Role of conserved glycosylation sites in maturation and transport of influenza A virus hemagglutinin.

The role of three N-linked glycans which are conserved among various hemagglutinin (HA) subtypes of influenza A viruses was investigated by eliminating the conserved glycosylation (cg) sites at asparagine residues 12 (cg1), 28 (cg2), and 478 (cg3) by site-directed mutagenesis. An additional mutant was constructed by eliminating the cg3 site and introducing a novel site 4 amino acids away, at position 482. Expression of the altered HA proteins in eukaryotic cells by a panel of recombinant vaccinia viruses revealed that rates and efficiency of intracellular transport of HA are dependent upon both the number of conserved N-linked oligosaccharides and their respective positions on the polypeptide backbone. Glycosylation at two of the three sites was sufficient for maintenance of transport of the HA protein. Conserved glycosylation at either the cg1 or cg2 site alone also promoted efficient transport of HA. However, the rates of transport of these mutants were significantly reduced compared with the wild-type protein or single-site mutants of HA. The transport of HA proteins lacking all three conserved sites or both amino-terminally located sites was temperature sensitive, implying that a polypeptide folding step had been affected. Analysis of trimer assembly by these mutants indicated that the presence of a single oligosaccharide in the stem domain of the HA molecule plays an important role in preventing aggregation of molecules in the endoplasmic reticulum, possibly by maintaining the hydrophilic properties of this domain. The conformational change observed after loss of all three conserved oligosaccharides also resulted in exposure of a normally mannose-rich oligosaccharide at the tip of the large stem helix that allowed its conversion to a complex type of structure. Evidence was also obtained suggesting that carbohydrate-carbohydrate interactions between neighboring oligosaccharides at positions 12 and 28 influence the accessibility of the cg2 oligosaccharide for processing enzymes. We also showed that terminal glycosylation of the cg3 oligosaccharide is site specific, since shifting of this site 4 amino acids away, to position 482, yielded an oligosaccharide that was arrested in the mannose-rich form. In conclusion, carbohydrates at conserved positions not only act synergistically by promoting and stabilizing a conformation compatible with transport, they also enhance trimerization and/or folding rates of the HA protein.     

Functional analysis of N-linked glycosylation mutants of the measles virus fusion protein synthesized by recombinant vaccinia virus vectors.

The role of N-linked glycosylation in the biological activity of the measles virus (MV) fusion (F) protein was analyzed by expressing glycosylation mutants with recombinant vaccinia virus vectors. There are three potential N-linked glycosylation sites located on the F2 subunit polypeptide of MV F, at asparagine residues 29, 61, and 67. Each of the three potential glycosylation sites was mutated separately as well as in combination with the other sites. Expression of mutant proteins in mammalian cells showed that all three sites are used for the addition of N-linked oligosaccharides. Cell surface expression of mutant proteins was reduced by 50% relative to the wild-type level when glycosylation at either Asn-29 or Asn-61 was abolished. Despite the similar levels of cell surface expression, the Asn-29 and Asn-61 mutant proteins had different biological activities. While the Asn-61 mutant was capable of inducing syncytium formation, the Asn-29 mutant protein did not exhibit any significant cell fusion activity. Inactivation of the Asn-67 glycosylation site also reduced cell surface transport of mutant protein but had little effect on its ability to cause cell fusion. However, when the Asn-67 mutation was combined with mutations at either of the other two sites, cleavage-dependent activation, cell surface expression, and cell fusion activity were completely abolished. Our data show that the loss of N-linked oligosaccharides markedly impaired the proteolytic cleavage, stability, and biological activity of the MV F protein. The oligosaccharide side chains in MV F are thus essential for optimum conformation of the extracellular F2 subunit that is presumed to bind cellular membranes.     

Analysis of N-linked glycosylation of hantaan virus glycoproteins and the role of oligosaccharide side chains in protein folding and intracellular trafficking

The membrane glycoproteins Gn and Gc of Hantaan virus (HTNV) (family Bunyaviridae) are modified by N-linked glycosylation. The glycoproteins contain six potential sites for the attachment of N-linked oligosaccharides, five sites on Gn and one on Gc. The properties of the N-linked oligosaccharide chains were analyzed by treatment with endoglycosidase H, peptide:N-glycosidase F, tunicamycin, and deoxynojirimycin and were confirmed to be completely of the high-mannose type. Ten glycoprotein gene mutants were constructed by site-directed mutagenesis, including six single N glycosylation site mutants and four double-site mutants. We determined that four sites (N134, -235, -347, and -399) on Gn and the only site (N928) on Gc in their ectodomains are utilized, whereas the fifth site on Gn (N609), which faces the cytoplasm, is not glycosylated. The importance of individual N-oligosaccharide chains varied with respect to folding and intracellular transport. The oligosaccharide chain on residue N134 was found to be crucial for protein folding, whereas single mutations at the other glycosylation sites were better tolerated. Mutation at glycosylation sites N235 and N399 together resulted in Gn misfolding. The endoplasmic reticulum chaperones calnexin and calreticulin were found to be involved in HTNV glycoprotein folding. Our data demonstrate that N-linked glycosylation of HTNV glycoproteins plays important and differential roles in protein folding and intracellular trafficking.     

Vesicular stomatitis virus G proteins with altered glycosylation sites display temperature-sensitive intracellular transport and are subject to aberrant intermolecular disulfide bonding

In this report we have extended our studies on a panel of vesicular stomatitis virus G proteins with altered glycosylation sites. These mutant proteins were generated by oligonucleotide-directed mutagenesis of the coding sequence to create new consensus sites for asparagine-linked oligosaccharide addition. We report that the intracellular transport of most of the mutant proteins is temperature-sensitive, implying a polypeptide folding step is affected. In addition, we find that the nonglycosylated G protein and those mutant proteins which lack oligosaccharides at the normal positions are subject to aberrant intermolecular disulfide bonding, leading to the accumulation of large complexes in the endoplasmic reticulum. These results imply that carbohydrate plays an indirect role in the intracellular transport of G protein.     

Influence of new glycosylation sites on expression of the vesicular stomatitis virus G protein at the plasma membrane

In this report, we have asked whether asparagine-linked oligosaccharides added to new sites in the polypeptide backbone of a model plasma membrane glycoprotein, the vesicular stomatitis virus G protein, can promote its intracellular transport. We modified the coding sequence of G protein lacking the two normal consensus sites for glycosylation by oligonucleotide-directed mutagenesis to create new consensus sites. The expression of the mutant proteins was then analyzed in transfected cells. Six of the eight new sites which were introduced were glycosylated, and an oligosaccharide at two of these new sites promoted transport of G protein which lacked the two normal sites. However, the efficiency of this process was reduced compared to the wild-type protein or to the proteins with only one oligosaccharide at either of the normal sites. In addition, an oligosaccharide at two of the other new sites caused inhibition of transport of the wild-type G protein. The data in this and the following report suggest that carbohydrate plays an indirect role in the intracellular transport of G protein.     

Functional analysis of individual oligosaccharide chains of Sendai virus hemagglutinin-neuraminidase protein

The roles of N-linked glycosylation in the intracellular transport and biological activities of the Sendai virus hemagglutinin-neuraminidase (HN) protein were studied. The protein contains four potential N-glycosylation sites: N77, N448, N499, and N511. By site-directed mutagenesis of these positions, the mature protein contained three N-linked oligosaccharides attached to N77, N499, and N511. The role of each added oligosaccharide in the structure and functions of the protein was identified by characterization of surface expression, hemadsorption, and neuraminidase activities of the corresponding mutant proteins. Elimination of the sites of N499 and N511 had the most detrimental effect, decreasing surface expression and hemadsorption. Elimination of the sites of N77 and N448 had similar but weaker effects. Mutants missing the sites of N499 and N511 were not able to induce syncytia formation in cells expressing mutant HN proteins and the fusion protein. Therefore, the N-linked oligosaccharides attached to N499 and N511 were important for intracellular transport and for the fusion promotion.     

Analysis of the glycosylation and phosphorylation of the lysosomal enzyme, beta-hexosaminidase B, by site-directed mutagenesis

Lysosomal enzymes require a mannose 6-phosphate recognition marker, constructed on asparagine-linked oligosaccharide chains, for targeting to lysosomes. We have identified the glycosylation sites of human beta-hexosaminidase B and have determined the influence of individual oligosaccharides on the phosphorylation, lysosomal targeting, and catalytic activity of the enzyme. The five potential glycosylation sites of the hexosaminidase beta-chain were modified individually by site-directed mutagenesis, and the constructs were expressed in COS 1 cells. By this analysis, we determined that four of the five potential sites were glycosylated. Two of the four oligosaccharides were preferentially phosphorylated. The absence of these two preferentially phosphorylated oligosaccharides resulted in greatly reduced amounts of the lysosomal form of the enzyme with increased secretion into the medium. The catalytic activity of beta-hexosaminidase B was not significantly altered by the absence of individual oligosaccharides suggesting the folding and assembly of the enzyme was not disrupted.     

Glycosylation increases potassium channel stability and surface expression in mammalian cells.

N-linked glycosylation is not required for the cell surface expression of functional Shaker potassium channels in Xenopus oocytes (Santacruz-Toloza, L., Huang, Y., John, S. A., and Papazian, D. M. (1994) Biochemistry 33, 5607-5613). We have now investigated whether glycosylation increases the stability, cell surface expression, and proper folding of Shaker protein expressed in mammalian cells. The turnover rates of wild-type protein and an unglycosylated mutant (N259Q,N263Q) were compared in pulse-chase experiments. The wild-type protein was stable, showing little degradation after 48 h. In contrast, the unglycosylated mutant was rapidly degraded (t(1/2) = approximately 18 h). Lactacystin slowed the degradation of the mutant protein, implicating cytoplasmic proteasomes in its turnover. Rapid lactacystin-sensitive degradation could be conferred on wild-type Shaker by a glycosylation inhibitor. Expression of the unglycosylated mutant on the cell surface, assessed using immunofluorescence microscopy and biotinylation, was dramatically reduced compared with wild type. Folding and assembly were analyzed by oxidizing intersubunit disulfide bonds, which provides a fortuitous hallmark of the native structure. Surprisingly, formation of disulfide-bonded adducts was quantitatively similar in the wild-type and unglycosylated mutant proteins. Our results indicate that glycosylation increases the stability and cell surface expression of Shaker protein but has little effect on acquisition of the native structure.     

Different roles of individual N-linked oligosaccharide chains in folding, assembly, and transport of the simian virus 5 hemagglutinin-neuraminidase.

The role of N-linked glycosylation in protein maturation and transport has been studied by using the simian virus 5 hemagglutinin-neuraminidase (HN) protein, a model class II integral membrane glycoprotein. The sites of N-linked glycosylation on HN were identified by eliminating each of the potential sites for N-linked glycosylation by oligonucleotide-directed mutagenesis on a cDNA clone. Expression of the mutant HN proteins in eucaryotic cells indicated that four sites are used in the HN glycoprotein for the addition of N-linked oligosaccharide chains. These functional glycosylation sites were systematically eliminated in various combinations from HN to form a panel of mutants in which the roles of individual carbohydrate chains and groups of carbohydrate chains could be analyzed. Alterations in the normal glycosylation pattern resulted in the impairment of HN protein folding and assembly which, in turn, affected the intracellular transport of HN. The severity of the consequences on HN maturation depended on both the number of deleted carbohydrate sites and their position in the HN molecule. Analysis of the reactivity pattern of HN conformation-specific monoclonal antibodies with the mutant HN proteins indicated that one specific carbohydrate chain plays a major role in promoting the correct folding of HN. Another carbohydrate chain, which is not essential for the initial folding of HN was found to play a role in preventing the aggregation of HN oligomers. The HN molecules which were misfolded, owing to their altered glycosylation pattern, were retained in the endoplasmic reticulum. Double-label immunofluorescence experiments indicate that misfolded HN and folded HN are segregated in the same cell. Misfolded HN forms disulfide-linked aggregates and is stably associated with the resident endoplasmic reticulum protein, GRP78-BiP, whereas wild-type HN forms a specific and transient complex with GRP78-BiP during its folding process.     

In vitro mutagenesis of potential N-glycosylation sites of arylsulfatase A. Effects on glycosylation, phosphorylation, and intracellular sorting.

The correct intracellular sorting of lysosomal enzymes such as arylsulfatase A depends on the presence of mannose 6-phosphate residues on high mannose type oligosaccharides. The arylsulfatase A cDNA contains three potential N-glycosylation sites, two of which are utilized. We have mutated one or two of the N-glycosylation sites and analyzed the glycosylation, phosphorylation, and intracellular sorting of the mutant arylsulfatase A polypeptides. The results show that each of the three glycosylation sites (I, II, and III) can be glycosylated, but glycosylation at sites I and II is mutually exclusive. In mutants with one oligosaccharide side chain at positions I, II, or III all side chains can acquire mannose 6-phosphate residues irrespective of their location. This demonstrates spatial flexibility of the phosphotransferase, which specifically recognizes lysosomal enzymes and initiates the addition of mannose 6-phosphate residues on oligosaccharide side chains. However, these mutants have different intracellular sorting efficiencies and seem to use different (mannose 6-phosphate receptor-dependent and -independent) sorting pathways.     

Addition of carbohydrate side chains at novel sites on influenza virus hemagglutinin can modulate the folding, transport, and activity of the molecule

We have constructed and expressed a series of mutant influenza virus hemagglutinins, each containing a new consensus site for glycosylation in addition to the seven sites found on the wild-type protein. Oligosaccharide side chains were added with high efficiency at four of the five novel sites, located on areas of the protein's surface that are not normally shielded by carbohydrate. Investigations of the structure, intracellular transport, and biological activities of the mutant hemagglutinin molecules indicated that (a) supernumerary carbohydrate side chains can be used to shield or disrupt functional epitopes on the surface of hemagglutinin, and (b) the presence of an additional oligosaccharide may cause temperature-dependent defects in the transport of the glycoprotein. We discuss the addition of supernumerary oligosaccharides as a general tool for shielding chosen areas of the surface of proteins that enter or traverse the secretory pathway.     

Identification of the N-linked glycosylation sites on the high density lipoprotein (HDL) receptor SR-BI and assessment of their effects on HDL binding and selective lipid uptake

The murine class B, type I scavenger receptor mSR-BI, a high density lipoprotein (HDL) receptor that mediates selective uptake of HDL lipids, contains 11 potential N-linked glycosylation sites and unknown numbers of both endoglycosidase H-sensitive and -resistant oligosaccharides. We have examined the consequences of mutating each of these sites (Asn --> Gln or Thr --> Ala) on post-translational processing of mSR-BI, cell surface expression, and HDL binding and lipid transport activities. All 11 sites were glycosylated; however, disruption of only two (Asn-108 and Asn-173) substantially altered expression and function. There was very little detectable post-translational processing of these two mutants to endoglycosidase H resistance and very low cell surface expression, suggesting that oligosaccharide modification at these sites apparently plays an important role in endoplasmic reticulum folding and/or intracellular transport. Strikingly, although the low levels of the 108 and 173 mutants that were expressed on the cell surface exhibited a marked reduction in their ability to transfer lipids from HDL to cells, they nevertheless bound nearly normal amounts of HDL. Indeed, the affinity of (125)I-HDL binding to the 173 mutant was similar to that of the wild-type receptor. Thus, N-linked glycosylation can influence both the intracellular transport and lipid-transporter activity of SR-BI. The ability to uncouple the HDL binding and lipid transport activities of mSR-BI by in vitro mutagenesis should provide a powerful tool for further analysis of the mechanism of SR-BI-mediated selective lipid uptake.     

Glycosylation is not needed for the intracellular transport of phytohemagglutinin in developing Phaseolus vulgaris cotyledons and for the maintenance of its biological activities

Approximately 10% of the total protein contained in Phaseolus vulgaris L. cv. Greensleeves seeds is composed of the glycoprotein lectin, phytohemagglutinin. We have investigated whether the presence of N-linked oligosaccharide side chains is a prerequisite for the correct intracellular transport of this protein and whether unglycosylated phytohemagglutinin maintains its biological activities. Excised developing cotyledons were incubated in the presence of tunicamycin to prevent glycosylation "in vivo", and the fate of the unglycosylated protein synthesized in such cotyledons determined. It was found that unglycosylated phytohemagglutinin reaches its normal site of accumulation, the protein bodies, and maintains erythro-agglutinating and mitogenic activities.     

Site-specific mutagenesis defines the intracellular role of the asparagine-linked oligosaccharides of chorionic gonadotropin beta subunit

The beta subunit of human chorionic gonadotropin contains two asparagine (N)-linked oligosaccharides. To examine the structural and functional roles of these oligosaccharide units in vivo, we constructed mutant genes containing alterations in either the asparagine or threonine codons of the two glycosylation consensus sequences and inserted them into a eukaryotic expression vector. Wild-type and mutant CG beta proteins were expressed in Chinese hamster ovary cells alone or in the presence of native alpha subunit. Pulse-chase analysis of the beta-expressing clones showed that absence of the second N-linked sugar but not the first slows secretion 1.6-1.8-fold; absence of both N-linked units slows secretion 2-2.4-fold. Analysis of dimer clones reveals that greater than 80% of the native and glycosylation mutant CG beta subunits are secreted as dimer. However, pulse-chase analysis of these clones also reveals that the mutants completely devoid of N-linked sugars but not the single site mutants are slow to assemble with the alpha subunit. Thus, in vivo the two N-linked oligosaccharides of CG beta are critical for efficient secretion and assembly with the alpha subunit and are likely important for proper folding of the CG beta subunit.     

Glycosylation of a Vesicular Monoamine Transporter: A Mutation in a Conserved Proline Residue Affects the Activity, Glycosylation, and Localization of the Transporter

Abstract: The role of N -glycosylation in the expression, ligand recognition, activity, and intracellular localization of a rat vesicular monoamine transporter (rVMAT1) was investigated. The glycosylation inhibitor tunicamycin induced a dose-dependent decrease in the rVMAT1-mediated uptake of [³H]serotonin. Part of this effect was due to a general toxic effect of the drug. Therefore, to assess the contribution of each of the glycosylation sites to the transporter activity, the three putative N -glycosylation sites were mutated individually, in combination, and in toto ("triple" mutant). Mutation of each glycosylation site caused a minor and additive decrease in activity, up to the triple mutant, which retained at least 50% of the wild-type activity. No significant differences were found either in the time dependence of uptake or the apparent affinity for ligands of the triple mutant compared with the wild-type protein. It is interesting that in contrast to plasma-membrane neurotransmitter transporters, the unglycosylated form of rVMAT1 distributed in the cell as the wild-type protein. Pro⁴³ is a highly conserved residue located at the beginning of the large loop in which all the potential glycosylation sites are found. A Pro⁴³Leu mutant transporter was inactive. It is remarkable that despite the presence of glycosylation sites, the mutant transporter was not glycosylated. Moreover, the distribution pattern of the Pro⁴³Leu mutant clearly differed from that of the wild type. In contrast, a Pro⁴³Gly mutant displayed an activity practically identical to the wild-type protein. As this replacement generated a protein with wild-type characteristics, we suggest that the conformation conferred by the amino acid at this position is essential for activity.     

Glycosylation requirements for intracellular transport and function of the hemagglutinin of influenza virus.

The contribution of each of the seven asparagine-linked oligosaccharide side chains on the hemagglutinin of the A/Aichi/68 (X31) strain of influenza virus was assessed with respect to its effect on the folding, intracellular transport, and biological activities of the molecule. Twenty mutant influenza virus hemagglutinins were constructed and expressed, each of which had one or more of the seven glycosylation sites removed. Investigations of these mutant hemagglutinins indicated that (i) no individual oligosaccharide side chain is necessary or sufficient for the folding, intracellular transport, or function of the molecule, (ii) at least five oligosaccharide side chains are required for the X31 hemagglutinin molecule to move along the exocytic pathway to the plasma membrane, and (iii) mutant hemagglutinins having less than five oligosaccharide side chains form intracellular aggregates and are retained in the endoplasmic reticulum.     

Independent and cooperative roles of N-glycans and molecular chaperones in the folding and disulfide bond formation of the low-density lipoprotein (LDL) receptor-related protein

The low-density lipoprotein receptor-related protein (LRP) is a large receptor that contains extensive glycosylation sites and disulfide bonds. Here we analyzed how N-linked glycosylation and molecular chaperones function during LRP folding. Treatment of cells with a glycosylation inhibitor tunicamycin significantly impaired LRP folding, although binding to receptor-associated protein (RAP), a specialized chaperone for LRP, was not affected. The effects of tunicamycin on LRP folding were not due to an inhibition of RAP glycosylation since a mutant RAP that harbors a mutation at its sole glycosylation site was still capable of promoting LRP folding. The roles of N-linked glycosylation and the lectin chaperone, calnexin, in LRP folding were further dissected using LRP minireceptors that carry mutations at individual glycosylation sites. Interestingly, we found that RAP interacts with oxidoreductase ERp57 and mediates its interaction with LRP. Since previous studies have shown that N-glycan-bound calnexin/calreticulin are also capable of recruiting ERp57, our results suggest that N-linked glycosylation and RAP can independently and cooperatively recruit oxidoreductases to facilitate protein folding and proper disulfide bond formation.     

Tissue-specific N-glycosylation, site-specific oligosaccharide patterns and lentil lectin recognition of rat Thy-1.

To examine the extent to which protein structure and tissue-type influence glycosylation, we have determined the oligosaccharide structures at each of the three glycosylation sites (Asn-23, 74 and 98) of the cell surface glycoprotein Thy-1 isolated from rat brain and thymus. The results show that there is tissue-specificity of glycosylation and that superimposed on this is a significant degree of site-specificity. On the basis of the site distribution of oligosaccharides, we find that no Thy-1 molecules are in common between the two tissues despite the amino acid sequences being identical. We suggest, therefore, that by controlling N-glycosylation a tissue creates an unique set of glycoforms (same polypeptide but with oligosaccharides that differ either in sequence or disposition). The structures at each of the three sites were also determined for the thymocyte Thy-1 that binds to lentil lectin (Thy-1 L+) and for that which does not (Thy-1 L-). Segregation of intact thymus Thy-1 into two distinct sets of glycoforms by lentil lectin was found to be due to the structures at site 74. Analysis of oligosaccharide structures at the 'passenger' sites (23 and 98) suggests that either Thy-1 L+ and Thy-1 L- molecules are made in different cell-types or that the biosynthesis of oligosaccharides at one site is influenced by the glycosylation at other sites.     

The role of asparagine-linked oligosaccharides in the subunit structure, steroid binding, and secretion of androgen-binding protein.

Testicular androgen-binding protein (ABP) and liver sex hormone-binding globulin are encoded by the same gene. These proteins have the same primary amino acid sequences, but they differ in attached oligosaccharides; the differences are presumably due to cell-specific glycosylation mechanisms. To investigate the role of oligosaccharides in ABP/sex hormone-binding globulin subunit structure, secretion, and steroid binding, mutant rat ABP proteins were constructed that eliminated one or both of the two potential sites of asparagine (Asn)-linked glycosylation. Immunoblot analysis of wild type recombinant ABP yielded the typical heterogeneous banding pattern. Secreted ABP was composed of two protomers of M(r) 46,000 and M(r) 43,000, while cellular ABP yielded three mol wt species (M(r) 43,000, 41,000, and 39,000). Substitution of the Asn residue in either consensus sequence for Asn-linked glycosylation with an Ile residue resulted in increased mobility of the immunoreactive ABP species. These changes are consistent with the loss of an Asn-linked oligosaccharide. Substitution of both Asn residues yielded a single immunoreactive species in the medium and cell extracts that migrated as a M(r) 39,000 protein. These results demonstrate that the mol wt heterogeneity of ABP is due to differential Asn-linked glycosylation of both potential sites. All three mutant forms of ABP were secreted by the COS cells. However, the amount of immunoreactive ABP and [3H]5 alpha-dihydrotestosterone binding in the medium was lower than wild type (100%) in one of the single mutants (65%) and in the double mutant (29%). Unlike the glycosylation mutants, alteration of other residues, not involved in glycosylation, yielded cellular ABP and no detectable medium ABP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Phosphorylation of Asn-linked oligosaccharides located at novel sites on the lysosomal enzyme cathepsin D.

We have examined the phosphorylation of Asn-linked oligosaccharides introduced at seven novel sites on human cathepsin D to determine whether the location of an oligosaccharide on a lysosomal enzyme affects its ability to serve as a substrate for UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (phosphotransferase), the enzyme that catalyzes the initial step in the biosynthesis of mannose 6-phosphate residues. The glycosylation sites were introduced into the cathepsin D cDNA by site-directed mutagenesis and were selected to be widely distributed over the surface of the molecule. When the constructs were expressed in Xenopus oocytes, the oligosaccharides at each glycosylation site were phosphorylated at levels considerably above background (19-70% phosphorylation versus < 0.4% for the secretory protein glycopepsinogen). However, oligosaccharides located closer to the essential components of the phosphotransferase recognition domain (lysine 203 and amino acids 265-292) were phosphorylated better than oligosaccharides located further away. Similar results were obtained for oligosaccharides at homologous sites on a pepsinogen/cathepsin D chimera containing only lysine 203 and residues 265-319 of cathepsin D, although the absolute levels of phosphorylation were lower. These results demonstrate that there is considerable flexibility in the placement of glycosylation sites on cathepsin D in terms of the ability of the oligosaccharides to serve as substrates for phosphotransferase, although oligosaccharides located closer to the phosphotransferase recognition determinant are preferentially phosphorylated.