Mechanism underlying nuclear encoded plant protein sorting into Mitochondria

Mechanism by which the cells increase the mitochondrial mass requires coordinated expression of both the nuclear and organellar genomes and specific intracellular protein targeting, processing and assembly machinery. Mitochondria imports hundreds of different proteins that are encoded by the nuclear genes. Almost 98% of mitochondrial proteins are synthesized as preproteins in the cytosol. Most of the nuclear encoded mitochondrial proteins are synthesised as precursor proteins containing an N-terminal extension (presequence) which functions as a targeting signal, which is then proteolytically cleaved off after import into mitochondria. These presequences are responsible for the specific targeting of proteins to the matrix of the mitochondria. This review sheds insight into the mechanism underlying the mitochondrial protein import system in plants which encompasses structure and properties of mitochondrial targeting signals, the role of cytoplasmic chaperons in mitochondrial protein targeting, targeting pathway of mitochondrial preproteins and energetics of import process. Serine content is high (17%) in plant presequence. The N-terminal and C-terminal region of the presequence was found to have more significance in mitochondrial protein targeting than the central region. Domain structure analysis of preprotein showed that it has an import domain and a processing domain. Various cytoplasmic chaperons are found to interact with these preproteins which assist in their folding, prevent aggregation and prevent missorting. Studies have shown that chloroplast preproteins but not mitochondrial preprotein are phosphorylated by plant specific cytoplasmic protein kinases which help in their sorting to chloroplast. Dephosphorylation is also needed for complete preprotein translocation to chloroplast. This phosphorylation and dephosphorylation cycle help in sorting of preprotein between mitochondria and chloroplast. Mitochondrial outer membrane has TOM complex which act as the only entry point for the preproteins. Once it crosses the outer membrane, these preproteins segregate for further import into the matrix through two inner membrane translocases. ATP dependent action of mtHSP70 helps in proper movement of preproteins to mitochondrial matrix. The MPP specifically catalyses the removal of the presequence of a range of different mitochondrial precursor proteins. Membrane potential and ATP hydrolysis play an important role in import of preproteins.