Targeting of ricin A chain into pea chloroplasts

On the impact of precursor unfolding during protein import into chloroplasts

Protein translocation across membranes is a fundamental cellular process. The majority of the proteins of organelles such as mitochondria and chloroplasts is synthesized in the cytosol and subsequently imported in a post-translational manner. The precursor proteins have to be unfolded at least for translocation, but it has also been assumed that they are unfolded during transport to the organelle in the cytosol. Unfolding is governed by chaperones and the translocon itself. At the same time, chaperones provide the energy for the import process. The energetic properties of the chloroplast translocon were studied by import of the Ig-like module of the muscle protein titin fused to the transit peptide of the chloroplast targeted oxygen evolving complex subunit of 33 kDa (OE33). Our results suggest that p(OE33)titin is folded prior to import and that translocation is initiated by unfolding after having bound to the translocon at the chloroplast surface. Using a set of stabilizing and destabilizing mutants of titin previously analyzed by atomic force microscopy and as passenger for mitochondrial translocation, we studied the unfolding force provided by the chloroplast translocon. Based on these results, a model for translocation is discussed.

Identification of a Role for an Azide-Sensitive Factor in the Thylakoid Transport of the 17-Kilodalton Subunit of the Photosynthetic Oxygen-Evolving Complex

We have examined the transport of the precursor of the 17-kD subunit of the photosynthetic O2-evolving complex (OE17) in intact chloroplasts in the presence of inhibitors that block two protein-translocation pathways in the thylakoid membrane. This precursor uses the transmembrane pH gradient-dependent pathway into the thylakoid lumen, and its transport across the thylakoid membrane is thought to be independent of ATP and the chloroplast SecA homolog, cpSecA. We unexpectedly found that azide, widely considered to be an inhibitor of cpSecA, had a profound effect on the targeting of the photosynthetic OE17 to the thylakoid lumen. By itself, azide caused a significant fraction of mature OE17 to accumulate in the stroma of intact chloroplasts. When added in conjunction with the protonophore nigericin, azide caused the maturation of a fraction of the stromal intermediate form of OE17, and this mature protein was found only in the stroma. Our data suggest that OE17 may use the sec-dependent pathway, especially when the transmembrane pH gradient-dependent pathway is inhibited. Under certain conditions, OE17 may be inserted across the thylakoid membrane far enough to allow removal of the transit peptide, but then may slip back out of the translocation machinery into the stromal compartment.

Ricin A chain fused to a chloroplast-targeting signal is unfolded on the chloroplast surface prior to import across the envelope membranes

The initial stages of chloroplast protein import involve the binding of precursor proteins to surface-bound receptors prior to translocation across the envelope membranes in a partially folded conformation. We have analyzed the unfolding process by examining the conformation of a construct, comprising the presequence of a chloroplast protein linked to ricin A chain, before and after binding to the chloroplast surface. We show that the presequence is highly susceptible to proteolysis in solution, probably reflecting a lack of tertiary structure, whereas the A chain passenger protein is resistant to extremely high concentrations of protease, unless deliberately unfolded using denaturant. The A chain moiety is furthermore active, indicating that the presence of the presequence does not prevent formation of a tightly folded, native state. In contrast, receptor-bound p33KRA (fusion protein comprising the 33-kDa presequence plus 22 residues of mature protein, linked to the A chain of ricin) is quantitatively digested by protease concentrations that have little effect on the A chain in solution. We conclude that protein unfolding can take place on the chloroplast surface in the absence of translocation and without the aid of soluble factors.

SecA is plastid-encoded in a red alga: implications for the evolution of plastid genomes and the thylakoid protein import apparatus

Partial sequence analysis of the plastid DNA (ptDNA) from a red alga, Antithamnion sp., revealed the presence of a homologue to the Escherichia coli secA gene as well as two open reading frames (ORF 510, ORF 179). In addition a sec Y homologue has been detected on the plastid genome by heterologous hybridization. None of these genes has been found in completely sequenced chlorophytic plastid genomes. SecA and secY gene copies were also detected in the ptDNA of a chromophytic alga, indicating that secA Y may be ubiquitous in rhodophytes and chromophytes. The significance of these findings for the evolution of plastid genomes and the thylakoid protein import mechanism is discussed.