τ binds and organizes Escherichia coli replication proteins through distinct domains -: Partial proteolysis of terminally tagged τ to determine candidate domains and to assign domain V as the α binding domain

The tau subunit dimerizes Escherichia coli DNA polymerase III core through interactions with the a:subunit. In addition to playing critical roles in the structural organization of the holoenzyme, tau mediates intersubunit communications required for efficient replication fork function. We identified potential structural domains of this multifunctional subunit by limited proteolysis of C-terminal biotin-tagged I proteins, The cleavage sites of each of eight different proteases were found to be clustered within four regions of the tau subunit, The second susceptible region corresponds to the hinge between domain II and III of the highly homologous S' subunit, and the third region is near the C-terminal end of the tau-delta' alignment (Guenther, B., Onrust, R., Sali, A., O'Donnell, M., and Kuriyan, J. (1997) Cell 91, 335-345), We propose a five-domain structure for the tau protein, Domains I and II are based on the crystallographic structure of delta' by Guenther and colleagues. Domains III-V are based on our protease cleavage results, Using this information, we expressed biotin-tagged tau proteins lacking specific protease-resistant domains and analyzed their binding to the a subunit by surface plasmon resonance, Results from these studies indicated that the alpha binding site of tau lies within its C-terminal 147 residues (domain V).