  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 subtle balance
between hydrophobic and coulomb interactions. The extent of population of a
fibril-prone structure in the spectrum of monomer conformations is the major
determinant of $\tau_{fib}$. This observation is used to determine the
aggregation-prone consensus sequences by exhaustively exploring the sequence
space. Our results provide a basis for genome wide search of fragments that are
aggregation prone.
