PHOSPHOENOLPYRUVATE CARBOXYKINASE OF TRYPANOSOMA-BRUCEI IS TARGETED TO THE GLYCOSOMES BY A C-TERMINAL SEQUENCE

Import of proteins into the glycosomes of T. brucei resembles the pero xisomal protein import in that C-terminal SKL-like tripeptide sequence s can function as targeting signals. Many of the glycosomal proteins d o not, however, possess such C-terminal tripeptide signals. Among thes e, phosphoenolpyruvate carboxykinase (PEPCK (ATP)) was thought to be t argeted to the glycosomes by an N-terminal or an internal targeting si gnal. A limited similarity to the N-terminal targeting signal of rat p eroxisomal thiolase exists al the N-terminus of T. brucei PEPCK. Howev er, we found that this peroxisomal targeting signal does not function for glycosomal protein import in T. brucei. Further studies of the PEP CK gene revealed that the C-terminus of the predicted protein does not correspond to the previously deduced protein sequence of 472 amino ac ids due to a -1 frame shift error in the original DNA sequence. Readju sting the reading frame of the sequence results in a predicted protein of 525 amino acids in length ending in a tripeptide serine-arginine-l eucine (SRL), which is a potential targeting signal for import into th e glycosomes. A fusion protein of firefly luciferase, without its own C-terminal SKL targeting signal, and T. brucei PEPCK is efficiently im ported into the glycosomes when expressed in procyclic trypanosomes. D eletion of the C-terminal SRL. tripeptide or the last 29 amino acids o f PEPCK reduced the import only by about 50%, while a deletion of the last 47 amino acids completely abolished the import. These results sug gest that T. brucei PEPCK may contain a second, internal glycosomal ta rgeting signal upstream of the C-terminal SRL