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.

Glossary

Ambiguous targeting signal:

A targeting signal that directs a protein to more than one location in a cell.

Amphiphilic (amphipathic):

With reference to targeting signals refers to the fact that they contain polar and apolar properties.

Dual-targeting:

A targeting process in which a protein is authentically targeted and accumulates in more than one location in a cell.

Intraorganelle sorting:

The requirement to sort a protein to the correct location inside the organelle once it has been targeted to that organelle. The default targeting signal for mitochondria targets protein to the matrix and thus all proteins that are not located in the matrix must be stopped on their ways to the matrix (stop-transfer) or retargeted from the matrix to their final location (conservative sorting).

Mis-targeting:

A process in which a protein is targeted to a location other than that where it is normally found. Mis-targeting can be observed in some in vitro uptake assays, in vivo targeting assays that tag proteins with artificial passenger proteins, when using heterologous systems or when using chimeric constructs.

Molecular chaperone:

A protein that binds transiently to unfolded domains of other proteins, preventing them for misfolding and conferring an import competent conformation.

Passenger protein:

A protein that when linked to a presequence will be directed to mitochondria. Passenger proteins have properties that allow them to be easily visualized, such as fluorescence (GFP) or can be manipulated to fold or unfold under altered experimental (DHFR).

Precursor processing:

Proteolytic cleavage of the presequence from a precursor protein by mitochondrial processing peptidases (MPP, IMP, ICP55, MIP) resulting in a mature protein and a free presequence that is further degraded by presequence protease, PreP.

Presequence:

N-terminal cleavable targeting signals that direct proteins to mitochondria.

Transit peptide:

N-terminal cleavable targeting signals that direct proteins to chloroplasts.

Translocase:

A multi-subunit membrane bound protein complex that mediates the recognition and insertion of proteins into or across a membrane.