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

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Abstract
1. An adenosine(5′)tetraphospho(5′)adenosine (Ap4A) binding protein has been purified from calf thymus. The protein is comprised of a single polypeptide of Mr 54000 and is capable of high‐affinity (Kd= 13 μM) binding of Ap4A with great substrate specificity. 2. The Ap4A 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 Ap4A binding protein contains associated adenosine(5′)tetraphospho(5′)adenosine phosphohydrolase (Ap4Aase) activity, while the form resolved from DNA polymerase α contains no such activity. 4. The Ap4Aase activity, which catalyzes the phosphohydrolysis of Ap4A to ATP and AMP, is strongly inhibited by low levels (50–100 μM) of Zn2+ without any effect on the Ap4A binding protein activity. 5. This difference in associated Ap4Aase activity between free and polymerase‐bound forms of the protein, plus the copurification mentioned above, indicate a specific association between Ap4A binding protein and DNA polymerase α.