Rate limiting step of the allosteric activation of the bacterial adhesin
FimH investigated by molecular dynamics simulations
Abstract
The bacterial adhesin FimH is a model for the study of protein allostery
because its structure has been resolved in multiple configurations,
including the active and the inactive state. FimH consists of a pilin
domain (PD) that anchors it to the rest of the fimbria and an
allosterically regulated lectin domain (LD) that binds mannose on the
surface of infected cells. Under normal conditions, the two domains are
docked to each other and LD binds mannose weakly. However, in the
presence of tensile force generated by shear the domains separate and
conformational changes propagate across LD resulting in a stronger bond
to mannose. Recently, the crystallographic structure of a variant of
FimH has been resolved, called FimH FocH, where PD
contains 10 mutations near the inter-domain interface. Although the
X-ray structures of FimH and FimH FocH are almost
identical, experimental evidence shows that FimH FocH
is activated even in the absence of shear. Here, molecular dynamics
simulations combined with the Jarzinski equality were used to
investigate the discrepancy between the crystallographic structures and
the functional assays. The results indicate that the free energy barrier
of the unbinding process between LD and PD is drastically reduced in
FimH FocH. Rupture of an inter-domain hydrogen bond
involving R166 constitutes a rate limiting step of the domains
separation process and occurs more readily in FimH
FocH than FimH. In conclusion, the mutations in FimH
FocH shift the equilibrium towards an equal occupancy
of bound and unbound states for LD and PD by reducing a rate limiting
step.