  Using lattice models we explore the factors that determine the tendencies of
polypeptide chains to aggregate by exhaustively sampling the sequence and
conformational space. The morphologies of the fibril-like structures and the
time scales ($\tau_{fib}$) for their formation depend on a balance between
hydrophobic and coulomb interactions. The extent of population of an ensemble
of \textbf{N$^*$} structures, which are fibril-prone structures in the spectrum
of conformations of an isolated protein, is the major determinant of
$\tau_{fib}$. This observation is used to determine the aggregating sequences
by exhaustively exploring the sequence space, thus providing a basis for genome
wide search of fragments that are aggregation prone.
