Adenosine(5′)tetraphospho(5′)adenosine‐binding protein of calf thymus

1. An adenosine(5′)tetraphospho(5′)adenosine (Ap 4 A) binding protein has been purified from calf thymus. The protein is comprised of a single polypeptide of M r 54000 and is capable of high‐affinity ( K d = 13 μM) binding of Ap 4 A with great substrate specificity. 2. The Ap 4 A binding protein has been isolated in two forms: a “free”, or non‐polymerase‐bound, form which predominates, and a similar form which copurifies with DNA polymerase α, but which can be resolved from it. 3. The free form of Ap 4 A binding protein contains associated adenosine(5′)tetraphospho(5′)adenosine phosphohydrolase (Ap 4 Aase) activity, while the form resolved from DNA polymerase α contains no such activity. 4. The Ap 4 Aase activity, which catalyzes the phosphohydrolysis of Ap 4 A to ATP and AMP, is strongly inhibited by low levels (50–100 μM) of Zn 2+ without any effect on the Ap 4 A binding protein activity. 5. This difference in associated Ap 4 Aase activity between free and polymerase‐bound forms of the protein, plus the copurification mentioned above, indicate a specific association between Ap 4 A binding protein and DNA polymerase α.