Presequence-Dependent Import

The early observation that amino-terminal sequences rich in hydroxylated and basic amino acids reminiscent of mitochondrial targeting signals were present in the conceptually translated products of the E. histolytica genes encoding mitochondrial-marker proteins Cpn60 and PNT provided the first 

A defining characteristic of mitochondrial and hydrogensomal protein import is the proteolytic removal of amino-terminal targeting presequences upon organelle import. In Giardia, two IscU bands have been observed by western blotting in the organelle fraction that differ in size by about 2 kDa, a 

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While it has been known for many years that the N-terminal presequence is sufficient to promote mitochondrial targeting and assembly, the subsequent interaction of the precursor molecule with the outer mitochondrial membrane and the uptake of the protein is still an area of active research. There seems little doubt, however, that there are proteins on the outer mitochondrial membrane which are required for the import process. The function of these proteins is uncertain, but they may act as receptors with the subsequent transfer through the membrane at proteinous pores located at contact sites between the inner and outer membranes. Several proteins have been identified which seem to play an important role as either receptor proteins or part of the import channel (Pfanner et al., 1991). Again, not all proteins seem to depend on this mechanism. Cytochrome c, which is loosely associated with the outer aspect of the inner mitochondrial membrane, can cross... 

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