Cyclic AMP-dependent phosphorylation of terminal deoxynucleotidyl transferase

Terminal deoxynucleotidyl transferase: the story of a misguided DNA polymerase

Nearly every DNA polymerase characterized to date exclusively catalyzes the incorporation of mononucleotides into a growing primer using a DNA or RNA template as a guide to direct each incorporation event. There is, however, one unique DNA polymerase designated terminal deoxynucleotidyl transferase that performs DNA synthesis using only single-stranded DNA as the nucleic acid substrate. In this chapter, we review the biological role of this enigmatic DNA polymerase and the biochemical mechanism for its ability to perform DNA synthesis in the absence of a templating strand. We compare and contrast the molecular events for template-independent DNA synthesis catalyzed by terminal deoxynucleotidyl transferase with other well-characterized DNA polymerases that perform template-dependent synthesis. This includes a quantitative inspection of how terminal deoxynucleotidyl transferase binds DNA and dNTP substrates, the possible involvement of a conformational change that precedes phosphoryl transfer, and kinetic steps that are associated with the release of products. These enzymatic steps are discussed within the context of the available structures of terminal deoxynucleotidyl transferase in the presence of DNA or nucleotide substrate. In addition, we discuss the ability of proteins involved in replication and recombination to regulate the activity of the terminal deoxynucleotidyl transferase. Finally, the biomedical role of this specialized DNA polymerase is discussed focusing on its involvement in cancer development and its use in biomedical applications such as labeling DNA for detecting apoptosis.

Isoforms of terminal deoxynucleotidyltransferase: developmental aspects and function

The immune system develops in a series of programmed developmental stages. Although recombination-activating gene (RAG) and nonhomologous end-joining (NHEJ) proteins are indispensable in the generation of immunoglobulins and T-cell receptors (TCRs), most CDR3 diversity is contributed by nontemplated addition of nucleotides catalyzed by the nuclear enzyme terminal deoxynucleotidyltransferase (TdT) and most nucleotide deletion is performed by exonucleases at V(D)J joins. Increasing TdT expression continuing into adult life results in N region addition and diversification of the T and B cell repertoires. In several species including mice and humans, there are multiple isoforms of TdT resulting from alternative mRNA splicing. The short form (TdTS) produces N additions during TCR and B-cell receptor (BCR) gene rearrangements. Other long isoforms, TdTL1 and TdTL2, have 3' --> 5' exonuclease activity. The two forms of TdT therefore have distinct and opposite functions in lymphocyte development. The enzymatic activities of the splice variants of TdT play an essential role in the diversification of lymphocyte repertoires by modifying the composition and length of the gene segments involved in the production of antibodies and T-cell receptors.

Terminal deoxynucleotidil transferase is a nuclear PKC substrate

Protein phosphorylation is the regulatory mechanism of many cellular events in response to changes in metabolic activity and environmental conditions. Seeing that PKC and TdT levels in cells are both regulated by PMA, we sought particularly intriguing to investigate TdT phosphorylation in vivo, utilizing KM-3 cells, a TdT-positive human pre-B cell line treated with PMA and in vitro, employing purified PKC and human recombinant TdT. Our data show that TdT is a substrate for PKC activity, suggesting that TdT phosphorylation could play a key role in the pathway affecting the control of gene transcription and protein synthesis during lymphoid cells differentiation.

Purification of high molecular mass species of calf thymus terminal deoxynucleotidyltransferase

We have developed a simplified column chromatographic procedure for the simultaneous purification of two high molecular mass forms (58 kd and 45 kd) and a standard two subunit 44 kd from of terminal deoxynucleotidyltransferase (TdT) from calf thymus chromatin. The procedure involves high salt extraction of the chromatin fraction followed by successive chromatographies on phosphocellulose, DEAE sephadex, and hydroxylapatite matrices. While all 3 species of TdT comigrate throughout these steps, separation of individual species is achieved on a single stranded DNA agarose column. The combined yield of the 45 kd and 58 kd TdTs is quite high (approximately 8 mg/5000g tissue), 45 kd being the major species (approximately 60%) and the 58 kd constituting about 30%. The 44 kd species containing two subunits usually represents under 10% of the total. All the three forms of TdT showed similar specific activity and preference for purine deoxynucleoside triphosphates (dNTPs). The Km for individual dNTP with all three species of TdT is quite similar and decreases in the order dCTP greater than dTTP greater than dATP greater than dGTP. The Km for both synthetic primer and activated DNA with the 3 TdTs was, in increasing order, two subunit 44 kd less than 45 kd less than 58 kd TdT. Both 58 kd and 45 kd TdT displayed two optima for Mn++ (0.1 mM and 1 mM) and a single sharp optimum for Mg++ (2.5 mM). The two subunit 44 kd TdT exhibited a single but broad optimum for Mn++ (1 mM) and for Mg++ (10 mM).

Molecular biology of terminal transferase

Terminal transferase is an unusual deoxynucleotide polymerizing enzyme found only in prelymphocytes. The protein was purified to homogeneity from calf thymus glands in 1971 as a 32 kDa protein with a two peptide structure. Subsequent biochemical and immunological analyses of terminal transferase protein in crude extracts from a number of animal species showed a single peptide with a molecular weight of about 58,000. The two peptide structure found earlier was caused by proteolysis. Homogeneous 58 kDa terminal transferase has now been produced from human lymphoblastoid cells and calf thymus glands by immunoaffinity chromatography. In vitro phosphorylation studies showed that the terminal transferase protein contains one phosphorylation site near one end of the polypeptide chain, and the phosphorylation of the enzyme has been confirmed by in vivo labeling experiments. Unambiguous demonstration of the molecular weight of the human terminal transferase was obtained by translation of the cloned human terminal transferase DNA sequence to a 58,308 Da protein. The translated amino acid sequence also provided a possible phosphorylation site near the amino-terminus of the protein. Preliminary analysis of the genomic structure shows a simple intron/exon pattern with the total human terminal transferase gene spanning at least 65 Kb.

Phosphorylation and immunology of poly(A) polymerase

Phosphorylation of poly(A) polymerase by protein kinase NI (a cyclic nucleotide-independent nuclear kinase closely associated with poly(A) polymerase at early stages of purification) resulted in as much as 7-fold activation of poly(A) polymerase. Phosphorylation causes an increase in the rate rather than the extent of polyadenylation. Antibodies raised in rabbits against purified poly(A) polymerase from Morris hepatoma 3924A reacted specifically with poly(A) polymerase following "Western" transfer of the enzyme onto diazobenzyloxyl methyl paper. Using iodinated enzyme, a competition radioimmunoassay for poly(A) polymerase was developed. Using the radioimmunoassay, it was shown that Morris hepatoma 3924A contains 100 micrograms of poly(A) polymerase/mg DNA or 10(7) molecules of the enzyme/cell nucleus. Nuclear poly(A) polymerase from fetal liver, but not from normal liver, was able to compete well with hepatoma enzyme in the radioimmunoassay. These data suggest that the tumor poly(A) polymerase is probably an oncofetal antigen, resulting from derepression of a gene not normally expressed in adult liver.