  With the help of a simple 20 letters, lattice model of heteropolymers, we
investigate the energy landscape in the space of designed good-folder
sequences. Low-energy sequences form clusters, interconnected via neutral
networks, in the space of sequences. Residues which play a key role in the
foldability of the chain and in the stability of the native state are highly
conserved, even among the chains belonging to different clusters. If, according
to the interaction matrix, some strong attractive interactions are almost
degenerate (i.e. they can be realized by more than one type of aminoacid
contacts) sequence clusters group into a few super-clusters. Sequences
belonging to different super-clusters are dissimilar, displaying very small
($\approx 10%$) similarity, and residues in key-sites are, as a rule, not
conserved. Similar behavior is observed in the analysis of real protein
sequences.
