On the Hydrophobic Nature of Signal Sequences

A number of signal sequences, prokaryotic as well as eukaryotic, have been analyzed in terms of gross amino acid composition and hydrophobicity. It is shown that the amino acid composition of the hydrophobic core can be well reproduced in a computer simulation of signal sequence 'evolution' with selection operating on the mean hydrophobicity of the sequence and the non-occurrence of charged residues. The calculated hydrophobicities are interpreted in terms of a model in which the hydrophobic part of the signal sequence partitions directly into the membrane interior, thereby making further translocation of the growing nascent chain possible.