The DnaB center dot DnaC complex: a structure based on dimers assembled around an occluded channel

Replicative helicases are motor proteins that unwind DNA at replication for ks. Escherichia coli DnaB is the best characterized member of this family o f enzymes. We present the 26 Angstrom resolution three-dimensional structur e of the DnaB hexamer in complex with its loading partner, DnaC, obtained f rom cryo-electron microscopy. Analysis of the volume brings insight into th e elaborate way the two proteins interact, and provides a structural basis for control of the symmetry state and inactivation of the helicase by DnaC, The complex is arranged on the basis of interactions among DnaC and DnaB d imers, DnaC monomers are observed for the first time to arrange as three du mb-bell-shaped dimers that interlock into one of the faces of the helicase, This could be responsible for the freezing of DnaB in a C-3 architecture b y its loading partner. The central channel of the helicase is almost occlud ed near the end opposite to DnaC, such that even single-stranded DNA could not pass through. We propose that the DnaB N-terminal domain is located at this face.