Signal peptide for peroxisomal targeting: replacement of an essential histidine residue by certain amino acids converts the amino-terminal presequence of peroxisomal 3-ketoacyl-CoA thiolase to a mitochondrial signal peptide.

The role of the histidine residue at position -17 of the amino-terminal signal peptide of rat peroxisomal 3-ketoacyl-CoA thiolase was studied in vivo, employing site-directed mutagenesis. Among the nine amino acids tested, only glutamine could partially substitute for the histidine. Mutants carrying basic amino acids, arginine and lysine, and hydrophobic residues, leucine and valine, in place of histidine were all translocated to mitochondria, but not to peroxisomes. These results indicate that the signal peptide of the thiolase is recognized by a mechanism totally different from that for the SKL motif, a known peroxisomal targeting signal. Relationship of the thiolase signal peptide to those of mitochondrial proteins is discussed.