Presequence translocase-associated motor subunits of the mitochondrial protein import apparatus are dual-targeted to mitochondria and plastids.

The import and assembly of most of the mitochondrial proteome is regulated by protein translocases located within the mitochondrial membranes. The Presequence Translocase-Associated Motor (PAM) complex powers the translocation of proteins across the inner membrane and consists of Hsp70, the J-domain containing co-chaperones, Pam16 and Pam18, and their associated proteins Tim15 and Mge1. In Arabidopsis, multiple orthologues of Pam16, Pam18, Tim15 and Mge1 have been identified and a mitochondrial localization has been confirmed for most. As the localization of Pam18-1 has yet to be determined and a plastid localization has been observed for homologues of Tim15 and Mge1, we carried out a comprehensive targeting analysis of all PAM complex orthologues using multiple in vitro and in vivo methods. We found that, Pam16 was exclusively targeted to the mitochondria, but Pam18 orthologues could be targeted to both the mitochondria and plastids, as observed for the PAM complex interacting partner proteins Tim15 and Mge1.

A Mitochondrial LYR Protein Is Required for Complex I Assembly.

Complex I biogenesis requires the expression of both nuclear and mitochondrial genes, the import of proteins, cofactor biosynthesis, and the assembly of at least 49 individual subunits. Assembly factors interact with subunits of Complex I but are not part of the final holocomplex. We show that in Arabidopsis (Arabidopsis thaliana), a mitochondrial matrix protein (EMB1793, At1g76060), which we term COMPLEX I ASSEMBLY FACTOR 1 (CIAF1), contains a LYR domain and is required for Complex I assembly. T-DNA insertion mutants of CIAF1 lack Complex I and the Supercomplex I+III. Biochemical characterization shows that the assembly of Complex I is stalled at 650 and 800 kD intermediates in mitochondria isolated from ciaf1 mutant lines.I. Yeast-two-hybrid interaction and complementation assays indicate that CIAF1 specifically interacts with the 23-kD TYKY-1 matrix domain subunit of Complex I and likely plays a role in Fe-S insertion into this subunit. These data show that CIAF1 plays an essential role in assembling the peripheral matrix arm Complex I subunits into the Complex I holoenzyme.

Plant mitochondrial protein import: the ins and outs.

The majority of the mitochondrial proteome, required to fulfil its diverse range of functions, is cytosolically synthesised and translocated via specialised machinery. The dedicated translocases, receptors, and associated proteins have been characterised in great detail in yeast over the last several decades, yet many of the mechanisms that regulate these processes in higher eukaryotes are still unknown. In this review, we highlight the current knowledge of mitochondrial protein import in plants. Despite the fact that the mechanisms of mitochondrial protein import have remained conserved across species, many unique features have arisen in plants to encompass the developmental, tissue-specific, and stress-responsive regulation in planta. An understanding of unique features and mechanisms in plants provides us with a unique insight into the regulation of mitochondrial biogenesis in higher eukaryotes.