tau binds and organizes Escherichia coli replication proteins through distinct domains - Domain III, shared by gamma and tau binds delta delta ' and x psi

The DnaX complex of the DNA polymerase holoenzyme assembles the beta (2) pr ocessivity factor onto the primed template enabling highly processive repli cation. The key ATPases within this complex are tau and gamma, alternative frameshift products of the dnaX gene. Of the five domains of tau, I-III are shared with gamma In vivo, gamma binds the auxiliary subunits delta delta' and chi psi (Glover, B. P., and McHenry, C. S. (2000) J. Biol. Chem. 275, 3017-3020), To localize delta delta' and chi psi binding domains within gam ma domains I-III, we measured the binding of purified biotin-tagged DnaX pr oteins lacking specific domains to delta delta' and chi psi by surface plas mon resonance. Fusion proteins containing either DnaX domains I-III or doma ins III-V bound delta delta' and chi psi subunits, A DnaX protein only cont aining domains I and II did not bind delta delta' or chi psi The binding af finity of chi psi for DnaX domains I-III and domains III-V was the same as that of chi psi for full-length tau, indicating that domain III contained a ll structural elements required for chi psi binding. Domain III of tau also contained delta delta' binding sites, although the interaction between del ta delta' and domains III-V of tau was 10-fold weaker than the interaction between SS' and full length tau. The presence of both S and chi psi strengt hened the delta'-C(0)tau interaction by at least 15-fold. Domain III was th e only domain common to all of tau fusion proteins whose interaction with d elta' was enhanced in the presence of S and chi psi. Thus, domain III of th e DnaX proteins not only contains the delta delta' and chi psi binding site s but also contains the elements required for the positive cooperative asse mbly of the DnaX complex.