DIVERSE AMINO-ACID-RESIDUES FUNCTION WITHIN THE TYPE-1 PEROXISOMAL TARGETING SIGNAL - IMPLICATIONS FOR THE ROLE OF ACCESSORY RESIDUES UPSTREAM OF THE TYPE-1 PEROXISOMAL TARGETING SIGNAL

The purpose of this study was to determine whether the plant type 1 pe roxisomal targeting signal (PTS1) utilizes amino acid residues that do not strictly adhere to the serine-lysine-leucine (SKL) motif (small-b asic-hydrophobic residues). Selected residues were appended to the C t erminus of chloramphenicol acetyltransferase (CAT) and were tested for their ability to target CAT fusion proteins to glyoxysomes in tobacco (Nicotiana tabacum L.) cv Bright Yellow 2 suspension-cultured cells. CAT was redirected from the cytosol into glyoxysomes by a wide range o f residues, i.e. A/C/G/S/T-H/K/L/N/R-I/L/M/Y. Although L and N at the -2 position (-SLL, -ANL) do not conform to the SKL motif, both functio ned, but in a temporally less-efficient manner. Other SKL divergent re sidues, however, did not target CAT to glyoxysomes, i.e. F or P at the -3 position (-FKL, -PKL), S or T at the -2 position (-SSI, STL), or D at the -1 position (-SKD). The targeting inefficiency of CAT-ANL coul d be ameliorated when K was included at the -4 position (-KANL). In su mmary, the plant PTS1 mostly conforms to the SKL motif. For those PTS1 s that possess nonconforming residue(s), other residues upstream of th e PTS1 appear to function as accessory sequences that enhance the temp oral efficiency of peroxisomal targeting.