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By equilibrium dialysis a disadenosine 5',5'''-P1,P2-tetraphosphate (Ap4A) binding activity is shown to be present in mammalian cells. The Ap4A binding activity copurifies with 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 seven distinct protein bands with apparent Mrs of 64,000, 63,000, 62,000, 60,000, 57,000, 55,000, and 52,000. By affinity labeling, the protein with Mr 57,000 has been shown to be 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. From these results, DNA polymerase seems to be the intracellular target of Ap4A. This is discussed in respect to the recently reported of Ap4A to trigger DNA replication in quiescent mammalian cells [Grummt, F. (1978) Proc. Natl. Acad. Sci. USA 75, 371-375].

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