Giardia mitosomal protein import machinery differentially recognizes mitochondrial targeting signals.

Giardia lamblia mitosomes are believed to be vestigial mitochondria which lack a genome. Similar to higher eukaryotes， mitosomal proteins possess either N-terminal or internal mitosomal targeting sequences. To date， some components of the higher eukaryote archetypal mitochondrial protein import apparatus have been identified and characterized in Giardia mitosomes; therefore， it is expected that mitochondrial signals will be recognized by the mitosomal protein import system. To further determine the level of conservation of the Giardia mitosome protein import apparatus， we expressed mitochondrial proteins from higher eukaryotes in Giardia. These recombinant proteins include Tom20 and Tom22; two components of the mitochondrial protein import machinery. Our results indicate that N-terminal mitochondrial targeting sequence is recognized by the mitosomal protein import machinery; however， interestingly the internal mitochondrial targeting sequences of higher eukaryotes are not recognized by the mitosome. Our results indicate that Giardia mitosome protein transport machinery shows differential recognition of higher eukaryotic mitochondria transfer signals， suggesting a divergence of the transport system in G. lamblia. Therefore， our data support the hypothesis that the protein import machinery in Giardia lamblia mitosome is an incomplete vestigial derivative of mitochondria components.