New functions of the chloroplast Preprotein and Amino acid Transporter (PRAT) family members in protein import

Abstract

Common principles of protein translocation across membranesCells contain different, membrane-limited compartments endowed with specific proteins. To maintain these compartments, cells have developed elaborate systems for transporting the cell's few thousand different proteins to their final location. Export and import systems have been identified in bacteria and eukaryotes that operate by a similar set of principles.1 Each protein carries NH 2 -terminal or internal targeting information that is recognized by cytoplasmic targeting factors and directs the protein to specific receptors on the target membrane; the targeting signal interacts with a hetero-oligomeric trans-membrane channel that is gated both across and within the plane of the membrane. Molecular chaperones act as nucleoside triphosphate-powered import motors and interact with both the translocating polypeptide chain and the protein-conducting channel. Methods that have led to the identification of these principles include protein biochemistry, cell biology, and genetics.2-8 For example, it was found that mitochondria contain 2 types of receptors on their outer surface to bind cytosolic precursors for import. Mitochondrial precursors containing cleavable NH 2 -terminal pre-sequences interact with TOM20. By contrast, carrier proteins and other hydrophobic membrane proteins lacking cleavable NH 2 -terminal pre-sequences bind TOM70. A third TOM protein, TOM22, was found to associate with both TOM70 and TOM20. All 3 TOM proteins are essential for the formation of intact mitochondria and cooperate in the passage of precursors into the TOM40 import channel in the outer mitochondrial membrane.2-4 Import of pre-sequence-containing and pre-sequence-less precursors in fact converges at TOM40. 2-4Numerous other TOM proteins assist in subsequent steps of mitochondrial protein import. 2-4Import of pre-sequence-containing chloroplast precursors is mediated by at least 3 proteins: the 2 GTP binding receptors TOC159 and TOC34, and the β-barrel protein and translocation channel TOC75 5-8 . TOC159 and TOC34 are encoded by small gene families in Arabidopsis thaliana, with AtTOC159 containing 4 members, designated Attoc159, Attoc132, Attoc120, and Attoc90, and AtTOC34 containing 2 members, named Attoc34 and Attoc33. Biochemical and genetic evidence suggests that there is a functional specialization among the different members of the guanosine triphosphate hydrolyzing GTPase receptor families. 5-8Import of most pre-sequence-containing cytosolic precursors is Keywords: Membrane transport, Protein translocation, Chloroplasts and mitochondria, TIC and TIM translocons at the inner chloroplast envelope and inner mitochondrial membrane, TOC and TOM translocons at outer chloroplast and outer mitochondrial membrane, PRAT protein evolutionPlant cells contain distinct compartments such as the nucleus, the endomembrane system comprising the endoplasmic reticulum and Golgi apparatus, peroxisomes, vacuoles, as well as mitochondria and chloroplasts. All of these compartments are surroun...

HP30‐2, a mitochondrial PRAT protein for import of signal sequence‐less precursor proteins in Arabidopsis thaliana

“…Our results reveal that a single PRAT protein, namely HP30‐2, can accomplish redundant roles both in chloroplasts and mitochondria which is not the case for either TIM23 or B14.7. In both organelle types, HP30‐2 likewise operates in the import of transit sequence‐less precursors and does so by interacting with distinct proteins to establish unique import sites (Rossig et al , ; Reinbothe et al , and this study). Hereby, HP30‐2 differentially bound two other members of the PRAT family, namely HP30 in chloroplasts, and TIM22 in mitochondria (Figure C).…”

“…HP20 and HP22 (HP20 subfamily) as well as HP30 and HP30‐2 (HP30 subfamily) all were originally discovered by proteomics studies on isolated plastid envelope membranes (Ferro et al ). We found HP20, HP30 and HP30‐2 to establish a new translocase for the import of cytosolic precursor proteins lacking cleavable NH 2 ‐terminal transit peptides into chloroplasts (Rossig et al ; Reinbothe et al ; see Rossig et al , for review).…”

“…Examination of the Arabidopsis genome annotations reveals the presence of 17 candidate genes encoding PRAT proteins (Rassow et al 1999;Drea et al 2006;Murcha et al 2007). Phylogenetic analyses identified four main clades designated TIM17, TIM22, TIM23 and OEP16-like, and two other clades annotated as HP20 and HP30 (Drea et al 2006;Murcha et al 2007;Rossig et al 2014). HP20 and HP30 constitute subfamilies, each comprising two closely related members.…”

“…These findings were reminiscent of results reported for the TIM22 translocase in yeast mitochondria that, as said before, operates in the import of presequence‐less proteins into the inner mitochondrial membrane. Together, these data identified the PRAT family as a widely used system of protein translocases in different membranes of endosymbiotic origin (Rossig et al ; Reinbothe et al ; see Rossig et al , for review).…”

A Protochlorophyllide (Pchlide) a Oxygenase for Plant Viability

“…Of the remainder, 37 were predicted to have a mitochondrial targeting signal, 40 to have a signal peptide for translocation into the endoplasmic reticulum, and 142 to possess no cleavable presequence. Evidence is emerging for the dual targeting of cytosolic proteins to mitochondria and chloroplasts (Peeters and Small, 2001), for the plastid import of transit peptide-less precursors (Miras et al, 2002, 2007; Nada and Soll, 2004; Rossig et al, 2013, 2014), and for the involvement of the secretory pathway in the import of certain precursors into chloroplasts (Villarejo et al, 2005; Baslam et al, 2016).…”