Phosphorylation and activation of poly (A)-endoribonuclease from calf thymus gland

Poly(A) metabolism and aging: a current view

Polyadenylation of mRNA is a key step in post-transcriptional control of gene expression. Therefore, age-dependent changes in poly(A) synthesis have to play a crucial role in the course of cellular aging. In this review, the importance of the signal sequence, poly(A), in determining mRNA stability and intracellular distribution of mRNA during aging is discussed.

Soluble cAMP-independent protein kinase from human spleen

A protein kinase (EC 2.7.1.37) was purified 2000-fold, from the soluble protein fraction of human spleen cells, using ion-exchange chromatography, ammonium sulfate fractionation, and gel filtration. This rapid procedure yielded 30% of the initial activity and an enzyme preparation with specific activity of 62 nmol min-1 mg-1 of protein. On the basis of disc gel electrophoresis in sodium dodecyl sulfate acrylamide gels and isoelectric focusing the enzyme preparation appears homogeneous and to consist of one polypeptide with a molecular weight of 43,000 and having a pI of 7.1. The purified enzyme activity is cyclic AMP and cGMP independent phosphorylates both alpha-casein and phosvitin, and uses Mg2+ ATP and Mg2+ GTP as phosphate donors, exhibiting an apparent Km of 2.0 and 6.6 X 10(-5)m, respectively. Furthermore, the enzyme activity is strongly inhibited by heparin (K50 = 0.1 micrograms/ml). These catalytic properties are characteristic of the enzyme casein kinase II, as described in several eukaryotic cells.

Increase in the levels of activity of polyadenylic acid-metabolizing enzymes following phytohaemagglutinin stimulation of human lymphocytes

Increased levels of soluble activity of all three enzymes involved in polyadenylic acid metabolism were measured in PHA-stimulated versus normal lymphocytes. Poly(A)-polymerase and poly(A)-exonuclease values increased significantly (from 25.7 +/- 4.2 (S.E.M.) to 53.5 +/- 10.6 (S.E.M.), and from 334.6 +/- 33.2 (S.E.M.) to 653.2 +/- 53.4 (S.E.M.) respectively), while a moderate increase was observed in poly(A)-endonuclease (from 299.2 +/- 33.8 (S.E.M.) to 403.0 +/- 77.1 (S.E.M.). The above differences persisted after two fractionations of the crude cell extracts by ion exchange chromatography and molecular sieving, and could not be attributed to the competitive action of all three enzymes in the untreated extracts. Fractionation of the extracts of resting and stimulated cells on Sephadex G-75 revealed two molecular forms of poly(A)-polymerase activity.

Isolation and characterization of cytoplasmic poly(A) polymerase from cryptobiotic gastrulae of Artemia salina

Poly(A) polymerase has been purified to near homogeneity from the cytoplasm of Artemia salina cryptobiotic gastrulae by ion-exchange chromatography on DEAE-cellulose, DEAE-Sepharose CL-6B and phosphocellulose P11, gel filtration on CL-Sepharose 6B, affinity chromatography on poly(A)-Sepharose 4B and ATP-agarose. The enzyme is fully dependent on exogeneous oligo(riboadenylic acid) and is free of any nuclease or other enzyme activities. In standard assay conditions the enzyme preparation has a specific activity of 5.6 mumol AMP . h-1 . (mg protein)-1. Sodium dodecyl sulphate/polyacrylamide gel electrophoresis reveals the presence of only two proteins with Mr 94 000 and 70 000. The Mr-70 000 protein has been identified as poly(A) polymerase. The enzyme is exclusively activated by Mn2+. Addition of Ca2+, Mg2+, Zn2+, NH4+, K+ or Na+ inhibits the enzymatic reaction. The activity is specific for ATP and competitive inhibition is observed in the presence of other ribonucleoside 5'-triphosphates. AMP incorporation is time-dependent and is increased non-linearly with protein and primer concentration.

Base-specific ribonucleases potentially involved in heterogeneous nuclear RNA processing and poly(A) metabolism

Polyadenylation and splicing of heterogeneous nuclear RNA, two crucial steps in mRNA processing, are apparently enzymatically mediated processes. This contribution summarizes the properties and the presumed functions of the known poly(A) catabolic enzymes (endoribonuclease IV and V, 2',3'- exoribonuclease ) as well as those of the pyrimidine-specific endoribonucleases associated with snRNP -hnRNP complexes (endoribonuclease VII, acidic pI 4.1 endoribonuclease and poly(U)-specific U1 snRNP -nuclease).

Occurrence of short-sized oligo(A) fragments during course of cell cycle and ageing

Affinity chromatography of nucleic acids precipitated by N-cetyl-N,N,N-trimethyl ammonium bromide on poly(U)-Sepharose has proved to be a suitable method for a nearly quantitative isolation of oligo(A) sequences down to a chain length of 4 nucleotide units. Analysis of short oligo(A) fragments in synchronized L5178y mouse lymphoma cells after labeling with [3H]Ado revealed that the percentage of A2-6 sequences on the total radioactivity amounted in S-phase cells to 1.6%, while the value obtained for the stationary L-cell system was 8.0%. The alterations of occurrence and chain length distribution of short oligo(A) fragments during ageing were studied in two age groups of female quails: mature (250-320 days old) and senescent animals (3-3.5 yr old). It was found that the amount of low molecular weight oligo(A) fragments gradually decreases during ageing of the animals; the amount in the mature animal group was significantly higher (6-fold) than in the old animal group. The decreased amounts of oligo(A) during S phase and ageing could in part be due to posttranslational modification of enzymes involved in poly(A) metabolism. It could be demonstrated that both homogeneous poly(A) anabolic poly(A) polymerase and homogeneous poly(A) catabolic endoribonuclease IV are phosphorylated by nuclear protein kinase NI.