Polymeric sequences reveal a functional interrelationship between hydrophobicity and length of signal peptides

We have examined the hydrophobicity component of signal peptide function using polymeric sequences in combination with cassette mutagenesis. Using homopolymeric units of either isoleucine, leucine, valine or alanine to replace the natural core segment of the Escherichia coli alkaline phosphatase signal peptide, the hydrophobicity requirements for export and processing were delineated. The transport properties of these mutants demonstrated that the net hydrophobicity determines the total extent of precursor processing, while a high mean hydrophobicity/residue is critical for complete, rapid processing and translocation. Moreover, alkaline phosphatase was converted from a periplasmic to an active membrane-anchored protein via a signal containing 20 leucine residues. This application of polymeric sequences allows systematic comparisons to be made, unambiguously revealing the hydrophobicity requirements governing specific steps in the transport process.