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J. Biol. Chem., Vol. 276, Issue 52, 48709-48715, December 28, 2001

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Carboxyl-terminal Domain III of the ' Subunit of DNA Polymerase III Holoenzyme Binds DnaX and Supports Cooperative DnaX Complex Assembly^*

Min-Sun Song and Charles S. McHenry

From the Department of Biochemistry and Molecular Genetics, University of Colorado Health Sciences Center, Denver, Colorado 80262

The ' subunit of the DNA polymerase-III holoenzyme is a key component of the DnaX complex; it is required for loading the ₂ processivity factor onto a primed template. The x-ray crystal structure of ' indicates a three domain C-shaped structure (Guenther, B., Onrust, R., Sali, A., O'Donnell, M., and Kuriyan, J. (1997) Cell 91, 335-345). In this study, we localized the DnaX-binding domain of ' to its carboxyl-terminal domain III by quantifying protein-protein interactions using a series of ' fusion proteins lacking specific domains. The fusion protein corresponding to domain III of ' bound to DnaX with an affinity approaching that of full-length '. In contrast, a construct bearing ' domains I-II did not bind DnaX at detectable levels. The presence of  and strengthened the interaction of DnaX with full-length ' and ' domain III. Thus, domain III of ' not only contains the DnaX-binding site, but also contains the elements required for positive cooperative assembly of the DnaX complex. A domain III-specific anti-' monoclonal antibody interfered with DnaX complex formation and abolished the replication activity of DNA polymerase III holoenzyme.

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