Partial structure and hormonal regulation of rabbit liver inhibitor-1; distribution of inhibitor-1 and inhibitor-2 in rabbit and rat tissues

Inhibitor-1 purified from rabbit liver could not be distinguished from the skeletal muscle protein by chromatographic, electrophoretic and immunological criteria. Amino acid sequences comprising 68% of rabbit liver inhibitor-1 were identical to the skeletal muscle protein indicating that they are products of a single gene. Total inhibitor-1 activity in heat-treated rabbit liver extracts was similar to that in skeletal muscle extracts, and the phosphorylation state of inhibitor-1 increased from 14% to 42% in rabbit liver in vivo after an intravenous injection of glucagon. Monospecific antibodies to rabbit skeletal muscle inhibitor-1 recognised a single major protein of identical electrophoretic mobility (26 kDa) in each rabbit tissue examined (skeletal muscle, liver, brain, heart, kidney, uterus and adipose). The antibodies also recognised a single major (30 kDa) protein in the same rat tissues, except liver. The results show that while there are interspecies differences in apparent molecular mass, inhibitor-1 is likely to be the same gene product in each mammalian tissue. Inhibitor-1 was not detected in rat liver, either by activity measurements or immunoblotting, irrespective of the age, sex or strain of the animals. Immunoblotting also failed to detect inhibitor-1 in mouse liver, although it was present in guinea pig, porcine and sheep liver. The absence of inhibitor-1 in rat liver indicates that phosphorylation of this protein cannot underlie the increased phosphorylation of hydroxymethylglutaryl-CoA reductase observed after stimulation by glucagon. Monospecific antibodies to rabbit skeletal muscle inhibitor-2 recognised a 31 kDa protein in each rabbit tissue, and a 33 kDa protein in all rat tissues including liver. The results suggest that inhibitor-2 is the same gene product in each mammalian tissue.     

Muscle proteins related to microtubule associated protein-2 are substrates for an insulin-stimulatable kinase

High molecular weight muscle protein(s), present as a "doublet" (approximately 320 and approximately 290 kDa apparent molecular weight) in partially purified preparations of inhibitor-2 from rabbit skeletal muscle, and homogeneous bovine brain microtubule associated protein-2 are both in vitro substrates for a soluble insulin-stimulatable serine/threonine kinase in 3T3-L1 adipocytes. The high molecular weight muscle substrate "doublet" was specifically immunoprecipitated by affinity-purified anti-microtubule associated protein-2 antibody.     

Phosphorylase phosphatase regulatory subunit. "Western" blotting with immunoglobulins against inhibitor-2 reveals a protein of Mr = 60,000

Rabbit muscle phosphorylase phosphatase has been isolated in different laboratories as an inactive complex of Mr = 70,000, composed of separate catalytic (Mr = 38,000) and regulatory (Mr = 31,000) proteins. The regulatory protein is identical to one of two heatstable inhibitors called inhibitor-2 (I2). Antiserum raised in sheep against I2 by repeated immunization potently blocked inhibitory activity, whereas preimmune serum did not. Immunoglobulins which blocked inhibitory activity were purified by affinity chromatography with I2 as the immobilized ligand. Using a "Western" immunoblotting procedure, as little as 1-5 ng of pure I2, obtained by electroelution of the Mr = 31,000 band of I2 from a polyacrylamide gel segment, were detected. Immunoblotting of the immunogen revealed only a band at Mr = 31,000, indicating the absence of contaminating antigenic proteins. When extracts of skeletal muscle and other rabbit tissues were denatured directly in dodecyl sulfate for immunoblotting the most intensely stained band was present at Mr = 60,000, rather than at Mr = 31,000 as expected. A small amount of I2 and other bands were detected, in particular at Mr = 36,000 and 25,000. Subsequent to heat treatment of the tissue extracts, there was an enrichment of I2 content relative to the Mr = 60,000 band. The results indicate the existence of a Mr = 60,000 protein related to I2. Activation of phosphorylase phosphatase in a muscle extract by treatment with Co2+ plus trypsin exactly coincided with digestion of the Mr = 60,000 immunoreactive protein. Available data indicate that this protein may function as a regulatory subunit of phosphorylase phosphatase.     

The phosphorylase kinase deficiency (Phk) locus in the mouse: Evidence that the mutant allele codes for an enzyme with an abnormal structure

Female (I/St X C57BL/St) F1 mice heterozygous at the sex-linked phosphorylase kinase deficiency locus (Phk) have phosphorylase kinase activities averaging 86% that of mice homozygous for the wild-type allele (C57BL/St), i.e., 72% greater than the sum of one-half the activities of the parental strains. Approximately one-half the phosphorylase kinase activity in the (I X C57BL) F1 muscle extracts had a stability at 42.5 C similar to that of the activity in C57BL extracts (t1/2 = 13.2 min); the other half of the activity in the F1 extracts was more labile (t1/2 = 3.9 min). Two species of phosphorylase kinase activity in F1 muscle extracts were also differentiated with an antiserum prepared in guinea pigs against purified rabbit skeletal muscle phosphorylase kinase. This anti-serum cross-reacted with phosphorylase kinase in C57BL muscle extracts but did not cross-react with skeletal muscle extracts of mice hemi- or homozygous for the mutant allele (I/LnJ). The guinea pig antiserum precipitated 52% as much protein from (I X C57BL)F1 muscle extracts compared to those of C57BL. However, an antiserum prepared against purified rabbit skeletal muscle phosphorylase kinase in the goat cross-reacted with the mutant phosphorylase kinase. The ratio C57BL:(I X C57BL)F1:I of immunoprecipitated protein from skeletal muscle extracts with this antiserum was 1:0.97:1.08. Polyacrylamide gel electrophoresis of the immunoprecipitates in the presence of 0.1% sodium dodecylsulfate showed three subunits for mouse phosphorylase kinase with molecular weights of 139,000, 118,000, and 41,000; these values are similar to the ones obtained with purified rabbit skeletal muscle phosphorylase kinase. These three subunits were also observed in immunoprecipitates from I/LnJ muscle extracts. These results offer substantial evidence (1) that in skeletal muscle extracts of mice heterozygous at the Phk locus the mutant phosphorylase kinase is active, (2) that the gene product of the mutant allele is an enzyme with an abnormal structure, and (3) that the phosphorylase kinase deficiency in I/LnJ skeletal muscle extracts is not the result of the absence of phosphorylase kinase or one of its subunits.     

Mammalian skeletal muscle myosin light chain kinases. A comparison by antiserum cross-reactivity

Purified myosin light chain kinases from skeletal muscle are reported to be significantly smaller (Mr = 75,000-90,000) than the kinases purified from smooth muscle (Mr = 130,000-155,000). It has been suggested that the smaller kinases from striated muscle are proteolytic fragments of a larger enzyme which is homologous, if not identical, to myosin light chain kinase from smooth muscle. Therefore, we have used an antiserum to rabbit skeletal muscle myosin light chain kinase and Western blot analysis to compare the subunit molecular weight of the kinase in skeletal muscle extracts of several mammalian species. In rabbit skeletal muscle, the antiserum only recognized a polypeptide of Mr = 87,000, with no indication that this polypeptide was a proteolyzed fragment of a larger protein. The apparent molecular weights observed in different animal species were 75,000 (mouse), 83,000 (guinea pig), 82,000 (rat), 87,000 (rabbit), 100,000 (dog), and 108,000 (steer). The molecular weight of myosin light chain kinase was constant within an animal species, regardless of skeletal muscle fiber type. The antiserum inhibited the catalytic activity of skeletal muscle myosin light chain kinase. Similar antibody dilution curves for inhibition of myosin light chain kinase activity in extracts were observed for all animal species (rabbit, rat, mouse, guinea pig, dog, cat, steer, and chicken) and different fibers (slow twitch oxidative, fast twitch oxidative glycolytic, and fast twitch glycolytic) tested. The antiserum did not inhibit the activity of rabbit smooth muscle myosin light chain kinase. These results suggest that there may be at least two classes of muscle myosin light chain kinase represented in skeletal and smooth muscles, respectively.     

Partial purification and characterization of heat stable protein phosphatase inhibitor-2 from rabbit reticulocytes

Previous studies indicated that the species of type 1 and type 2 protein phosphatases (PP-1, PP-2) in rabbit reticulocytes are similar to those of rabbit skeletal muscle and rabbit liver. Reticulocyte PP-1 was found to be selectively inhibited by the heat stable protein phosphatase inhibitor-2 (I-2) from rabbit skeletal muscle. Of interest was the observation that muscle I-2 appeared to regulate protein synthesis in reticulocyte lysates by inhibiting an eIF-2 alpha phosphatase with type 1 properties. In this study we have characterized reticulocyte inhibitor-2 (I-2) and find that its properties are similar to those of skeletal muscle I-2. (i) Both I-2 species are stable to boiling and to acid treatment, and have similar chromatographic profiles on DEAE-cellulose and on Blue Sepharose CL-6B. (ii) The two I-2 species migrate electrophoretically as 26-28,000 dalton polypeptides in SDS-acrylamide gels. (iii) Both skeletal muscle I-2 and reticulocyte I-2 selectively inhibit isolated reticulocyte PP-1 and endogenous PP-1 in the lysate. (iv) Reticulocyte I-2 co-chromatographs with PP-1 on DEAE-cellulose, and over 90% of lysate I-2 can be isolated from this partially purified PP-1. (v) Both inhibitor-2 species are active in the unphosphorylated state, but upon addition to lysates, both are phosphorylated by endogenous cAMP-independent protein kinase(s). In addition a preliminary analysis using a polyclonal antibody against muscle inhibitor-1 confirmed biochemical analyses which indicate that lysates are deficient in inhibitor-1.     

The protein phosphatases involved in cellular regulation. Evidence that dephosphorylation of glycogen phosphorylase and glycogen synthase in the glycogen and microsomal fractions of rat liver are catalysed by the same enzyme: protein phosphatase-1

Glycogen synthase (labelled in sites-3) and glycogen phosphorylase from rabbit skeletal muscle were used as substrates to investigate the nature of the protein phosphatases that act on these proteins in the glycogen and microsomal fractions of rat liver. Under the assay conditions employed, glycogen synthase phosphatase and phosphorylase phosphatase activities in both subcellular fractions could be inhibited 80-90% by inhibitor-1 or inhibitor-2, and the concentrations required for half-maximal inhibition were similar. Glycogen synthase phosphatase and phosphorylase phosphatase activities coeluted from Sephadex G-100 as broad peaks, stretching from the void volume to an apparent molecular mass of about 50 kDa. Incubation with trypsin decreased the apparent molecular mass of both activities to about 35 kDa, and decreased their I50 for inhibitors-1 and -2 in an identical manner. After tryptic digestion, the I50 values for inhibitors-1 and -2 were very similar to those of the catalytic subunit of protein phosphatase-1 from rabbit skeletal muscle. The glycogen and microsomal fractions of rat liver dephosphorylated the beta-subunit of phosphorylase kinase much faster than the alpha-subunit and dephosphorylation of the beta-subunit was prevented by the same concentrations of inhibitor-1 and inhibitor-2 that were required to inhibit the dephosphorylation of phosphorylase. The same experiments performed with the glycogen plus microsomal fraction from rabbit skeletal muscle revealed that the properties of glycogen synthase phosphatase and phosphorylase phosphatase were very similar to the corresponding activities in the hepatic glycogen fraction, except that the two activities coeluted as sharp peaks near the void volume of Sephadex G-100 (before tryptic digestion). Tryptic digestion of the hepatic glycogen and microsomal fractions increased phosphorylase phosphatase about threefold, but decreased glycogen synthase phosphatase activity. Similar results were obtained with the glycogen plus microsomal fraction from rabbit skeletal muscle or the glycogen-bound form of protein phosphatase-1 purified to homogeneity from the same tissue. Therefore the divergent effects of trypsin on glycogen synthase phosphatase and phosphorylase phosphatase activities are an intrinsic property of protein phosphatase-1. It is concluded that the major protein phosphatase in both the glycogen and microsomal fractions of rat liver is a form of protein phosphatase-1, and that this enzyme accounts for virtually all the glycogen synthase phosphatase and phosphorylase phosphatase activity associated with these subcellular fractions.     

Isolation and characterization of two 70 kDa modulator-complexes from rabbit skeletal muscle

The activation as well as the inactivation of the ATP,Mg-dependent protein phosphatase has been shown to be totally dependent upon the presence of the modulator subunit. This modulator (inhibitor-2) is a heat stable protein and its isolation in pure form (32 kDa) always includes a boiling step. The boiled modulator fractions are known to be inhibitory to the phosphatase activity. Unboiled rabbit skeletal muscle preparations do not contain "free modulator", but two higher molecular weight complexes (70 kDa) can be isolated which have the 32 kDa modulator together with a 38 kDa protein. One complex is the already characterized inactive ATP,Mg-dependent phosphatase [FCM] while the second one, [MX], although seemingly of identical composition, does not exhibit phosphatase activity when measured under the usual conditions. The MX-complex does not inhibit the phosphatase activity unless subjected to a boiling step which dissociates the modulator subunit. The unboiled [MX] exhibits the activation as well as the inactivation characteristics of the free modulator.     

Isolation and characterization of glycogen branching enzyme from rabbit liver

Glycogen branching enzyme was isolated from rabbit liver. The highly purified enzyme shows a monomer molecular weight of 71 000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and apparent molecular weights of 93 000 by sucrose density gradient sedimentation and 52 000 by gel-exclusion chromatography on Sephacryl S-300. No glucosamine, mannosamine, galactosamine, or sialic acid was detected in the protein. An amino acid analysis is reported. The spectrum of branching enzyme is that of a simple polypeptide, with A1%280nm = 24.6. Highly purified branching enzyme consists of several closely related active enzyme forms that can be resolved by isoelectric focusing in polyacrylamide gel. The major species of pI 5.7 is flanked by less abundant forms of pI 5.6 and 5.8. Seemingly identical enzyme forms are observed in crude extracts of rabbit liver, skeletal muscle, brain, and heart, although the absolute and relative concentrations vary among the tissues. Branching enzyme apparently does not exhibit tissue-specific isoenzymes.     

The heat labile phosphatase modulator (inhibitor-2) complex from rabbit skeletal muscle

The heat stable phosphatase modulator protein (inhibitor-2) has been shown to play a crucial role in the reversible ATP, Mg-dependent activation of a multisubstrate protein phosphatase. The modulator activity is acid and heat stable and resides in a small asymmetrical protein which, after boiling migrates in sucrose density gradient centrifugation with a molecular weight of 17K. The present report shows that in unboiled rabbit skeletal muscle preparations all the modulator activity is found associated with a heat labile protein component, which imposes an important regulatory feature on the heat stable activity. The heat labile complex migrates in sucrose density gradient centrifugation as a M_r = 70K protein.     

Structural characterization of the 1,4-dihydropyridine receptor of the voltage-dependent Ca2+ channel from rabbit skeletal muscle. Evidence for two distinct high molecular weight subunits

The 1,4-dihydropyridine receptor purified from rabbit skeletal muscle triads was shown to contain four protein components of 175,000, 170,000, 52,000, and 32,000 Da when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions. Monoclonal antibodies capable of specifically immunoprecipitating the [3H]PN200-110-labeled dihydropyridine receptor from digitonin-solubilized triads recognized the 170,000-Da protein on nitrocellulose transfers of skeletal muscle triads, transverse tubular membranes, and purified dihydropyridine receptor. Wheat germ agglutinin peroxidase stained the 175,000-Da protein on similar nitrocellulose transfers, demonstrating that the 175,000-Da protein is the glycoprotein subunit of the purified dihydropyridine receptor. The apparent molecular weight of the Mr 170,000 protein remained unchanged with reduction, whereas the apparent molecular weight of the glycoprotein subunit shifted from 175,000 to 150,000 upon reduction. These results demonstrate that the 1,4-dihydropyridine receptor of the voltage-dependent Ca2+ channel from rabbit skeletal muscle contains two distinct high molecular weight subunits of 175,000 and 170,000.     

Separation and characterization of two phosphorylase phosphatase inhibitors from rabbit skeletal muscle.

Two heat-stable and trypsin-labile inhibitors of phosphorylase phosphatase, designated inhibitor-1 and inhibitor-2, were partially purified from extracts of rabbit skeletal muscle by heating and coloumn chromatography using DEAE-dellulose and Bio-gel P-60. Inhibitor-1 exists in an active phosphorylated form and an inactive dephosphorylated form. The interconversion of phosphorylated inhibitor-1 and dephosphorylated inhibitor-1 is mediated by protein kinase dependent on adenosine 3':5'-monophosphate (cyclic AMP) and a Mn2+-stimulated phosphoprotein phosphatase. Inhibitory activity of inhibitor-2 is not influenced by treatment with either the kinase or the Mn2+-stimulated phosphatase. The molecular weights of inhibitor-1 and inhibitor-2 estimated by sodium dodecylsulfate-polyacrylamide gel electrophoresis are 26000 and 33000 respectively. Both inhibitor-1 and inhibitor-2 inhibit phosphorylase phosphatase by a mechanism which appears to be non-competitive with respect to the substrate phosphorylase a. Inhibitor fractions at early stages of purification also inhibit cyclic-AMP-dependent histone phosphorylation, but this kinase inhibitory activity resides with a protein moiety which is separable from inhibitor-1 and inhibitor-2.     

Comparative studies on muscle AMP-deaminase--I. Purification, molecular weight, subunit structure and metal content of the enzymes from rat, rabbit, hen, frog and pikeperch.

1. AMP-deaminase (EC 3.5.4.6) from skeletal muscle of frog and pikeperch was purified to homogeneity and compared with the homogeneous enzymes purified from rat, rabbit and hen skeletal muscle. 2. Their molecular weight was close to 280,000, every enzyme consisted of four identical subunits of molecular weight about 70,000. 3. All enzymes were found to contain about two atoms of zinc per molecule. 4. Minor differences of u.v.-absorption spectra between amphibian and fish muscle enzyme as compared with mammalian and bird muscle enzyme were found.     

Adipose tissue protein phosphatase inhibitor-2

Rat fat cells contain three species of spontaneously active inhibitor proteins of protein phosphatase 1, as resolved by SDS-PAGE, with apparent molecular masses of 40 kDa, and 28 kDa respectively. The 33-kDa, thermostable inhibitor was highly purified from bovine adipose tissue and shown to be very similar to inhibitor-2 of skeletal muscle. It was phosphorylated, on threonine only, by glycogen synthase kinase 3. It formed an inactivated complex with protein phosphatase 1, that was reactivated by incubation with ATP-Mg and glycogen synthase kinase 3. By gel filtration it had a Stokes radius of 3.4 nm. Peptide and phosphopeptide maps, generated by Staphylococcus aureus V8 proteinase, trypsin or thermolysin, of the inhibitor and of the skeletal muscle inhibitor-2 were similar. The 40-kDa inhibitor, which was denatured by boiling, represents a novel protein phosphatase inhibitor protein or an undegraded precursor of inhibitor-2. The total activity of inhibitor-2-like material (thermostable and macromolecular) in an adipocyte cytosol extract corresponded to an intracellular concentration of 0.3 microM inhibitor-2.     

Dihydropyridine-sensitive calcium channels in cardiac and skeletal muscle membranes: studies with antibodies against the alpha subunits

Polyclonal antibodies (PAC-2) against the purified skeletal muscle calcium channel were prepared and shown to be directed against alpha subunits of this protein by immunoblotting and immunoprecipitation. These polypeptides have an apparent molecular weight of 162,000 without reduction of disulfide bonds. Under conditions where the functional properties of the purified skeletal muscle calcium channel are retained, beta subunits (Mr 50,000) and gamma subunits (Mr 33,000) are coprecipitated, demonstrating specific noncovalent association of these three polypeptides in the purified skeletal muscle channel. PAC-2 immunoprecipitated cardiac calcium channels labeled with [3H]isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5- (methoxycarbonyl)pyridine-3-carboxylate ([3H]PN200-110) at a 3-fold higher concentration than skeletal muscle channels. Preincubation with cardiac calcium channels blocked only 49% of the immunoreactivity of PAC-2 toward skeletal muscle channels, indicating that these two proteins have both homologous and distinct epitopes. The immunoreactive component of the cardiac calcium channel was identified by immunoprecipitation and polyacrylamide gel electrophoresis as a polypeptide with an apparent molecular weight of 170,000 before reduction of disulfide bonds and 141,000 after reduction, in close analogy with the properties of the alpha 2 subunits of the skeletal muscle channel. It is concluded that these two calcium channels have a homologous, but distinct, alpha subunit as a major polypeptide component.     

Degradation of myofibrillar proteins by cathepsins B and D

1. The procedure of Barrett [(1973) Biochem. J.131, 809-822] for isolating cathepsins B and D from human liver was modified for use with rat liver and skeletal muscle. The purified enzymes appeared to be similar to those reported in other species. 2. Sephadex G-75 chromatography of concentrated muscle extract resolved two peaks of cathepsin B inhibitory activity, corresponding to molecular weights of 12500 and 62000. 3. The degradation of purified myofibrillar proteins by cathepsins B and D was clearly demonstrated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. After incubation with enzyme, the polypeptide bands representing the substrates decreased in intensity and lower molecular weight products appeared. 4. Cathepsins B and D, purified from either rat liver or skeletal muscle, were shown to degrade myosin, purified from either rabbit or rat muscle. Soluble denatured myosin was degraded more extensively than insoluble native myosin. Degradation by cathepsin B was inhibited by lack of reducing agent, or by myoglobin, iodoacetic acid and leupeptin, but not by pepstatin. The same potential modifiers were applied to cathepsin D, and only pepstatin produced inhibition. 5. Rat liver cathepsin B had a pH optimum of 5.2 on native rabbit myosin. The pH optimum of cathepsin D was 4.0, with a shoulder of activity about 1pH unit above the optimum. 6. Rat liver cathepsins B and D were demonstrated to degrade rabbit F-actin at pH5.0, and were inhibited by leupeptin and pepstain, respectively. 7. The degradation of myosin and actin by cathepsin D was more extensive than that by cathepsin B.     

Glycogen debranching enzyme: purification, antibody characterization, and immunoblot analyses of type III glycogen storage disease.

Type III glycogen storage disease is caused by a deficiency of glycogen debranching-enzyme activity. Many patients with this disease have both liver and muscle involvement, whereas others have only liver involvement without clinical or laboratory evidence of myopathy. To improve our understanding of the molecular basis of the disease, debranching enzyme was purified 238-fold from porcine skeletal muscle. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis the purified enzyme gave a single band with a relative molecular weight of 160,000 that migrated to the same position as purified rabbit-muscle debranching enzyme. Antiserum against porcine debranching enzyme was prepared in rabbit. The antiserum reacted against porcine debranching enzyme with a single precipitin line and demonstrated a reaction having complete identity to those of both the enzyme present in crude muscle and the enzyme present in liver extracts. Incubation of antiserum with purified porcine debranching enzyme inhibited almost all enzyme activity, whereas such treatment with preimmune serum had little effect. The antiserum also inhibited debranching-enzyme activity in crude liver extracts from both pigs and humans to the same extent as was observed in muscle. Immunoblot analysis probed with anti-porcine-muscle debranching-enzyme antiserum showed that the antiserum can detect debranching enzyme in both human muscle and human liver. The bands detected in human samples by the antiserum were the same size as the one detected in porcine muscle. Five patients with Type III and six patients with other types of glycogen storage disease were subjected to immunoblot analysis. Although anti-porcine antiserum detected specific bands in all liver and muscle samples from patients with other types of glycogen storage disease (Types I, II, and IX), the antiserum detected no cross-reactive material in any of the liver or muscle samples from patients with Type III glycogen storage disease. These data indicate (1) immunochemical similarity of debranching enzyme in liver and muscle and (2) that deficiency of debranching-enzyme activity in Type III glycogen storage disease is due to absence of debrancher protein in the patients that we studied.     

Latent forms of type-1 protein phosphatase in rabbit skeletal muscle

We have examined the characteristics of partially purified forms of rabbit skeletal muscle type-1 protein phosphatase (PP-1). Over 90% of PP-1 in unfractionated extracts and in glycogen particles was inactive, but could be activated by the divalent cations, Mn2+ or Co2+ (Me2+) plus trypsin. Gel filtration of muscle extracts revealed two inactive forms of PP-1; one activated by Me2+ alone or Me2+ plus trypsin, and a second containing inhibitor-2 and activated only by Me2+ plus trypsin. The kinetics of Me2+ plus trypsin activation revealed that after DEAE-chromatography, PP-1 was altered from the forms in crude extracts, gel filtered extracts or glycogen particles. The results indicate that the purified form of PP-1 catalytic subunit has properties which distinguish it from the forms of the enzyme in muscle extracts.     

Isolation and characterization of actin from Entamoeba histolytica

Actin has been identified and purified partially from trophozoites of Entamoeba histolytica HMI-IMSS by a procedure that minimizes proteolysis. In cellular extracts, Entamoeba actin would copolymerize with muscle actin, but would not bind to DNase I or form microfilaments. Fractionation of the extracts by DEAE-cellulose and Sephadex G-150 chromatography yielded a purified actin that would copolymerize with rabbit skeletal muscle actin or polymerize alone into long filaments at 24 degrees C upon addition of 100 mM KC1 and 2 mM MgCl2. These filaments are not cold-stable and will depolymerize at 4 degrees C in 1 or 2 h. Entamoeba actin filaments bind phallotoxin with the same affinity as muscle actin and decorate with rabbit skeletal muscle heavy meromyosin. Entamoeba actin filaments activate the Mg2+ ATPase of heavy meromyosin to the same Vmax as muscle actin, but the Kapp is 2.8 times higher. Entamoeba actin is a single species with a slightly higher molecular weight than muscle actin (45,000) and a more acidic pI (5.4). The purified actin does not bind to DNase I, produce inhibition of the enzymatic activity, or block the binding of muscle actin. Comparison of the peptides obtained by limit digest with protease V8 from Staphylococcus aureus shows sequences with common mobility between alpha-actin and Entamoeba actin, but additional peptides are present which may account for the different properties of the Entamoeba actin. Finally, in vitro translation of mRNA from trophozoites produces a single polypeptide equivalent to the molecule purified from Entamoeba extracts.