Human interferon omega 1: isolation of the gene, expression in Chinese hamster ovary cells and characterization of the recombinant protein.

A gene encoding human interferon omega-1 (IFN-omega 1) was isolated from a cosmid library, sequenced and expressed in Chinese hamster ovary (CHO) cells under the control of an SV40-derived promoter/enhancer sequence. Culture supernatants of stably transfected cell clones contained biologically active IFN-omega 1 at concentrations up to 10 micrograms/l. Amplification of the expression vector containing a dhfr gene under methotrexate selection pressure resulted in yields up to 200 micrograms/l. Production of IFN-omega 1 was further enhanced 2- to 3-fold by propagation of the cells in the presence of n-butyrate. IFN-omega 1 was purified from culture supernatants by monoclonal antibody affinity chromatography. The resulting protein was at least 95% pure as determined by reverse-phase HPLC and size-exclusion HPLC. Sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) showed two bands of about the same intensity with apparent molecular masses of 24.5 and 22.5 kDa. Upon treatment with peptide:N-glycosidase F, both bands were shifted to lower molecular masses (20.5 and 18.5 kDa), indicating that CHO cell-derived IFN-omega 1 is glycosylated; Asn-78 was identified as the glycosylation site. Analysis of the carbohydrate moiety using glycosidases and lectins revealed the presence of biantennary complex oligosaccharides containing neuraminic acid. Amino acid sequencing showed that only about 40% of the molecules have the expected N-terminus, whereas the others carry two additional amino acids derived from the signal sequence. C-terminal amino acid sequencing using carboxypeptidase P demonstrated that the smaller form of the protein lacks nine amino acids. Disulfide bridges were shown to connect Cys residues 1 and 99 as well as 29 and 139, respectively, as in IFN-alpha. The specific antiviral activity of recombinant, glycosylated human IFN-omega 1 on human cells was 2.6 x 10(8) IU/mg, not significantly different from that of the authentic, human leukocyte-derived protein.     

Identification of the glycosylation site of the adenovirus type 5 fiber protein

The fiber protein purified from the pool of nonincorporated viral protein after infection of cells with adenovirus type 5 exists as two forms separable by reverse-phase HPLC. As determined by mass spectrometry, this heterogeneity results from a difference in one O-linked N-acetylglucosamine (GlcNac). A western blot analysis using a monoclonal antibody directed against the GlcNac motif showed that only one of the two forms reacted with the antibody, suggesting that one form carries a single GlcNac and the other form has none. The ratio of glycosylated to nonglycosylated forms of fiber, which is about 1, is conserved in assembled viruses. After digestion of glycosylated fiber with endoproteinase GluC, isolation of the glycosylated peptide by reverse-phase HPLC, and chemical derivatization using dimethylamine, the site of glycosylation was located in the fiber shaft at serine 109 by mass spectrometry. Elimination of glycosylation by site-directed mutagenesis of fiber should help to understand the function of this postranslational modification.     

Carbohydrate composition of Carcinus aestuarii hemocyanin

The hemocyanin of the crab Carcinus aestuarii contains a carbohydrate moiety that represents 1.6% of protein mass. This carbohydrate content is higher than that exhibited by other arthropod hemocyanins so far investigated. By combination of FPLC ion exchange chromatography and reverse-phase HPLC, the native oligomeric protein can be resolved into three major and one minor electrophoretically pure fractions that are found to be homogeneous by N-terminal sequencing and correspond to the subunit polypeptide chains. Sugar analysis on the different subunits reveals that the subunit referred to as Ca2 is glycosylated, with a carbohydrate content of 6.3%. By Ca2 trypsin digestion, separation of glycopeptides, and amino acid sequencing, three consensus sequences for O-glycosylation and one for N-glycosylation were found. MALDI-MS was applied for the determination of the molecular masses of the various glycopeptides and peptides after removal of carbohydrates by neuraminidase and alpha-N-acetylgalactosaminidase.     

Identification of three pertussis toxin substrates (41, 40 and 39 kDa proteins) in mammalian brain. Comparison of predicted amino acid sequences from G-protein alpha-subunit genes and cDNAs with partial amino acid sequences from purified proteins

We have determined the partial amino acid sequences of the 40 kDa protein, one of the three pertussis toxin substrates in porcine brain. Purified 40 kDa protein from porcine brain was completely digested with TPCK-trypsin. Digested peptides were separated by reverse-phase HPLC and subjected to analysis by gas-phase protein sequencing. Several sequences of porcine brain 40 kDa protein completely matched with those which were deduced from the nucleotide sequences of the human Gi2 alpha gene and rat Gi2 alpha cDNA. On the other hand, the previously determined sequences of the rat brain 41 and 39 kDa proteins were in complete agreement with the predicted amino acid sequences of rat Gi1 alpha and Go alpha cDNAs, respectively.     

Serotriflin, a CRISP family protein with binding affinity for small serum protein-2 in snake serum

Habu (Trimeresurus flavoviridis) serum contains 3 small serum proteins (SSP-1, SSP-2, and SSP-3) with molecular masses of 6.5 to 10 kDa. Gel filtration analysis showed that all the SSPs exist in high molecular mass forms of approximately 60 kDa in the serum. Ultrafiltration of Habu serum showed that SSPs dissociated from the complex below a pH of 4. An SSP-binding protein was purified from Habu serum by gel filtration, ion exchange, and reverse-phase HPLC. N-terminal sequencing yielded a 39-amino acid sequence, similar to the N-terminal region of triflin, which is a snake venom-derived Ca2+ channel blocker that suppresses smooth muscle contraction. The amino acid sequence of this protein, termed serotriflin, was established by peptide analysis and cDNA cloning. Serotriflin is a glycosylated protein and consists of 221 amino acids. Among the 3 SSPs, only SSP-2 formed a noncovalent complex with serotriflin. It was bound to triflin and serotriflin with high affinity, as evidenced by surface plasmon resonance. SSP-2 is considered to be a protein that prevents self injury by accidental leaking of venom into the blood.     

Isolation of fibronectin type III like peptide from human placental extract used as wound healer

A peptide of around 7.4 kDa has been purified from the aqueous extract of human placenta used as wound healer. Derived partial amino acid sequence from mass spectrometric analysis showed its homology with human fibronectin type III. Under nondenaturing condition, it formed aggregate, the elution pattern of which from reverse-phase HPLC was identical with that of fibronectin type III. Immuno-blot of the peptide with reference fibronectin type III-C showed strong cross reactivity. Since fibronectin type III plays important roles in wound healing, similar peptide in the extract is likely to take part in curing process.     

N-linked glycosylation of a proenkephalin A-derived peptide. Evidence for the glycosylation of an NH2-terminally extended Met-enkephalin Arg6-Gly7-Leu8 variant

To investigate the possibility that the opioid peptide precursor proenkephalin A was glycosylated, we utilized an antiserum raised against the COOH terminus of Met-enkephalin Arg6-Gly7-Leu8 (MERGL) to identify and characterize enkephalin-containing peptides from extracts of bovine adrenal medulla. Sephadex G-50 gel filtration separated two immunoreactive peaks which had apparent masses of 9 and 6 kDa. Anion-exchange chromatography and reverse-phase high pressure liquid chromatography (HPLC) revealed that the 9-kDa material was a heterogenous mixture of immunoreactive peptides, of which one (9K-MERGL Ia) was purified to homogeneity. The 6-kDa material separated into two major immunoreactive peaks (6K-MERGL I and 6K-MERGL II) on anion-exchange chromatography, and these were obtained in an homogenous form after reverse-phase HPLC. Amino acid sequencing, together with immunological characterization, indicated that the three peptides were identical in chain length, and corresponded to proenkephalin A 116-165. They contained the sequence Asn-Ser-Ser which is a potential N-glycosylation site. In 9K-MERGL Ia, but not the others, automated Edman amino acid sequencing was unable to detect the relevant asparagine residue, suggesting that this residue has been chemically modified. Further investigation of the 9K-MERGL material using lectin affinity chromatography provided direct evidence of glycosylation. Verification of this result was obtained using the specific enzyme glycopeptidase F (glycopeptide-N-glycosidase) which demonstrated that 9K-MERGL contained, in part, N-linked oligosaccharide chains. These results show that an NH2 terminally extended Met-enkephalin Arg6-Gly7-Leu8 variant was N-glycosylated, and hence indicate that the precursor polypeptide proenkephalin A can be glycosylated during translation in the rough endoplasmic reticulum.     

Insulin-like growth factor-binding protein from human plasma. Purification and characterization

Insulin-like growth factor (IGF)-binding protein (BP) has been purified from Cohn fraction IV of human plasma by acidification, ion exchange to remove endogenous ligands, and affinity chromatography on agarose-IGF-II. The pure protein appeared as a single peak by high performance reverse-phase and gel permeation chromatography (molecular mass, 45-50 kDa), but on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave a major band at 53 kDa and a minor band at 47 kDa, unreduced, or 43 and 40 kDa, respectively, reduced. The two bands stained for both protein and carbohydrate. After storage at 2 degrees C for 5 months at pH 3, two additional bands, at 26 and 22 kDa on unreduced gels, were also present. Autoradiography after affinity labeling with IGF-I or IGF-II tracer revealed a single labeled band of 61 kDa. BP, quantitated using a specific radioimmunoassay, was retained by agarose-immobilized IGF-I, IGF-II, concanavalin A, and wheat germ lectin, but not Helix pomatia lectin. Competitive binding curves using pure BP and human IGF-I and IGF-II as both labeled and unlabeled ligands indicated association constants of 2-3 X 10(10) liters/mol for both peptides, with a slightly higher affinity for IGF-II than IGF-I, and 0.9 binding sites for either peptide per 53-kDa protein. The exact relationship of this acid-stable IGF BP to the 150-kDa complex from which it is derived remains to be determined.     

S layer protein of Clostridium tetani: purification and properties.

S layer protein of Clostridium tetani strain AO 174, a nontoxigenic derivative of strain Harvard A 47, was prepared from the cell walls by 4 M urea extraction and purified by DEAE-Sepharose CL-6B chromatography followed by a combination of anion-exchange chromatography and reverse-phase chromatography using an HPLC system. The molecular weight of the S layer protein was estimated to be 140 kilodaltons (kDa) by SDS-PAGE. The amino acid composition of the 140 kDa protein was very similar to those of S layer proteins from the other bacterial species: it was rich in acidic amino acid and lacked cysteine. Also, the protein was unique in its extremely low content of proline (0.02 to 0.03 mol%). Multiple isoelectric forms ranging from pH 4.0 to 4.5 were observed in the purified preparation. Immunodiffusion analysis showed that the 140 kDa protein was a common antigen to the three strains of C. tetani tested.     

Purification and Characterization of a Glycine-rich Polypeptide Tightly Bound to Cell Walls from Soybean Aleurone Layers

A part of cell walls in soybean aleurone layers remained undigested after pectinase and cellulase treatments, and the features of the undigested cell walls were similar to those of Casparian strips. Glycine-rich polypeptides (GRPP) were extracted with 0.4 n NaOH from the undigested cell walls, Casparian strip-like tissues. Approximately 6.5–kDa GRPP obtained by gel-permeation chromatography from the extract was purified by anion-exchange HPLC and reverse-phase HPLC. The major amino acids of GRPP were glycine (69%) and serine (13%). The n-terminal amino acid sequence of GRPP was the same polyglycine as 30 kDa glycine-rich protein (GRP). GRPP would participate in adhesion between neighboring cells in aleurone layers because of tight binding to the cell wall.     

Stabilization and improvement of catalytic activity of a low molar mass cellobiase by cellobiase-sucrase aggregation in the culture filtrate of Termitomyces clypeatus

The extracellular cellobiase (EC 3.2.1.21) of Termitomyces clypeatus separated in two protein fractions when culture filtrate or ammonium sulfate precipitated proteins were chromatographed on BioGel P-200 column. During purification of cellobiase (CBS) from the lower molar mass (LMM) protein fraction, the enzyme behaved like a low molecular weight multimeric protein. The purified enzyme gave a single 56 kDa band in SDS-PAGE but ladderlike bands (14, 28, 42, and 56 kDa) on denaturation by reducing-SDS and urea. The protein, however, dissociated on dilution and protomeric (14 kDa) and multimeric forms (28 and 60 kDa) were eluted separately during HPGPLC. Specific activity of CBS gradually decreased as the molar mass of the enzyme was lowered in different eluted peaks. Protein present in all CBS pool fractions had the same amino acid composition and all displayed the same, single protein peak in reverse-phase HPLC and 56 kDa band in SDS-PAGE. Thus, T. clypeatus CBS was a multimeric 14 kDa protein that was optimally active as a tetramer. CBS purified from the higher molar mass fraction (HMM) as a SDS-PAGE homogeneous 110-kDa protein did not dissociate on dilution or by SDS-urea. The purified protein was a protein aggregate as CBS consistently contained 20 +/- 5% sucrase (SUC) Units in the preparation. The aggregate resolved during reverse-phase chromatography on a C(4) column, and an additional protein peak other than CBS was detected. The aggregated CBS had a higher temperature optimum and was more stable toward thermal and chemical denaturations than SUC-free CBS. Increase of stability and catalytic activity of CBS by aggregation with SUC was much higher than those by the multimerization of CBS itself. All of these observations for the first time suggested that the heterologous protein-protein aggregation, observed for a long time for fungal enzymes, might have a significant role in modulating physicochemical properties of the extracellular enzyme.     

Identification of three pertussis toxin substrates (41, 40 and 39 kDa proteins) in mammalian brain Comparison of predicted amino acid sequences from G-protein α-subunit genes and cDNAs with partial amino acid sequences from purified proteins

We have determined the partial amino acid sequences of the 40 kDa protein, one of the three pertussis toxin substrates in porcine brain. Purified 40 kDa protein from porcine brain was completely digested with TPCK-trypsin. Digested peptides were separated by reverse-phase HPLC and subjected to analysis by gas-phase protein sequencing. Several sequences of porcine brain 40 kDa protein completely matched with those which were deduced from the nucleotide sequences of the human G_i2α gene and rat G_i2α cDNA. On the other hand, the previously determined sequences of the rat brain 41 and 39 kDa proteins were in complete agreement with the predicted amino acid sequences of rat G_i1α and G_oα cDNAs, respectively.     

gp160, the envelope glycoprotein of human immunodeficiency virus type 1, is a dimer of 125-kilodalton subunits stabilized through interactions between their gp41 domains.

The molecular masses, carbohydrate contents, oligomeric status, and overall molecular structure of the env glycoproteins of human immunodeficiency virus type 1--gp120, gp160, and gp41--have been determined by quantitative electron microscopy. Using purified gp160s, a water-soluble form of env purified from a recombinant vaccinia virus expression system, we have measured the masses of several hundred individual molecules by dark-field scanning transmission electron microscopy. When combined with sequence-based information, these mass measurements establish that gp160s is a dimer of subunits with an average monomer mass of 123 kDa, of which approximately 32 kDa is carbohydrate and 91 kDa is protein. Similarly, gp120 was found to be a monomer of 89 kDa and to contain virtually all of env's glycosylation. gp41 is glycosylated only slightly, if at all, and is responsible for the interactions that stabilize the gp160s dimer. A molecular mass map of gp160s derived by image processing depicts an asymmetric dumbbell whose two domains have masses of approximately 173 and approximately 73 kDa, corresponding to a gp120 dimer and a gp41 dimer, respectively. We infer that the average monomer mass of native gp160 is 125 kDa and that in situ, env is either a dimer or a tetramer but is most unlikely to be a trimer.     

Purification and characterization of apolipoprotein J

Apolipoprotein J (apoJ), a unique 70-kDa component of high density lipoproteins in human plasma, consists of two disulfide-linked subunits designated apoJ alpha (34-36 kDa), and apoJ beta (36-39 kDa) which share pI values of 4.9-5.4 and which are recognized by a monoclonal antibody (mAb) 11. ApoJ and its subunits were purified to homogeneity from plasma by a combination of immunoaffinity chromatography, using mAb11 linked to Affi-Gel, and reverse-phase high performance liquid chromatography. ApoJ alpha and apoJ beta are both glycoproteins. When deglycosylated, the molecular mass of apoJ alpha is 24 kDa and that of apoJ beta is 28 kDa, suggesting that approximately 30% of the mass of each subunit is carbohydrate. The amino acid compositions of apoJ alpha and apoJ beta are very similar; however, the sequences of the first 30-amino acid residues are distinct. A comparison of peptide maps suggests that apoJ alpha and apoJ beta are not identical but share limited regions of homology. This possibility is supported by immunochemical data. Five additional mAb specific for apoJ were characterized. One of the mAb, like mAb11, reacts with both apoJ alpha and apoJ beta; the others react with apoJ alpha only. All mAb, including those which recognize both apoJ alpha and apoJ beta and those which recognize apoJ alpha only, immunoprecipitate a approximately 50-kDa protein synthesized from a liver mRNA template translated in a rabbit reticulocyte lysate. We propose that the apoJ alpha and apoJ beta subunits, which have limited homology, are derived by proteolytic cleavage of a common precursor.     

Functional expression, purification, and characterization of human Flt3 ligand in the Pichia pastoris system

Flt3 ligand (FL) is a potent hematopoietic cytokine that affects the growth and differentiation of hematopoietic progenitor and stem cells both in vivo and in vitro. Pichia pastoris transformants secreting high-level rhFL were obtained using 'yeastern blotting' method and the expression level in liquid was about 30 mg/L. rhFL was purified to about 95% purity with overnight dialysis, filtration and an anion-exchange step. Further purification steps employing Sephacryl S-200 and reverse-phase HPLC raised the purity to over 99%. The purified rhFL possessed correct N-terminal amino acid sequence and positive Western blotting bands. SDS-PAGE and mass spectrometry analysis showed molecular weight of rhFL was about 21 and 34 kDa, suggesting that rhFL was glycosylated. The result of capillary electrophoresis showed that its pI is 3.12-4.72. Endo H deglycosylation analysis indicated that there was O-glycosylation besides N-glycosylation in rhFL secreted from P. pastoris. Bioactivity assay showed that the purified rhFL had dose-dependent expansion activity on bone marrow nucleated cells.     

Plasmodium falciparum synthesizes O-glycosylated glycoproteins containing O-linked N-acetylglucosamine.

Asexual blood forms of the human malaria parasite, Plasmodium falciparum, synthesize a major glycosylated 195 kDa protein that has been considered for the development of a vaccine. beta-Elimination-borohydride reduction of the 195 kDa glycoprotein and its 16 kDa processed product after metabolic labeling of their carbohydrates, showed the presence of derived, labeled glucosaminitol and alanine. This suggests that the 195 and 16 kDa glycoproteins contain distinct O-glycosyl linkages and that N-acetylglucosamine and serine residues are involved in the attachment of carbohydrate moieties to the protein core. Endo-O-glycanase treatment of total glycoproteins shows that O-glycosidycally-linked sugars represent a major carbohydrate moiety in P. falciparum glycoproteins.     

Structural analysis of N-linked oligosaccharides by a combination of glycopeptidase, exoglycosidases, and high-performance liquid chromatography

A simple, sensitive, and rapid method for the analysis of structures of N-linked carbohydrates is reported. The method involves four steps: preparation of carbohydrate chains from glycopeptides by N-oligosaccharide glycopeptidase digestion; derivatization of the reducing ends of carbohydrate chains with a fluorescent reagent, 2-aminopyridine, by using sodium cyanoborohydride; separation of oligosaccharide derivatives by reverse-phase high-performance liquid chromatography; and structural analysis of oligosaccharides by sequential exoglycosidase digestion. The elution positions of 50 standard oligosaccharide derivatives were determined by HPLC. The structure of an unknown oligosaccharide can be characterized by comparison of its elution position with those of the standard compounds. The method was applied to elucidate the structures of oligosaccharides in the myeloma IgG protein, Yot.     

Stimulation of the biosynthesis of lactosamine repeats in glycoproteins in differentiating U937 cells and its suppression in the presence of NH4Cl.

We prepared a mouse monoclonal antibody, 2D5, which recognized a highly glycosylated human lysosomal membrane antigen. The apparent molecular mass of this antigen was cell type dependent and ranged between 100 kDa and 130 kDa. The difference was due to a variation in the carbohydrate moiety, since upon removal of the N-linked oligosaccharides the size of the glycoprotein was reduced to approximately 50 kDa in all cases. The high carbohydrate contents, subcellular localization and N-terminal sequence indicated a high similarity or identity of this antigen with the lamp-2 protein. In U937 cells several agents known to elicit differentiation induced synthesis of a larger form of the lamp antigen. Thus, treatment of cells with calcitriol resulted in a shift in its average molecular mass from 115 kDa to 130 kDa. The difference was due to an increase in the contents of lactosamine repeats. In subcellular membranes from calcitriol-treated cells the specific activity of the UDP-N-acetylglucosamine: N-acetyllactosamine N-acetylglucosaminyltransferase was enhanced 3-fold. The enhancement was accompanied with an elongation of lactosamine repeats in N-linked oligosaccharides in the 46 kDa mannose 6-phosphate receptor and the homing receptor, the leucocyte antigen CD44. In contrast, the apparent size of the leucocyte antigen CD43 which bears numerous O-linked oligosaccharides was not changed indicating a selectivity in the modulation of the formation of lactosamine repeats in N- and O-linked carbohydrates. It is shown further that the synthesis of lactosamine repeats in U937 cells is impeded in the presence of NH4Cl.     

The asparagine-linked oligosaccharides at individual glycosylation sites in human thyrotrophin.

The asparagine-linked carbohydrate structures at each of the three glycosylation sites of human thyrotrophin were investigated by 400 MHz 1H-NMR spectroscopy. Highly purified, biologically active human thyrotrophin (hTSH) was dissociated into its subunits hTSH alpha (glycosylated at Asn 52 and Asn 78) and hTSH beta (glycosylated at Asn 23). The alpha-subunit was further treated with trypsin which gave two glycopeptides that were subsequently separated by reverse-phase HPLC and identified by amino acid sequence analysis. The oligosaccharides were liberated from hTSH alpha glycopeptides and from intact hTSH beta by hydrazinolysis, and were fractionated as alditols by anion-exchange and ion-suppression amine-adsorption HPLC preparatory to structural analysis. The N-glycans present on hTSH were mainly diantennary complex-type structures with a common Man alpha 1-3 branch that terminated with 4-O-sulphated GalNAc. The Man alpha 1-6 branch displayed structural heterogeneity in the terminal sequence, with chiefly alpha 2-3-sialylated Gal and/or 4-O-sulphated GalNAc. The relative amounts of the two major complete diantennary oligosaccharides and their core fucosylation differed according to glycosylation site; the sulphated/sialylated diantennary oligosaccharide was most abundant at the two sites on the alpha-subunit, whereas the disulphated, core-fucosylated oligosaccharide was more plentiful on the beta-subunit. Some interesting structural features, not previously reported for the N-glycans of hTSH, included 3-O-sulphated galactose (SO4-3Gal) and peripheral fucose (Fuc alpha 1-3GlcNAc) in the Man alpha 1-6 branch of some diantennary structures; the former suggests the presence of a hitherto uncharacterized galactose-3-O-sulphotransferase in thyrotroph cells of the human anterior pituitary gland.     

Inhibitors of protein phosphatase-1. Inhibitor-1 of bovine adipose tissue and a dopamine- and cAMP-regulated phosphoprotein of bovine brain are identical

Protein phosphatase inhibitor-1 was purified from bovine adipose tissue. The protein had an apparent molecular mass of 32 kDa by SDS/PAGE and a Stokes' radius of 3.4 nm. It was phosphorylated by cAMP-dependent protein kinase on a threonyl residue; this phosphorylation was necessary for inhibition of protein phosphatase-1. Bovine adipose tissue inhibitor-1 was compared directly with rabbit skeletal muscle inhibitor-1 and with a 32000-Mr, dopamine- and cAMP-regulated phosphoprotein from bovine brain (DARPP-32), also an inhibitor of protein phosphatase-1. By the following biochemical and immunochemical criteria, bovine adipose tissue inhibitor-1 was found to be very similar and possibly identical to DARPP-32 and was clearly distinct from skeletal muscle inhibitor-1: molecular mass by SDS/PAGE; Stokes' radii; phosphorylation on threonine residues; Staphylococcus-aureus-V8-protease-generated peptide patterns analyzed by SDS/PAGE; tryptic phosphopeptide maps analysed by two-dimensional thin-layer electrophoresis/chromatography; elution on reverse-phase HPLC; chymotryptic peptide maps as analysed by reverse-phase HPLC; amino acid composition; antibody recognition by immunoprecipitation and immunoblotting; effect of cyanogen bromide cleavage on protein phosphatase inhibitor activity. Based on these results we conclude that bovine brain and adipose tissue contain an identical phosphoprotein inhibitor of protein phosphatase-1 (DARPP-32), which is distinct from that of skeletal muscle (inhibitor-1).