A new definition for the consensus sequence of the peroxisome targeting signal type 2

All organisms except the nematode Caenorhabditis elegans have been shown to possess an import system for peroxisomal proteins containing a peroxisome targeting signal type 2 (PTS2). The currently accepted consensus sequence for this amino-terminal nonapeptide is -(R/K)(L/V/I)X(5)(H/Q)(L/A)-. Some C.elegans proteins contain putative PTS2 motifs, including the ortholog (CeMeK) of human mevalonate kinase, an enzyme known to be targeted by PTS2 to mammalian peroxisomes. We cloned the gene for CeMeK (open reading frame Y42G9A.4) and examined the subcellular localization of CeMeK and of two other proteins with putative PTS2s at their amino termini encoded by the open reading frames D1053.2 and W10G11.11. All three proteins localized to the cytosol, confirming and extending the finding that C.elegans lacks PTS2-dependent peroxisomal protein import. The putative PTS2s of the proteins encoded by D1053.2 and W10G11.11 did not function in targeting to peroxisomes in yeast or mammalian cells, suggesting that the current PTS2 consensus sequence is too broad. Analysis of available experimental data on both functional and nonfunctional PTS2s led to two re-evaluated PTS2 consensus sequences: -R(L/V/I/Q)XX(L/V/I/H)(L/S/G/A)X(H/Q)(L/A)-, describes the most common variants of PTS2, while -(R/K)(L/V/I/Q)XX(L/V/I/H/Q)(L/S/G/A/K)X(H/Q)(L/A/F)-, describes essentially all variants of PTS2. These redefined PTS2 consensus sequences will facilitate the identification of proteins of unknown cellular localization as possible peroxisomal proteins.