A functional decaisoleucine-containing signal sequence. Construction by cassette mutagenesis

An alkaline phosphatase signal sequence optimized for formation of a hydrophobic alpha-helix functions very efficiently in the transport process. This mutant contained a core region comprised of 9 consecutive leucine residues (Kendall, D. A., Bock, S. C., and Kaiser, E. T. (1986) Nature 321, 706-708). We have now constructed a second mutant containing a decaisoleucine core region. Isoleucine was chosen because it is an isomer of leucine with comparable hydrophobicity but in synthetic peptides isoleucine favors beta-sheet formation. Surprisingly, this mutant precursor was also processed efficiently, and mature alkaline phosphatase was correctly targeted to the Escherichia coli periplasm. Since the effective length of a beta-strand is extended relative to an alpha-helix, conformational differences should be mirrored by the relative effectiveness of shortened polyisoleucine and polyleucine core regions. However, analysis of two additional mutants containing truncated segments of either polyleucine or polyisoleucine did not reveal any differences and both accumulate as precursors. We conclude that these mutants do not adopt critically different structures. This comparative analysis was facilitated by construction of a new plasmid, CASS3. This plasmid contains unique restriction sites flanking the DNA region coding for the signal sequence hydrophobic core segment. Consequently, the wild type core-encoding region can be readily replaced with synthetic oligonucleotides coding for new structural units and multiple amino acid substitutions can be made without the need for step-wise mutagenesis.