Toc64--a preprotein-receptor at the outer membrane with bipartide function

Protein translocation across membranes is assisted by translocation machineries present in the membrane targeted by the precursor proteins. Translocon subunits can be functionally divided into receptor proteins warranting the specificity of this machine and a translocation channel. At the outer envelope of chloroplasts two sets of receptor proteins regulate protein translocation facing the cytosol or acting in the intermembrane space. One, Toc64 is a receptor of the translocon at the outer envelope of chloroplasts (Toc complex) with dual function. Toc64 recognizes Hsp90 delivered precursor proteins via a cytosolic exposed domain containing three tetratrico-peptide repeat motifs and as demonstrated in here, Toc64 functions also as a major component of a complex facing the intermembrane space. The latter complex is composed of an Hsp70 localized in the intermembrane space, its interaction partner Toc12, a J-domain containing protein and the intermembrane space protein Tic22. We analyzed the intermembrane space domain of Toc64. This domain is involved in preprotein recognition and association with the Toc-complex independent of the cytosolic domain of the Toc64 receptor. Therefore, Toc64 is involved in preprotein translocation across the outer envelope at both sites of the membrane.     

Type II NAD(P)H dehydrogenases are targeted to mitochondria and chloroplasts or peroxisomes in Arabidopsis thaliana

We found that four type II NAD(P)H dehydrogenases (ND) in Arabidopsis are targeted to two locations in the cell; NDC1 was targeted to mitochondria and chloroplasts, while NDA1, NDA2 and NDB1 were targeted to mitochondria and peroxisomes. Targeting of NDC1 to chloroplasts as well as mitochondria was shown using in vitro and in vivo uptake assays and dual targeting of NDC1 to plastids relies on regions in the mature part of the protein. Accumulation of NDA type dehydrogenases to peroxisomes and mitochondria was confirmed using Western blot analysis on highly purified organelle fractions. Targeting of ND proteins to mitochondria and peroxisomes is achieved by two separate signals, a C-terminal signal for peroxisomes and an N-terminal signal for mitochondria.     

Differential uptake of photosynthetic and non-photosynthetic proteins by pea root plastids

The photosynthetic proteins RuBiSCO, ferredoxin I and ferredoxin NADP(+)-oxidoreductase (pFNR) were efficiently imported into isolated pea chloroplasts but not into pea root plastids. By contrast non-photosynthetic ferredoxin III and heterotrophic FNR (hFNR) were efficiently imported into both isolated chloroplasts and root plastids. Chimeric ferredoxin I/III (transit peptide of ferredoxin I attached to the mature region of ferredoxin III) only imported into chloroplasts. Ferredoxin III/I (transit peptide of ferredoxin III attached to the mature region of ferredoxin I) imported into both chloroplasts and root plastids. This suggests that import depends on specific interactions between the transit peptide and the translocon apparatus.