Can a signal sequence become too hydrophobic?

We have characterized several mutants that contain alterations in the hydrophilic domain (N region) of the pseudorabies virus glycoprotein gC signal sequence. In general, our results agree with previous findings and indicate that basic residues in the N region are not essential for efficient export of gC in infected cells. While reducing the N region to a net neutral charge led to a slight impairment of membrane translocation, a substantial gC export defect was not observed until a net negative charge was introduced. However, there was one exception to this pattern. The substitution of a leucine for an arginine at the carboxyl terminus of the N region led to a considerable export defect despite maintaining a net positive charge. As a consequence of the substitution, the mutant signal sequence was 1.5 times more hydrophobic than wild type, but we found that the defect could be largely corrected if an additional alteration that lessened the overall hydrophobicity of the gC signal sequence was incorporated. We suggest that an upper limit of hydrophobicity may exist for eukaryotic signal sequences; exceeding this value could lead to an export defect.