Structure, dynamics and function of replisomal Protein complex
Knowledge of the structure of a protein can be very informative in determining its function, however, a structure alone does not tell the whole story. In the case of large, dynamic multi-protein systems where a coordinated network of interactions and enzymatic functions are involved, structural and biochemical information may be gainfully combined with knowledge of dynamics . One such system is the bacterial replisome, which consists of more than a dozen interacting proteins and is capable of rapid DNA replication. As such, it is a good model system and a ripe target for antibiotic development. We have focussed on an interaction that involves one of the key organisational centres of the Gram-positive bacterial replisome, the DnaB helicase and its loading partner, DnaI, which together form a tight 6:6 complex prior to loading of the hexameric helicase onto DNA, after which DnaI dissociates . We have separately measured the dynamics of DnaI, DnaB and their complex using elastic, quasielastic and inelastic neutron scattering on IN6 at ILL (Grenoble, France). These measurements have established that the molecular fl exibility and diffusive motions are signifi cantly decreased when DnaI and DnaB are complexed, with DnaB having the greatest fl exibility when free. Further to this, complexation results in an increase in structural rigidity with the DnaB-DnaI complex having a signifi cantly higher effective force constant both above and below the temperature of dynamical transition. The vibrational density of states was also extracted, revealing that the DnaI-DnaB complex has less inelastic vibrational modes than DnaB and DnaI below the absolute value of 20 meV.