Cell-free biosynthesis of the O-acetylated N-acetylneuraminic acid capsular polysaccharide of group C meningococci.

A cell-free system was established to study the biosynthesis of group C meningococcal capsular polysaccharide, an alpha-2 leads to 9-linked N-acetylneuraminic acid (NeuAc) homopolymer containing O-acetyl groups at either C7 or C8. Sialyltransferase activity, isolated from group C meningococcus strain C-11, catalyzed incorporation of [14C]NeuAc from CMP (CMP--[14C]NeuAc) into polymeric form. This sialyltransferase was stimulated by addition of meningococcus group C and Escherichia coli K92 capsular polysaccharides, the latter being an alpha-2 leads to 8- and alpha-2 leads to 9-linked NeuAc heteropolymer. Group C meningococcal sialyltransferase did not require divalent ions but was stimulated by Mn2+. Attempts to demonstrate a lipid-soluble intermediate in the biosynthesis of this NeuAc polymer were unsuccessful. Meningococcal group C sialyltransferase incorporated NeuAc into a membrane-associated product. The polysaccharide can be extracted from the membrane-bound fraction with Triton X-100. The newly synthesized polysaccharide coprecipitates with authentic group C antigen in meningococcal group C antiserum and is degraded by sodium metaperiodate, indicating that the NeuAc polymer synthesized by the cell-free system consists of alpha-2 leads to 9 linkage. Meningococcal group C spheroplast membranes contain an O-acetylase that can catalyze the transfer of acetyl groups from acetyl coenzyme A to the in vitro-synthesized polysaccharide.     

Production of cytidine 5'-monophosphate N-acetylneuraminic acid using recombinant Escherichia coli as a biocatalyst

An Escherichia coli strain expressing three recombinant enzymes, i.e., cytidine 5'-monophosphate (CMP) kinase, sialic acid aldolase and cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-NeuAc) synthetase, was utilized as a biocatalyst for the production of CMP-NeuAc. Both recombinant E. coli extract and whole cells catalyzed the production of CMP-NeuAc from CMP (20 mM), N-acetylmannosamine (40 mM), pyruvate (60 mM), ATP (1 mM), and acetylphosphate (60 mM), resulting in 90% conversion yield based on initial CMP concentration used. It was confirmed that endogenous acetate kinase can catalyze not only the ATP regeneration in the conversion of CMP to CDP but also the conversion of CDP to CTP. On the other hand, endogenous pyruvate kinase and polyphosphate kinase could not regenerate ATP efficiently. The addition of exogenous acetate kinase to the reaction mixture containing the cell extract increased the conversion rate of CMP to CMP-NeuAc by about 1.5-fold, but the addition of exogenous inorganic pyrophosphatase had no influence on the reaction. This E. coli strain could also be employed as an enzyme source for in situ regeneration of CMP-NeuAc in a sialyltransferase catalyzed reaction. About 90% conversion yield of alpha2,3-sialyl-N-acetyllactosamine was obtained from N-acetyllactosamine (20 mM), CMP (2 mM), N-acetylmannosamine (40 mM), pyruvate (60 mM), ATP (1 mM), and acetyl phosphate (80 mM) using the recombinant E. coli extract and alpha2,3-sialyltransferase.     

Stimulation of sialyltransferase activity of melanoma cells by retinoic acid

Retinoic acid (RA) treatment of murine S91-C2 melanoma cells has been found to augment the activity of glycoprotein: sialyltransferase in a dose-dependent and time-dependent process. The enzymatic activity in cells treated with 10 microM RA reached a maximal level, 3-fold higher than in untreated cells, 72 h after initiation of treatment. In contrast, the addition of RA directly into the reaction mixture had no stimulatory effect on sialyltransferase. The endogenous glycoproteins to which sialic acid is transferred from cytidine monophosphate (CMP)-[14C] sialic acid by the action of sialyltransferase have been identified by fluorography after polyacrylamide gel electrophoresis. One of these acceptors, a glycoprotein of Mr 160 000, comigrated in gel electrophoresis with a cell surface sialoglycoprotein that can be labeled by the periodate-tritiated borohydrate procedure more intensely on intact RA-treated than on untreated cells. Removal of sialic acid residues exposed on the surface of either control or RA-treated cells enhanced 2- to 3-fold the transfer of sialic acid to endogenous acceptors. These results suggest that the increased sialyltransferase activity in RA-treated melanoma cells may be responsible for the enhanced sialylation of certain cell surface glycoproteins. RA treatment of several other tumor cell lines also resulted in stimulation of sialyltransferase activity indicating that this effect of RA is not limited to the S91-C2 melanoma cells.     

In vitro synthesis of colominic acid by membrane-bound sialyltransferase of Escherichia coli K-235. Kinetic properties of this enzyme and inhibition by CMP and other cytidine nucleotides

The membrane-bound sialyltransferase obtained from Escherichia coli K-235 grown in a chemically defined medium (ideal for colominic acid production) was studied. The in vivo half-life calculated for this enzyme was 20 h. Kinetic tests revealed (at 33 degrees C and pH 8.3) hyperbolic behaviour with respect to CMP-Neu5Ac (Km250 microM) and a transition temperature at 31.3 degrees C. The enzyme was inhibited by NH4+, some divalent cations and by several agents that react with thiol groups. Detergents and fatty acids also inhibited the sialyltransferase activity. In vitro synthesis of colominic acid is strongly inhibited by CMP by blocking the incorporation of [14C]Neu5Ac into a protein-complex intermediate and therefore into free polymer. CDP and CTP also inhibited (91% and 84%) this enzyme activity whereas cytosine and cytidine had no effect. CMP inhibition corresponded to a competitive model the calculated Ki was 30 microM. Incubations of protein[14C]Neu5Ac with CMP, CDP and CTP led to de novo synthesis of CMP-[14C]Neu5Ac. The presence of colominic acid, which usually displaces the reaction equilibrium towards polymer synthesis, did not affect this de novo CMP-[14C]Neu5Ac formation. CMP also inhibited in vivo colominic acid biosynthesis.     

Characterization of CMP-N-acetylneuraminic acid synthetase of group B streptococci.

The capsular polysaccharide is a critical virulence factor for group B streptococci associated with human infections, yet little is known about capsule biosynthesis. We detected CMP-Neu5Ac synthetase, the enzyme which activates N-acetylneuraminic acid (Neu5Ac, or sialic acid) for transfer to the nascent capsular polysaccharide, in multiple group B streptococcus serotypes, all of which elaborate capsules containing Neu5Ac. CMP-Neu5Ac synthetase isolated from a high-producing type Ib strain was purified 87-fold. The enzyme had apparent Km values of 7.6 for Neu5Ac and 1.4 for CTP and a pH optimum of 8.3 to 9.4, required magnesium, and was stimulated by dithiothreitol. This is the first characterization of an enzyme involved in group B streptococcus capsular polysaccharide biosynthesis.     

Role of capsular sialic acid in virulence and resistance to phagocytosis of Streptococcus suis capsular type 2

Abstract Streptococcus suis capsular type 2 has a capsule rich in sialic acid (NANA). Sialic acid, known to be an antiphagocytic factor for many bacterial species, inhibits the activation of the alternative complement pathway. The role of capsular NANA in virulence, resistance to phagocytosis and intracellular survival of S. suis capsular type 2 was evaluated. In general, a low concentration of NANA was observed for all the S. suis strains tested. In addition, no difference could be found in NANA concentrations between strains of different virulence degrees. Sialic acid concentration increased in the virulent strain 89–1591 and the avirulent strain 90–1330 after in vivo growth with an increased capsular material thickness compared to growth in vitro. No significant difference could be found in the phagocytosis rate by porcine blood monocytes of either strain and strain 89–1591 treated with sialidase or the sialic acid-binding lectin from Sambucus nigra (SNA I). Intracellular survival of strain 89–1591 decreased after treatments with sialidase or lectin, becoming comparable to that of strain 90–1330. Finally, no difference could be seen in virulence using a murine model, even if strain 89–1591 was treated with the enzyme or the lectin. Thus, NANA does not seem to be a critical virulence factor for S. suis capsular type 2.     

Characterization of a CMPNeuAc: lactosylceramide alpha 2----3sialyltransferase from rainbow trout hepatoma (RTH-149) cells

1. The rainbow trout (Oncorhynchus mykiss) CMPNeuAc:lactosylceramide alpha 2----3sialytransferase enzyme from RTH-149 cells has been characterized. 2. Transfer of sialic acid to lactosylceramide was optimal at a pH of 5.9, temperature of 25 degrees C, and in the pressure of 0.3% CF-54, 10 mM Mn2+, 0.1 M sodium cacodylate, and 2 mM ATP. 3. Golgi-rich membrane fractions of RTH-149 cells were found to be enriched in sialidase activity and as such the addition of 40 microM 2,3-dehydro-2-deoxy-N-acetylneuraminic acid was necessary to assay alpha 2----3sialyltransferase activity optimally. 4. Apparent Km for donor (CMPNeuAc) and acceptor (lactosylceramide) were found to be 243 microM and 34 microM, respectively. 5. The alpha 2----3sialyltransferase characterized was found to be primarily specific for lactosylceramide though minor activity with other glycolipid acceptors was observed. 6. The presence of another sialyltransferase with differing substrate specificity was noted. 7. Properties of this enzyme, compared to analogous mammalian enzymes, are discussed.     

Regulation of the lipopolysaccharide-specific sialyltransferase activity of gonococci by the growth state of the bacteria, but not by carbon source, catabolite repression or oxygen supply

The enzyme sialyltransferase (STase) of Neisseria gonorrhoeae is a major pathogenicitiy determinant. Using a refined method for assaying the STase activity, the Km for CMP-NANA was shown to be 14 +/– 2 M, higher than that reported previously. Rates of sialylation by Nonidet extracts, prepared under conditions that optimise solubilisation of the membrane-bound enzyme, were 6 to 20 nmol of NANA transferred from CMP-¹⁴C-NANA onto isolated lipopolysaccharide/min./mg of extracted protein, far higher than the previously reported rates of less than 1 nmol of NANA transferred/min./mg of extracted protein. Gonococci grew more slowly with lactate or pyruvate than with glucose as the carbon source. Although growth with a mixture of limiting concentrations of both glucose and lactate was biphasic, diauxic growth was also found in the control culture supplied with glucose alone. The growth rate in the presence of lactate alone was slower than with glucose. The growth rate increased slightly relative to the glucose culture when both substrates were available; lactate was consumed more rapidly than glucose. Higher STase activities were found in bacteria harvested in the exponential than in the stationary phase of aerobic growth: the activity in aerated cultures was higher than those of oxygen-limited or anaerobic cultures. Similar STase activities were found in bacteria that had been grown with glucose, lactate or pyruvate as the carbon and energy source. Sialyltransferase synthesis is essentially constitutive: it is not regulated by glucose repression or by induction by lactate or anaerobiosis.     

A surface polysaccharide forms when gonococci are converted to serum resistance by cytidine 5''-monophospho-N-acetyl neuraminic acid

A serum-susceptible, guinea-pig chamber-passaged, laboratory strain (BS4 (agar)) of Neisseria gonorrhoeae was converted to serum resistance by incubation with cytidine 5-monophospho-N-acetyl neuraminic acid (CMP-NANA) and examined by electron microscopy after staining with ruthenium-red. In contrast to serum susceptible gonococci incubated without CMP-NANA, the majority (60-70%) of the serum resistant organisms showed a surface accumulation of polysaccharide. This surface polysaccharide was enhanced on all the resistant gonococci after incubation with fresh human serum. Control susceptible gonococci were devoid of the polysaccharide after incubation with heated human serum. Identical results were obtained with a fresh gonococcal isolate which had lost serum resistance on subculture but which, in common with 3 other isolates, was restored to serum resistance by incubation with CMP-NANA.     

Molecular cloning and analysis of genes for sialic acid synthesis in Neisseria meningitidis group B and purification of the meningococcal CMP-NeuNAc synthetase enzyme.

The gene encoding for the CMP-NeuNAc synthetase enzyme of Neisseria meningitidis group B was cloned by complementation of a mutant of Escherichia coli defective for this enzyme. The gene (neuA) was isolated on a 4.1-kb fragment of meningococcal chromosomal DNA. Determination of the nucleotide sequence of this fragment revealed the presence of three genes, termed neuA, neuB, and neuC, organized in a single operon. The presence of a truncated ctrA gene at one end of the cloned DNA and a truncated gene encoding for the meningococcal sialyltransferase at the other confirmed that the cloned DNA corresponded to region A and part of region C of the meningococcal capsule gene cluster. The predicted amino acid sequence of the meningococcal NeuA protein was 57% homologous to that of NeuA, the CMP-NeuNAc synthetase encoded by E. coli K1. The predicted molecular mass of meningococcal NeuA protein was 24.8 kDa, which was 6 kDa larger than that formerly predicted (U. Edwards and M. Frosch, FEMS Microbiol. Lett. 96:161-166, 1992). Purification of the recombinant meningococcal NeuA protein together with determination of the N-terminal amino acid sequence confirmed that this 24.8-kDa protein was indeed the meningococcal CMP-NeuNAc synthetase. The predicted amino acid sequences of the two other encoded proteins were homologous to those of the NeuC and NeuB proteins of E. coli K1, two proteins involved in the synthesis of NeuNAc. These results indicate that common steps exist in the biosynthesis of NeuNAc in these two microorganisms.     

Identification and characterization of N-acetyl-2,3-didehydro-2-deoxyneuraminic acid as a metabolite in mammalian brain

We have identified N-acetyl-2,3-didehydro-2-deoxyneuraminic acid (NADNA) in bovine and in rat brain. Identification was made by mass spectrometric and gas-liquid chromatographic analysis of the per(trimethylsilyl) derivative of the purified brain compound. Central nervous system NADNA hitherto has escaped detection; it behaves chromogenically and chromatographically during purification on ion-exchange chromatography as free N-acetylneuraminic acid (NANA) that also occurs in brain. Although NADNA is a dehydro analogue of NANA, we have ascertained that brain NANA does not give rise to NADNA as an artifact during its purification from brain. Three hours after intracranial injection of [14C]-N-acetylmannosamine [( 14C]ManNAc), we detected [14C]NANA but no [14C]NADNA in rat brain. ManNAc is a brain NANA precursor, and at this time, formation of cytidine 5'-phosphate (CMP)-[14C]NANA from [14C]ManNAc is at a maximum. This finding precludes decomposition of CMP-NANA as a source of brain NADNA. Upon intracranial injection of [14C]ManNAc, [14C]NADNA became detectable at 19 h and reached a maximum level around 40 h later; this maximum of labeling of NADNA coincides with the maximum label in brain sialo conjugate-NANA. These findings clearly demonstrate the occurrence of NADNA in mammalian brain. From the evidence, NADNA may derive enzymatically from brain sialo conjugates.     

Synthesis of group-specific polysaccharide by different strains of Neisseria meningitidis serogroup B under various conditions

Sonicated lysates of 5 N. meningitidis strains, serogroup B, obtained from two solid serum-free culture media (a medium with casamino acids and a medium with Hottinger's hydrolysate) were studied with the aim of comparing the capacity of different group B meningococcal strains for the accumulation of group-specific polysaccharide. In the lysates obtained after 7-hour growth no sialic acid was found. After 20-hour cultivation, group-specific polysaccharide was detected in the lysates obtained from 4 out of 5 strains. All sonicated lysates obtained in these experiments were serologically active. The lysates obtained from the medium containing casamino acid had a higher content of group-specific polysaccharide. N. meningitidis strain 125, obtained at the Mechnikov Central Research Institute for Vaccines and Sera (Moscow) by selection, showed the highest content of capsular polysaccharide in microbial cells and the stable yield of biomass.     

One-pot multi-enzyme (OPME) chemoenzymatic synthesis of sialyl-Tn-MUC1 and sialyl-T-MUC1 glycopeptides containing natural or non-natural sialic acid

A series of STn-MUC1 and ST-MUC1 glycopeptides containing naturally occurring and non-natural sialic acids have been chemoenzymatically synthesized from Tn-MUC1 glycopeptide using one-pot multienzyme (OPME) approaches. In situ generation of the sialyltransferase donor cytidine 5′-monophosphate-sialic acid (CMP-Sia) using a CMP-sialic acid synthetase in the presence of an extra amount of cytidine 5′-triphosphate (CTP) and removal of CMP from the reaction mixture by flash C18 cartridge purification allow the complete consumption of Tn-MUC1 glycopeptide for quantitative synthesis of STn-MUC1. A Campylobacter jejuni β1–3GalT (CjCgtBΔ30-His₆) mutant has been found to catalyze the transfer of one or more galactose residues to Tn-MUC1 for the synthesis of T-MUC1 and galactosylated T-MUC1. Sialylation of T-MUC1 using Pasteurella multocida α2–3-sialyltransferase 3 (PmST3) with Neisseria meningitidis CMP-sialic acid synthetase (NmCSS) and Escherichia coli sialic acid aldolase in one pot produced ST-MUC1 efficiently. These glycopeptides are potential cancer vaccine candidates.     

Role of lipid intermediate(s) in the synthesis of serogroup B Neisseria meningitidis capsular polysaccharide.

Neisseria meningitidis serogroup B strain M986 was examined for the involvement of lipid intermediate(s) participating in the biosynthesis of the sialic acid capsular polysaccharide. The addition of exogenous undecaprenyl phosphate, phosphatidylethanolamine, or phosphatidylglycerol to particulate membranes, in the presence of cytidine 5'-monophosphosialic acid, resulted in the stimulation of sialyltransferase activity specifically by undecaprenyl phosphate. Sialyltransferase activity, after delipidation of particulate membrane proteins, was specifically reconstituted by undecaprenyl phosphate. After the addition of 14C-labeled cytidine 5'-monophosphosialic acid to particulate membranes, the level of labeled lipid intermediate(s), extracted by chloroform-methanol (2:1), increased up to a maximum level between 3.75 and 5.0 min, which subsequently decreased to a lower steady-state level. Pulse-chase experiments revealed a transient, solvent-extractable, lipid-linked component. The extracted N-acetylneuraminic acid was in polymeric form. Sequential oxidation and reduction of the extracted radioactivity followed by neuraminidase treatment revealed an average degree of polymerization of four or five N-acetylneuraminic acid residues. Bacitracin-sensitive peptidoglycan was synthesized in vitro by particulate membranes. Cross-competition experiments between peptidoglycan and capsular polysaccharide synthesis by preincubation of precursors of one pathway during synthesis of the other revealed a competitive effect for a common component. This component was believed to be a common pool of undecaprenyl phosphate. A model for the production and regulation of the capsular polysaccharide is proposed.     

Regulation of rat-liver glycoprotein: N-acetylneuraminic acid transferase activity by pyrimidine nucleotides.

CMP-N-acetylneuraminic acid: glycoprotein sialyltransferase activities were assayed in rat liver microsomal fractions using desialylated fetuin as the substrate acceptors for N-acetylneuraminic acid. It was found that cytidine nucleotides specifically depressed enzyme activities. CMP was shown to act as a competitive inhibitor with an apparent Ki of 0.62 mM. N-Acetylneuraminic acid at 1.15 mM had no effect on enzyme activities. Uridine nucleotides at 1.15 mM, especially UDP, increased enzyme activities. UDP may act as an allosteric activating agent increasing the apparent V. Other nucleotides, sugars and nucleotide-sugars at similar concentrations affected sialyltransferase activities only slightly. A general mechanism is proposed for the regulation of glycosyltransferase activities by free nucleotides.     

Structural studies on the sialic acid polysaccharide antigen of Escherichia coli strain Bos-12.

A polysaccharide, antigenically related to group C meningococcus, has been isolated from Escherichia coli strain Bos-12 (016; K92; NM). Like groups B and C meningococcal polysaccharide, the Bos-12 antigen is a pure polymer of sialic acid. 13C NMR studies on the meningococcal group B and C polysaccharides have indicated that the former consists of sialic acid units linked 2 leads to 8- alpha, whereas the latter contains the sialic acid residues linked 2 leads to 9-alpha (Bhattacharjee, A.K., Jennings, H.J., Kenny, C.P., Martin, A., and Smith, I.C.P. (1975), J. Biol. Chem. 250, 1926). Comparison of natural abundance 13C NMR spectra of the Bos-12 polysaccharide with group B and C meningococcal polysaccharides established that Bos-12 was either (a) an equimolar mixture of 2 leads to 8-alpha linked sialic acid homopolymers or (b) a 2 leads to 8-alpha/2 leads to 9-alpha heteropolymer. These possibilities were distinguished in the following manner. The fact that Bos-12 polysaccharide precipitated with anti-group C serum but not with anti-group B serum would seem to exclude a. Further, chemical studies (periodate oxidation followed by tritiated NaBH4 reduction) gave saccharides with a radioactive-labeling pattern expected for alternating 2 leads to 8-alpha/2 leads to 9-alpha sialic acid linkages. Bos-12 is thus an 2 leads to 8/2 lead to 9-alpha heteropolymer.     

Sialyltransferase, sialic acid and sialoglycoconjugates in metastatic tumor and human liver tissue

1. The specific activities of sialytransferase in metastatic tumor sites were 5-37% of those of uninvolved non-cancerous tissue. The non-cancerous host tissues had a slightly lower average sialyltransferase activity than that of non-pathological control livers (986 vs 1194 dpm/min/mg protein). 2. The levels of total and bound sialic acid are increased 1.4 to 7.2-fold in homogenates, supernatants and resuspended pellets of metastatic tumor sites compared to non-cancerous and non-pathological control livers. For all these tissues, 24-29% of the bound sialic acid is found on soluble components (in the 16,300 g supernatant). 3. Soluble sialoglycoconjugates from most metastatic tumor sites give gel filtration profiles different from those of non-cancerous and non-pathological control livers.     

Cytidine 5'-monophosphate (CMP)-induced structural changes in a multifunctional sialyltransferase from Pasteurella multocida

Sialyltransferases catalyze reactions that transfer a sialic acid from CMP-sialic acid to an acceptor (a structure terminated with galactose, N-acetylgalactosamine, or sialic acid). They are key enzymes that catalyze the synthesis of sialic acid-containing oligosaccharides, polysaccharides, and glycoconjugates that play pivotal roles in many critical physiological and pathological processes. The structures of a truncated multifunctional Pasteurella multocida sialyltransferase (Delta24PmST1), in the absence and presence of CMP, have been determined by X-ray crystallography at 1.65 and 2.0 A resolutions, respectively. The Delta24PmST1 exists as a monomer in solution and in crystals. Different from the reported crystal structure of a bifunctional sialyltransferase CstII that has only one Rossmann domain, the overall structure of the Delta24PmST1 consists of two separate Rossmann nucleotide-binding domains. The Delta24PmST1 structure, thus, represents the first sialyltransferase structure that belongs to the glycosyltransferase-B (GT-B) structural group. Unlike all other known GT-B structures, however, there is no C-terminal extension that interacts with the N-terminal domain in the Delta24PmST1 structure. The CMP binding site is located in the deep cleft between the two Rossmann domains. Nevertheless, the CMP only forms interactions with residues in the C-terminal domain. The binding of CMP to the protein causes a large closure movement of the N-terminal Rossmann domain toward the C-terminal nucleotide-binding domain. Ser 143 of the N-terminal domain moves up to hydrogen-bond to Tyr 388 of the C-terminal domain. Both Ser 143 and Tyr 388 form hydrogen bonds to a water molecule, which in turn hydrogen-bonds to the terminal phosphate oxygen of CMP. These interactions may trigger the closure between the two domains. Additionally, a short helix near the active site seen in the apo structure becomes disordered upon binding to CMP. This helix may swing down upon binding to donor CMP-sialic acid to form the binding pocket for an acceptor.     

Cell surface sialic acid influences tumor cell recognition in the mixed lymphocyte reaction

The Ia+ B cell lymphoma, AKTB-1b, fails to stimulate thymic lymphocytes in a one-way mixed lymphocyte reaction unless pretreated with sialidase or inhibitors of N-linked oligosaccharide processing. A comparison of different sialidases and sialyltransferases suggests that the removal of only a subset of total surface sialic acid, rather than net desialylation of the cell surface, is required. Three sialidases were compared, including Vibrio cholerae (VC) and Clostridium perfringens (CP), which will cleave alpha 2-3, alpha 2-6, and alpha 2-8, sialic acid linkages, and Newcastle Disease virus (NDV), which will remove only alpha 2-3 and alpha 2-8 linked sialic acid. When treated with equivalent units of sialidase, CP-, VC-, and NDV-treated cells were 24-fold, sixfold, and threefold better stimulators than untreated cells. In contrast, VC released 1.3-fold and 2.5-fold more sialic acid per cell than did CP or NDV, respectively. Furthermore, VC was superior in reducing the levels of binding of the sialic acid-specific lectin, Limulus polyphemus agglutinin, in exposing Gal beta 1-3GalNAc and Gal beta 1-4GlcNAc residues, and in desialylating gangliosides. Two-dimensional gel analysis indicated that VC and CP were both equal and superior to NDV in the desialylation of iodinatable cell-surface proteins, including H-2Kk, I-A beta k, and a highly sialylated 65,000 dalton protein of unknown identity. Maximal resialylation of CP-treated cells with exogenously added CMP-NANA and either the alpha 2-3(Gal beta 1-3GalNAc) or alpha 2-6(Gal beta 1-4GlcNAc) sialyltransferase did not reduce the stimulatory capacity of these cells. However, resialylation of VC-treated cells with just CMP-NANA alone resulted in 49% reversal of their stimulatory capacity, and no additional reversal could be achieved with either of the sialyltransferases. Although the alpha 2-6(Gal beta 1-4GlcNAc) sialyltransferase was capable of adding back approximately 10% of the sialic acid removed, the endogenous activity added back approximately 0.1% of the total sialic acid removed. SDS-PAGE gels of the sialylated cells indicated that the exogenously added sialyltransferase labeled many different proteins, whereas the endogenous activity labeled far fewer proteins, predominantly in 46,000 and 25,000 m.w. range. Both the desialylation and resialylation data suggest that the sialidase-dependent stimulation is due to the desialylation of specific membrane structures. Together with previous studies, these data suggest that the sialic acids involved are probably alpha 2-6 linked to N-linked glycosyl moieties.