Understanding protein folding is
crucial for protein sciences.
The conformational spaces and energy landscapes of cold (unfolded)
protein states, as well as the associated transitions, are hardly
explored. Furthermore, it is not known how structure relates to the
cooperativity of cold transitions, if cold and heat unfolded states
are thermodynamically similar, and if cold states play important roles
for protein function. We created the cold unfolding 4-helix bundle
DCUB1 with a de novo designed bipartite hydrophilic/hydrophobic core
featuring a hydrogen bond network which extends across the bundle
in order to study the relative importance of hydrophobic versus hydrophilic
protein–water interactions for cold unfolding. Structural and
thermodynamic characterization resulted in the discovery of a complex
energy landscape for cold transitions, while the heat unfolded state
is a random coil. Below ∼0 °C, the core of DCUB1 disintegrates
in a largely cooperative manner, while a near-native helical content
is retained. The resulting cold core-unfolded state is compact and
features extensive internal dynamics. Below −5 °C, two
additional cold transitions are seen, that is, (i) the formation of
a water-mediated, compact, and highly dynamic dimer, and (ii) the
onset of cold helix unfolding decoupled from cold core unfolding.
Our results suggest that cold unfolding is initiated by the intrusion
of water into the hydrophilic core network and that cooperativity
can be tuned by varying the number of core hydrogen bond networks.
Protein design has proven to be invaluable to explore the energy landscapes
of cold states and to robustly test related theories.