Import of a mitochondrial presequence into protein-free phospholipid vesicles

Presequence-mediated intermembrane contact formation and lipid flow. A model membrane study

The ability of a synthetic peptide corresponding to the presequence of cytochrome c oxidase subunit IV from yeast to cause intermembrane contacts was investigated using monolayer techniques. The presequence inserted efficiently into the monolayer with a specificity for the mitochondrial cardiolipin. In the inserted form, the peptide strongly promoted the formation of close contacts with large unilamellar lipid vesicles present in the subphase, a property which was also specific for cardiolipin. The contacts formed were stable and tight and resulted in the flow of lipids from the vesicles to the monolayer. These results led to new suggestions on the involvement of intermembrane contact formation in mitochondrial protein import and membrane biogenesis.

Import of a mitochondrial presequence into P. denitrificans. Insight into the evolution of protein transport

According to the endosymbiont hypothesis, mitochondria are descended from ancient aerobic bacteria that were engulfed by protoeukaryotic cells. Experiments described here show that a synthetic peptide corresponding to a yeast mitochondrial targeting sequence can be imported into Paracoccus denitrificans, a soil bacterium thought to be closely related to the protomitochondrion. The import is very similar to that observed with isolated yeast mitochondria. The results suggest that the protomitochondrion may have been inherently able to translocate mitochondrial presequences. This ability may partly explain the development of the protein import process during the evolution of the mitochondrion.

The secondary structure of the ferredoxin transit sequence is modulated by its interaction with negatively charged lipids

Import of proteins into chloroplasts depends on an N-terminal transit sequence. Transit sequences contain little primary sequence similarity and therefore recognition of these sequences is thought to involve specific folding. To assess the conformational flexibility of the transit sequence, we studied the transit peptide of preferredoxin (trfd) by circular dichroism. In buffer, trfd is in a random coil conformation. A large increase in alpha-helix was induced in the presence of micelles or vesicles formed by anionic lipids. Less pronounced changes in secondary structure were induced by zwitterionic detergents but no changes were observed in the presence of neutral detergents or vesicles composed of phosphatidylcholine.