of a full-length expression construct of the inositol 1,4,5-trisphosphate receptor into COS cells resulted in the biosynthesis of a 260-kDa protein that bound inositol 1,4,5-trisphosphate and formed high molecular weight complexes similar to the native receptor as analyzed by sucrose gradient centrifugations. On the other hand, the protein product synthesized by a mutant receptor construct in which the amino-terminal 418 amino acids were deleted failed to bind inositol 1,4,5-trisphosphate. The mutant receptor still formed high molecular weight complexes, suggesting that it folded normally and that the amino-terminal sequences of the receptor are part of the ligand binding domain.     

Evidence that the hamster tunicamycin resistance gene encodes UDP-GlcNAc:dolichol phosphate N-acetylglucosamine-1-phosphate transferase.

A cDNA clone isolated from Chinese hamster ovary cells conferred elevated GlcNAc-1-P-transferase (GPT) activity and resistance to tunicamycin in transfected cells (Zhu, X., and Lehrman, M. A. (1990) J. Biol. Chem. 265, 14250-14255). It had been assumed that this cDNA, termed TRG for tunicamycin resistance gene, encoded GPT enzyme. However, other functions were not ruled out. Thus, by one of several mechanisms, the TRG protein could have instead functioned by activation of the transfected host's endogenous GPT enzyme. To analyze the biochemical function of the TRG protein, hamster TRG cDNA was stably expressed at high levels in Chinese hamster ovary cells. In addition, several antipeptide polyclonal antibodies directed against the predicted TRG protein were obtained. With these tools in hand, experiments were performed to test the hypothesis that the TRG encodes GPT enzyme, as well as to rule out other possible functions for the TRG protein. These experiments included examination of the effects of solubilization of membranes on TRG-dependent GPT activity, the apparent binding of tunicamycin to the TRG protein, and the immunoadsorption of GPT activity with TRG protein-specific antibodies. From these results, we conclude that the hamster TRG most likely encodes GPT enzyme.