Signal sequences containing multiple aromatic residues

Area of Science:

• Molecular Biology
• Protein Biochemistry
• Cellular Transport

Background:

• Signal peptides are essential for protein secretion and processing in bacteria.
• Hydrophobicity of the signal peptide's core region is a key determinant of its function.
• Previous studies indicated the importance of aliphatic residues for signal peptide efficiency.

Purpose of the Study:

• To investigate the role of phenylalanine and tryptophan homopolymers in the signal peptide of Escherichia coli alkaline phosphatase.
• To further elucidate the hydrophobicity requirements for protein export and processing.
• To establish a function-based assay for evaluating amino acid hydrophobicity in signal peptides.

Main Methods:

• Construction of mutant signal peptides using polyphenylalanine and polytryptophan sequences.
• Analysis of protein export and processing of mutant precursors in Escherichia coli.
• Evaluation of membrane insertion and translocation of polytryptophan-containing precursors.

Main Results:

• Polyphenylalanine-containing signal peptides exhibited efficient function, comparable to the wild-type.
• Polytryptophan-containing signal peptides were dysfunctional, showing membrane insertion but no translocation.
• Results confirm that high mean hydrophobicity per residue is critical for efficient protein translocation and precursor processing.

Conclusions:

• A high degree of hydrophobicity in the signal peptide core is essential for proper protein export and processing.
• Phenylalanine's bulky, aromatic nature or high hydrophobicity does not impede signal peptide function.
• Tryptophan's hydrophobicity is insufficient for efficient translocation, suggesting it's a weakly hydrophobic residue in this context.