Effects of conformational changes in the N-terminal domain of RfaH on
domain dissociation and fold switching
Abstract
RfaH is a two-domain metamorphic protein involved in transcription
regulation and translation initiation. To carry out dual functions, RfaH
relies on two coupled structural changes: domain dissociation and fold
switching. In the free state, the C-terminal domain (CTD) of RfaH adopts
an all- α fold and is tightly associated with the N-terminal
domain (NTD). Upon binding to RNA polymerase (RNAP), the domains
dissociate and the CTD completely transforms into an all- β fold,
while the NTD remains largely, but not entirely, unchanged. We test the
idea that a change in the conformation of an extended β-hairpin (
β3- β4) located on the NTD, helps trigger domain
dissociation. To this end, we use homology modelling to construct a
structure, H 1 , which is similar to free RfaH but with a remodeled
β3- β4 hairpin. We then use an all-atom physics-based
model enhanced with a dual-basin structure-based potential to simulate
domain separation driven by thermal unfolding of the CTD with NTD in a
fixed, folded conformation. We apply our model to both free RfaH and H 1
. For H 1 we find, in line with our hypothesis, that the CTD exhibits a
lower stability and the domains dissociate at a lower temperature (
T), as compared to free RfaH. We do not, however, observe
complete refolding to the all- β state in these simulations,
suggesting that a change in β3- β4 orientation aid in, but
is not sufficient for, domain dissociation. In addition, we study the
reverse fold switch in which RfaH returns from a domain-open all-
β state to its domain-closed all- α state. We observe a
T-dependent transition rate; fold switching is slow at low
T, where the CTD tend to be kinetically trapped in its all-
β state, and at high- T, where the all- α state
becomes unstable. Consequently, our simulations suggest an optimal
T at which fold switching is most rapid. At this T, the
stabilities of both folds are reduced. Overall, our study suggests that
both inter-domain interactions and conformational changes within NTD may
be important for proper functioning of RfaH.