Diadenosine 5',5'''-P1,P4-tetraphosphate, a ligand of the 57-kilodalton subunit of DNA polymerase alpha.

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Abstract
By equilibrium dialysis, a diadenosine 5'',5"''-P1,P2-tetraphosphate (Ap4A) binding activity is shown in mammalian cells. The Ap4A binding activity copurifies with [calf thymus] DNA polymerase .alpha. during the isolation procedure, which includes chromatography on phospho-, DEAE- and DNA-cellulose; gel filtration; sucrose gradient centrifugation; and electrophoresis in nondenaturing polyacrylamide gels. After these purification steps, DNA polymerase .alpha. appears to be homogeneous in nondenaturing polyacrylamide gels. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of such a purified DNA polymerase .alpha. preparation reveals 7 distinct protein bands with apparent MW of 64,000, 63,000, 62,000, 60,000, 57,000, 55,000 and 52,000. By affinity labeling, the protein with MW 57,000 was the Ap4A-binding constituent of DNA polymerase .alpha.. The binding activity of DNA polymerase .alpha. for Ap4A is highly specific because neither structural analogs nor several other adenine nucleotides compete effectively with Ap4A for its binding site. The Ap4A binding site is lost in neuronal cells during maturation of rat brains concomitantly with the loss of DNA polymerase .alpha. and mitotic activity in those cells. DNA polymerase seems to be the intracellular target of Ap4A. This is discussed in respect to the recently reported capability of Ap4A to trigger DNA replication in quiescent mammalian cells.