Protein import into plant mitochondria.

Abstract

The presence of plastids in plant cells requires a higher level of precursor recognition by the mitochondrial protein import apparatus than in nonplant organisms. Although the plant presequences display the overall features observed in yeast and mammals, they are generally longer and more hydrophilic. Most of them are highly organelle specific, but some have ambiguous targeting specificity delive-ring a protein to both mitochondria and chloroplasts. Many components of plant protein import apparatus appear different to that in yeast and mammalian systems. The three outer membrane mitochondrial proteins characterized to play role as receptors in plants – Tom20, OM64, and metaxin – are plant specific. However, the channel forming units of the TOM and SAM complexes, Tom40 and Sam50, respectively, are orthologous to these components in yeast. While components of the MIA and TIM complexes also display high levels of orthology, functional studies indicate divergences in function and mechanism. Differences exist also in terms of intraorganellar localization of proteolytic events, e.g., the location of the mitochondrial processing peptidase, MPP, involved in removing targeting signals is different, whereas the function and location of the presequence protease, PreP, degrading targeting peptides, is well conserved. Overall, although the protein import machinery of mitochondria from all organisms appears to have coopted and uses the channel forming subunits from the endosymbiont that gave rise to mitochondria, there is a greater diversity in plant components in comparison to those from nonplant species.