  In a similar way in which the folding of single--domain proteins provide an
important test in the study of self--organization, the folding of homodimers
constitute a basic challenge in the quest for the mechanisms which are at the
basis of biological recognition. Dimerization is studied by following the
evolution of two identical 20--letter amino acid chains within the framework of
a lattice model and using Monte Carlo simulations. It is found that when design
(evolution pressure) selects few, strongly interacting (conserved) amino acids
to control the process, a three--state folding scenario follows, where the
monomers first fold forming the halves of the eventual dimeric interface
independently of each other, and then dimerize ("lock and key" kind of
association). On the other hand, if design distributes the control of the
folding process on a large number of (conserved) amino acids, a two--state
folding scenario ensues, where dimerization takes place at the beginning of the
proces, resulting in an "induced type" of association. Making use of
conservation patterns of families of analogous dimers, it is possible to
compare the model predictions with the behaviour of real proteins. It is found
that theory provides an overall account of the experimental findings.
