  Metamorphic proteins like Lymphotactin are a notable exception of the
empirical principle that structured natural proteins possess a unique three
dimensional structure. In particular, the human chemokine lymphotactin protein
(Ltn) exists in two distinct conformations (one monomeric and one dimeric)
under physiological conditions. In this work we use a Ca Go-model to show how
this very peculiar behavior can be reproduced. From the study of the
thermodynamics and of the kinetics we characterize the interconversion
mechanism. In particular, this takes place through the docking of the two
chains living in a third monomeric, partially unfolded, state which shows a
residual structure involving a set of local contacts common to the two native
conformations. The main feature of two-fold proteins appears to be the sharing
of a common set of local contacts between the two distinct folds as confirmed
by the study of two designed two-fold proteins. Metamorphic proteins may be
more common than expected.
