Phosphorylation of ribosomal-associated proteins in reticulocyte lysates

Stereospecificity of acetylene reduction catalyzed by nitrogenase

In addition to catalyzing the reduction of dinitrogen to ammonia, the metalloenzyme nitrogenase catalyzes the reduction of a number of alternative substrates, including acetylene (C(2)H(2)) to ethylene (C(2)H(4)) and, in certain cases, to ethane (C(2)H(6)). The stereochemistry of proton addition for C(2)D(2) reduction to C(2)D(2)H(2) catalyzed by the Mo-dependent nitrogenase has been used to probe substrate binding and proton addition mechanisms. In the present work, the C(2)D(2) reduction stereospecificity of altered MoFe proteins having amino acid substitutions within the active site FeMo-cofactor environment was examined by Fourier transform infrared (FTIR) spectroscopy. Altered MoFe proteins examined included those having the alpha-subunit 96(Arg) residue substituted by Gln, Leu, or Ala, the alpha-subunit 69(Gly) residue substituted by Ser, and the alpha-subunit 195(His) residue substituted by Asn. The stereochemistry of proton addition to C(2)D(2) does not correlate with the measured K(m) values for C(2)H(2) reduction, or with the ability of the enzyme to reduce C(2)H(2) by four electrons to yield C(2)H(6). Instead, the electron flux through nitrogenase was observed to significantly influence the ratio of cis- to trans-1,2-C(2)H(2)D(2) formed. Finally, the product distribution observed for reduction of C(2)H(2) in D(2)O is not consistent with an earlier proposed enzyme-bound intermediate. An alternative model that accounts for the stereochemistry of C(2)H(2) reduction by nitrogenase based on a branched reaction pathway and an enzyme-bound eta(2)-vinyl intermediate is proposed.

2,3-Bisphosphoglycerate inhibits hemoglobin synthesis and phosphorylation of initiation factor 2 by casein kinase II in reticulocyte lysates

2,3-Bisphosphoglycerate inhibited protein synthesis in reticulocyte lysates with 50% inhibition at 2 mM. Glycerate 2,3-P2 increased the Mg2+ optimum for protein synthesis by chelation of Mg2+, but Mg2+ addition did not completely reverse the inhibition, suggesting an additional site of action. eIF-2 has been used to examine the activity of casein kinase II in reticulocyte lysates in response to glycerate 2,3-P2. When glycerate 2,3-P2 was increased to 4mM, phosphorylation of eIF-2 beta was increasingly inhibited. Thus inhibition of phosphorylation of translational components by casein kinase II can be correlated with inhibition of globin synthesis at physiological concentrations of glycerate 2,3-P2.

Regulation of MPF activity in vitro

We have developed a soluble, cell-free system from premeiotic Xenopus oocytes that executes the post-translational activation of a precursor form of maturation promoting factor (MPF). We have distinguished at least two components of this ATP-dependent reaction: pre-MPF, a precursor to MPF that activates independently of added MPF and whose apparent molecular weight changes from 400 kd to 260 kd upon activation; and INH, an inhibitor of pre-MPF activation that confers MPF dependence on the reaction. We present evidence suggesting that INH is a phosphatase and that the activation of pre-MPF occurs via phosphorylation. INH activity itself seems to be regulated by another phosphatase, protein phosphatase-1. We have directly examined the pattern of protein phosphorylation during the activation reaction and have found 92 and 140 kd proteins whose phosphorylation increases when MPF activity appears. This system makes possible a direct examination of the regulation of MPF activity during the cell cycle.

Differential effect of Mn2+ on the hemin-controlled translational repressor and the double-stranded RNA-activated inhibitor

The inhibition of protein synthesis that occurs when rabbit reticulocyte lysate is incubated in the absence of hemin is due to the activation of a protein kinase termed the hemin-controlled translational repressor, and that occurring when reticulocyte lysate is incubated with a low level of double-stranded RNA is mediated by the activation of a separate protein kinase termed the double-stranded RNA-activated inhibitor. Both the hemin-controlled translational repressor and the double-stranded RNA-activated inhibitor act by phosphorylating the Mr = 35,000 (alpha) subunit of eIF-2. MnCl2 (0.5 mM) partly reverses the inhibition of protein synthesis produced by hemin deficiency but not that induced by double-stranded RNA. In addition, Mn2+ reverses the inhibition of binding of [35S]Met-tRNAf to reticulocyte ribosomal components, isolated on Sepharose 6B, produced by the hemin-controlled translational repressor but not by the double-stranded RNA-activated inhibitor. The effect of Mn2+ is mediated at the level of activation and eIF-2 alpha kinase activity of these two regulatory protein kinases. Specifically, Mn2+ inhibits activation of the hemin-controlled translational repressor in the absence of hemin and the phosphorylation of eIF-2 alpha by pre-activated translational repressor. In contrast, the phosphorylation of eIF-2 alpha by the double-stranded RNA-activated inhibitor is not suppressed by Mn2+, and the activation and autophosphorylation of this inhibitor is enhanced by Mn2+. Finally, while the activation and inactivation of the hemin-controlled translational repressor does not appear to be mediated by autophosphorylation and dephosphorylation, the activation of the double-stranded RNA-activated inhibitor does appear to require autophosphorylation.

Insulin-mediated phosphorylation of ribosomal protein S6 in mouse melanoma cells and melanoma x fibroblast hybrid cells in relation to cell proliferation

The possible role of insulin-mediated phosphorylation of ribosomal protein S6 in the control of cell proliferation was examined in insulin-unresponsive mouse melanoma calls (PG19) and insulin-responsive melanoma x fibroblast clone 100A. In the hybrid cells, under conditions of growth arrest in medium with low serum, ribosomal protein S6 was rapidly phosphorylated in response to insulin or serum. The phosphorylation of the S6 protein increased over a wide range of insulin concentrations, suggesting that insulin stimulated the phosphorylation by interacting with both high- and low-affinity receptors. In contrast, in growth-arrested melanoma cells, an intermediate level of S6 phosphorylation was observed. Insulin caused only a marginal increase and serum caused a small but consistent increase in the level of S6 phosphorylation in the melanoma cells. Cell cycle analysis revealed that both cell lines arrested growth to a similar degree in the G1 phase of the cell cycle; thus, the higher baseline level of S6 phosphorylation observed in the melanoma cells was not attributable to less complete growth arrest of these cells in medium with low serum. The S6 phosphorylation results correlate well with previous results suggesting that the hybrid cells, but not the parental melanoma cells, can become growth-limited for processes regulated by insulin.

Effects of cyclic AMP and fluoride on phosphorylation of ribosomal protein S6 and on protein synthesis in rabbit reticulocytes

The effects of N6,O2-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) and sodium fluoride on the phosphorylation of ribosomal proteins S6 and on protein synthesis were examined. Rabbit reticulocytes were incubated in a nutritional medium containing 32Pi in the presence and absence of Bt2cAMP (1mM) and 3-isobutyl-1-methyl-xanthine (1mM). In the control cells, four phosphorylated derivatives of S6 were observed, with most of the radioactivity in the monophosphorylated form. Upon addition of cyclic nucleotide, a twofold increase in the phosphorylation of ribosomal protein S6 was observed. This was accompanied by an increase of radioactive phosphate in the diphosphorylated derivative. No alteration in protein synthesis was observed upon addition of cAMP and analogues of cAMP in conjunction with 3-isobutyl-1-methyl-xanthine or theophylline. The effects of sodium fluoride on phosphorylation of S6 and on protein synthesis were examined also. At 5 mM sodium fluoride, protein synthesis was inhibited by 85%. A 2.5-fold increase in the phosphorylation of ribosomal protein S6 was observed with an accumulation of 32Pi in the diphosphorylated, triphosphorylated and tetraphosphorylated derivatives. Inhibition of protein synthesis coincided with an increase in the more highly phosphorylated derivatives, whereas an increase of radioactive phosphate in the diphosphorylated derivative could not be correlated with an alteration in globin synthesis.

Purification of messenger ribonucleoprotein particles from rabbit reticulocytes by zonal centrifugation in metrizamide

A large-scale purification procedure for messenger ribonucleoprotein (mRNP) particles from rabbit reticulocyte polysomes is described. The mRNP particles were dissociated from polysomes by treatment with urea and separated by differential centrifugation under conditions of high ionic strength. Zonal centrifugation in a metrizamide buoyant density gradient was the final purification step. One major class of mRNA particle was observed. The RNA was defined as mRNA by polyacrylamide gel electrophoresis and by globin production in a cell-free protein-synthesizing system. The twenty-three different proteins associated with the particle were a discrete set of proteins, which ranged in molecular weight from 175,000 to 23,500. The relative amount of each peptide in the particle was determined from a gel scan of the stained protein by computer simulation. None of the polypeptides comigrated with proteins from the 40-S and 60-S ribosomal subunits when analyzed by two-dimensional polyacrylamide gel electrophoresis.

Identification of initiation factors and ribosome-associated phosphoproteins by two-dimensional polyacrylamide gel electrophoresis

A two-dimensional polyacrylamide gel electrophoresis procedure has been used to identify initiation factors rapidly in the high-salt-wash fraction from reticulocyte ribosomes. Initiation factors are identified by relative mobility and by co-electrophoresis with purified factors. A creatine phosphate/ATP/GTP/Pi exchange system is described which has been used to maintain [gamma-32P]ATP and [gamma-32P]GTP at constant specific activity in the cell-free protein-synthesizing system. Phosphorylated proteins associated with the protein-synthesizing complex have been identified using a combination of the two procedures. The salt-wash fraction contains eight major phosphorylated proteins and a number of minor ones. Two phosphorylated proteins are observed to comigrate with two of the three subunits of eukaryotic initiation factor 2 (eIF-2), the initiation factor involved in binding Met-tRNAf onto the 40-S subunit and promoting dissociation of 80-S ribosomes. eIF-4B, one of the proteins involved in binding mRNA to 40-S subunits is also phosphorylated. The remainder of phosphorylated proteins in the high-salt-wash fraction are not previously characterized initiation factors and have not been identified further. Two of the six phosphoproteins associated with the salt-washed ribosomes comigrate with ribosomal proteins; one is the major phosphorylated protein in 40-S ribosomal subunits, the other is an acidic protein.