  It is established that prion protein is the sole causative agent in a number
of diseases in humans and animals. However, the nature of conformational
changes that the normal cellular form PrPC undergoes in the conversion process
to a self-replicating state is still not fully understood. The ordered
C-terminus of PrPC proteins has three helices (H1, H2, and H3). Here, we use
the Statistical Coupling Analysis (SCA) to infer co-variations at various
locations using a family of evolutionarily related sequences, and the response
of mouse and human PrPCs to mechanical force to decipher the initiation sites
for transition from PrPC to an aggregation prone PrP* state. The sequence-based
SCA predicts that the clustered residues in non-mammals are localized in the
stable core (near H1) of PrPC whereas in mammalian PrPC they are localized in
the frustrated helices H2 and H3 where most of the pathogenic mutations are
found. Force-extension curves and free energy profiles as a function of
extension of mouse and human PrPC in the absence of disulfide (SS) bond between
residues Cys179 and Cys214, generated by applying mechanical force to the ends
of the molecule, show a sequence of unfolding events starting first with
rupture of H2 and H3. This is followed by disruption of structure in two
strands. Helix H1, stabilized by three salt-bridges, resists substantial force
before unfolding. Force extension profiles and the dynamics of rupture of
tertiary contacts also show that even in the presence of SS bond the
instabilities in most of H3 and parts of H2 still determine the propensity to
form the PrP* state. In mouse PrPC with SS bond there are about ten residues
that retain their order even at high forces.
