Chloroplast FtsY, chloroplast signal recognition particle, and GTP are required to reconstitute the soluble phase of light-harvesting chlorophyll protein transport into thylakoid membranes.

The integration of light-harvesting chlorophyll proteins (LHCPs) into the thylakoid membrane proceeds in two steps. First, LHCP interacts with a chloroplast signal recognition particle (cpSRP) to form a soluble targeting intermediate called the transit complex. Second, LHCP integrates into the thylakoid membrane in the presence of GTP, at least one other soluble factor, and undefined membrane components. We previously determined that cpSRP is composed of 43- and 54-kDa polypeptides. We have examined the subunit stoichiometry of cpSRP and find that it is trimeric and composed of two subunits of cpSRP43/subunit of cpSRP54. A chloroplast homologue of FtsY, an Escherichia coli protein that is critical for the function of E. coli SRP, was found largely in the stroma unassociated with cpSRP. When chloroplast FtsY was combined with cpSRP and GTP, the three factors promoted efficient LHCP integration into thylakoid membranes in the absence of stroma, demonstrating that they are all required for reconstituting the soluble phase of LHCP transport.     

An imported thylakoid protein accumulates in the stroma when insertion into thylakoids is inhibited

The light-harvesting chlorophyll a/b protein (LHCP) is synthesized in the cytosol as a precursor (pLHCP) that is imported into chloroplasts and assembled into thylakoid membranes. Under appropriate conditions, either pLHCP or LHCP will integrate into isolated thylakoids. We have identified two situations that inhibit integration in this assay. Ionophores and uncouplers inhibited integration up to 70%. Carboxyl-terminal truncations of pLHCP also interfered with integration. A 22-residue truncation reduced integration to about 25% of control, whereas a 93 residue truncation completely abolished it. When pLHCP was imported into chloroplasts in the presence of uncouplers or when truncated forms of pLHCP were used, significant amounts of the imported proteins failed to insert into thylakoids and instead accumulated in the aqueous stroma. Accumulation of stromal LHCP occurred at uncoupler concentrations required to dissipate the trans-thylakoid proton electrochemical gradient and was enhanced at reduced levels of ATP. The latter effect may be a secondary consequence of a reduction in ATP-dependent degradation within the stroma. These results indicate that the stroma is an intermediate location in the LHCP assembly pathway and provide the first evidence for a soluble intermediate during biogenesis of a chloroplast membrane protein.     

Early events in the import/assembly pathway of an integral thylakoid protein

The light-harvesting chlorophyll a/b protein (LHCP) is nuclear-encoded and must traverse the chloroplast envelope before becoming integrally assembled into thylakoid membranes. Previous studies implicated a soluble stromal form of LHCP in the assembly pathway, but relied upon assays in which the thylakoid insertion step was intentionally impaired [Cline, K., Fulsom, D. R. and Viitanen, P. V. (1989) J. Biol. Chem. 264, 14225-14232]. Here we have developed a rapid-stopping procedure, based upon the use of HgCl2, to analyze early events of the uninhibited assembly process. With this approach, we have found that proper assembly of LHCP into thylakoids lags considerably behind trans-envelope translocation. During the first few minutes of import, two distinct populations of mature-size LHCP accumulate within the chloroplast. One is the aforementioned soluble stromal intermediate, while the other is a partially (or improperly) assembled thylakoid species. Consistent with precursor/product relationships, both species reach peak levels at a time when virtually none of the imported molecules are correctly assembled. These results confirm and extend our previous interpretation, that upon import, preLHCP is rapidly processed to its mature form, giving rise to a soluble stromal intermediate. They further suggest that the stromal intermediate initially inserts into the thylakoid bilayer in a partially assembled form, which eventually becomes properly assembled into the light-harvesting complex.     

Requirement for three membrane-spanning alpha-helices in the post-translational insertion of a thylakoid membrane protein.

The insertion of a protein into a lipid bilayer usually involves a short signal sequence and can occur either during or after translation. A light-harvesting chlorophyll a/b-binding protein (LHCP) is synthesized in the cytoplasm of plant cells as a precursor and is post-translationally imported into chloroplasts where it subsequently inserts into the thylakoid membrane. Only mature LHCP is required for insertion into the thylakoid. To define which sequences of the mature protein are necessary and sufficient for thylakoid integration, fusion and deletion proteins and proteins with internal rearrangements were synthesized and incubated with isolated thylakoids and stroma. No evidence is found for the existence of a short signal sequence within LHCP, and, with the exception of the amino terminus and a short lumenal loop, the entire mature protein with consecutively ordered alpha-helices is required for insertion into thylakoid membranes. The addition of positive charges into stromal but not lumenal segments permits the insertion of mutant LHCPs into isolated thylakoids. Replacement of the LHCP transit peptide with the transit peptide from plastocyanin has no effect on LHCP insertion and does not restore insertion of the lumenal charge addition mutants.     

A hydrophobic, carboxy-proximal region of a light-harvesting chlorophyll a/b protein is necessary for stable integration into thylakoid membranes.

Proteins synthesized as soluble precursors in the cytoplasm of eukaryotic cells often cross organellar membrane barriers and then insert into lipid bilayers. One such polypeptide, the light-harvesting chlorophyll a/b-binding protein (LHCP), must also associate with pigment molecules and be assembled into the photosystem II light-harvesting complex in the chloroplast thylakoid membrane. A study of the import of mutant LHCPs into isolated chloroplasts has shown that a putative alpha-helical membrane-spanning domain near the carboxy terminus (helix 3) is essential for the stable insertion of LHCP in the thylakoid. Protease digestion experiments are consistent with the carboxy terminus of the protein being in the lumen. This report also shows that helix 3, when fused to a soluble protein, can target it to the thylakoids of isolated, intact chloroplasts. Although helix 3 is required for the insertion of LHCP and mutant derivatives into the thylakoid, the full insertion of helix 3 itself requires additionally the presence of other regions of LHCP. Thus, LHCP targeting and integration into thylakoid membranes requires a complex interaction involving a number of different domains of the LHCP polypeptide.     

ATP stimulates signal recognition particle (SRP)/FtsY-supported protein integration in chloroplasts

The signal recognition particle (SRP) and its receptor (FtsY in prokaryotes) are essential for cotranslational protein targeting to the endoplasmic reticulum in eukaryotes and the cytoplasmic membrane in prokaryotes. An SRP/FtsY-like protein targeting/integration pathway in chloroplasts mediates the posttranslational integration of the light-harvesting chlorophyll a/b-binding protein (LHCP) into thylakoid membranes. GTP, chloroplast SRP (cpSRP), and chloroplast FtsY (cpFtsY) are required for LHCP integration into thylakoid membranes. Here, we report the reconstitution of the LHCP integration reaction with purified recombinant proteins and salt-washed thylakoids. Our data demonstrate that cpSRP and cpFtsY are the only soluble protein components required for LHCP integration. In addition, our studies reveal that ATP, though not absolutely required, remarkably stimulates LHCP integration into salt-washed thylakoids. ATP stimulates LHCP integration by a mechanism independent of the thylakoidal pH gradient (DeltapH) and exerts no detectable effect on the formation of the soluble LHCP-cpSRP-targeting complex. Taken together, our results indicate the participation of a thylakoid ATP-binding protein in LHCP integration.     

Chloroplast Oxa1p homolog albino3 is required for post-translational integration of the light harvesting chlorophyll-binding protein into thylakoid membranes

Multiple sorting pathways operate in chloroplasts to localize proteins to the thylakoid membrane. The signal recognition particle (SRP) pathway in chloroplasts employs the function of a signal recognition particle (cpSRP) to target light harvesting chlorophyll-binding protein (LHCP) to the thylakoid membrane. In assays that reconstitute stroma-dependent LHCP integration in vitro, the stroma is replaceable by the addition of GTP, cpSRP, and an SRP receptor homolog, cpFtsY. Still lacking is an understanding of events that take place at the thylakoid membrane including the identification of membrane proteins that may function at the level of cpFtsY binding or LHCP integration. The identification of Oxa1p in mitochondria, an inner membrane translocase component homologous to predicted proteins in bacteria and to the albino3 (ALB3) protein in thylakoids, led us to investigate the potential role of ALB3 in LHCP integration. Antibody raised against a 50-amino acid region of ALB3 (ALB3-50aa) identified a single 45-kDa thylakoid protein. Treatment of thylakoids with antibody to ALB3-50aa inhibited LHCP integration, whereas the same antibody treatment performed in the presence of antigen reversed the inhibition. In contrast, transport by the thylakoid Sec or Delta pH pathways was unaffected. These data support a model whereby a distinct translocase containing ALB3 is used to integrate LHCP into thylakoid membranes.     

Protein-specific energy requirements for protein transport across or into thylakoid membranes. Two lumenal proteins are transported in the absence of ATP.

Cytosolically synthesized thylakoid proteins must be translocated across the chloroplast envelope membranes, traverse the stroma, and then be translocated into or across the thylakoid membrane. Protein transport across the envelope requires ATP hydrolysis but not electrical or proton gradients. The energy requirements for the thylakoid translocation step were studied here for the light-harvesting chlorophyll a/b protein (LHCP), an integral membrane protein, and for several thylakoid lumen-resident proteins: plastocyanin and OE33, OE23, and OE17 (the 33-, 23-, and 17-kDa subunits of the oxygen-evolving complex, respectively). Dissipation of the thylakoid protonmotive force during an in organello protein import assay partially inhibited the thylakoid localization of LHCP and OE33, totally inhibited localization of OE23 and OE17, and had no effect on localization of plastocyanin. We used reconstitution assays for LHCP insertion and for OE23 and OE17 transport into isolated thylakoids to investigate the energy requirements in detail. The results indicated that LHCP insertion absolutely requires ATP hydrolysis and is enhanced by a transthylakoid delta pH and that transport of OE23 and OE17 is absolutely dependent upon a delta pH. Surprisingly, OE23 and OE17 transport occurred maximally in the complete absence of ATP. These results establish the thylakoid membrane as the only membrane system in which a delta pH can provide all of the energy required to translocate proteins across the bilayer. They also demonstrate that the energy requirements for integration into or translocation across the thylakoid membranes are protein-specific.     

Integration of nuclear-encoded proteins into pea thylakoids with different pigment contents

The in vitro membrane integration of the light-harvesting protein of photosystem II (LHCP), the Rieske FeS protein of the cytochrome (Cyt) blf-complex, and the NADPH:protochlorophyllide oxidoreductase (Pchlide reductase) into pea thylakoids with different pigment composition was studied. Pea plants (Pisum sativum L. cv. Kelvedon Wonder) with different contents of chlorophyll (Chl) and carotenoids were obtained by growing the seedlings in a greenhouse or in weak red light with or without the herbicide Norflurazon, an inhibitor of carotenoid biosynthesis. Chloroplasts from untreated and Norflurazon-treated plants grown in weak red light contained approximately 29 and 14% of Chl compared to chloroplasts from untreated plants grown in the greenhouse. The corresponding carotenoid contents were 66 and 5%. Following an integration reaction using LHCP precursor protein and chloroplast lysate, thylakoids from untreated and Norflurazon-treated plants grown in weak red light contained approximately 30 and 5% of protease-protected LHCP, respectively, compared to thylakoids of untreated plants grown in a greenhouse. In contrast to LHCP, the in vitro assembly of the Pchlide reductase was only sligthly reduced in chloroplast lysates of plants grown in weak red light compared to greenhouse-grown plants. In chloroplast lysates of Norflurazon-treated plants, however, the amount of membrane associated, protease-protected Pchlide reductase was reduced to 32% of the amount in untreated plants grown under the same light conditions. In contrast, the integration of the Rieske FeS protein occurred to almost similar levels irrespective of light conditions and herbicide treatments. Reconstitution assays where stroma from Norflurazon-treated plants was added to thylakoids from untreated plants, showed that the herbicide did not affect any stromal component(s) vital for the insertion reaction. Removal of samples during the integration reaction of LHCP showed that no degradation of the protein occurred during the assay. Neither was the assembled protein degraded up to 24 h after the termination of the assay. This indicates that growing plants in weak red light, with or without Norflurazon treatment, mainly affected the primary step in thylakoid assembly of LHCP, i.e. the insertion reaction into the membrane. The results further indicate that proteins normally bound to pigments also require pigments for membrane recognition or integration.     

The chlorophyll a/b-binding protein inserts into the thylakoids independent of its cognate transit peptide

In order to determine if the cognate transit peptide of the light-harvesting chlorophyll a/b-binding protein (LHCP) is essential for LHCP import into the chloroplast and proper localization to the thylakoids, it was replaced with the transit peptide of the small subunit (S) of ribulose-1,5-bisphosphate carboxylase/oxygenase, a stromal protein. Wheat LHCP and S genes were fused to make a chimeric gene coding for the hybrid precursor, which was synthesized in vitro and incubated with purified pea chloroplasts. My results show that LHCP is translocated into chloroplasts by the S transit peptide. The hybrid precursor was processed; and most importantly, mature LHCP did not remain in the stroma, but was inserted into thylakoid membranes, where it normally functions. Density gradient centrifugation showed no LHCP in the envelope fraction. Hence, the transit peptide of LHCP is not required for intraorganellar routing, and LHCP itself contains an internal signal for localization to the correct membrane compartment.     

Regulation of the GTPase cycle in post-translational signal recognition particle-based protein targeting involves cpSRP43

The chloroplast signal recognition particle consists of a conserved 54-kDa GTPase and a novel 43-kDa chromodomain protein (cpSRP43) that together bind light-harvesting chlorophyll a/b-binding protein (LHCP) to form a soluble targeting complex that is subsequently directed to the thylakoid membrane. Homology-based modeling of cpSRP43 indicates the presence of two previously identified chromodomains along with a third N-terminal chromodomain. Chromodomain deletion constructs were used to examine the role of each chromodomain in mediating distinct steps in the LHCP localization mechanism. The C-terminal chromodomain is completely dispensable for LHCP targeting/integration in vitro. The central chromodomain is essential for both targeting complex formation and integration because of its role in binding the M domain of cpSRP54. The N-terminal chromodomain (CD1) is unnecessary for targeting complex formation but is required for integration. This correlates with the ability of CD1 along with the ankyrin repeat region of cpSRP43 to regulate the GTPase cycle of the cpSRP-receptor complex.     

Functional interaction of chloroplast SRP/FtsY with the ALB3 translocase in thylakoids: substrate not required

Integration of thylakoid proteins by the chloroplast signal recognition particle (cpSRP) posttranslational transport pathway requires the cpSRP, an SRP receptor homologue (cpFtsY), and the membrane protein ALB3. Similarly, Escherichia coli uses an SRP and FtsY to cotranslationally target membrane proteins to the SecYEG translocase, which contains an ALB3 homologue, YidC. In neither system are the interactions between soluble and membrane components well understood. We show that complexes containing cpSRP, cpFtsY, and ALB3 can be precipitated using affinity tags on cpSRP or cpFtsY. Stabilization of this complex with GMP-PNP specifically blocks subsequent integration of substrate (light harvesting chl a/b-binding protein [LHCP]), indicating that the complex occupies functional ALB3 translocation sites. Surprisingly, neither substrate nor cpSRP43, a component of cpSRP, was necessary to form a complex with ALB3. Complexes also contained cpSecY, but its removal did not inhibit ALB3 function. Furthermore, antibody bound to ALB3 prevented ALB3 association with cpSRP and cpFtsY and inhibited LHCP integration suggesting that a complex containing cpSRP, cpFtsY, and ALB3 must form for proper LHCP integration.     

Chloroplast YidC homolog Albino3 can functionally complement the bacterial YidC depletion strain and promote membrane insertion of both bacterial and chloroplast thylakoid proteins

A new component of the bacterial translocation machinery, YidC, has been identified that specializes in the integration of membrane proteins. YidC is homologous to the mitochondrial Oxa1p and the chloroplast Alb3, which functions in a novel pathway for the insertion of membrane proteins from the mitochondrial matrix and chloroplast stroma, respectively. We find that Alb3 can functionally complement the Escherichia coli YidC depletion strain and promote the membrane insertion of the M13 procoat and leader peptidase that were previously shown to depend on the bacterial YidC for membrane translocation. In addition, the chloroplast Alb3 that is expressed in bacteria is essential for the insertion of chloroplast cpSecE protein into the bacterial inner membrane. Surprisingly, Alb3 is not required for the insertion of cpSecE into the thylakoid membrane. These results underscore the importance of Oxa1p homologs for membrane protein insertion in bacteria and demonstrate that the requirement for Oxa1p homologs is different in the bacterial and thylakoid membrane systems.     

Evidence for a stromal GTP requirement for the integration of a chlorophyll a/b-binding polypeptide into thylakoid membranes.

The integration of chlorophyll a/b-binding (LHCP) polypeptides and the translocation of the 33-kD oxygen-evolving enhancer protein (OEE33) have been previously shown to occur in chloroplast extracts containing stroma, thylakoids, ATP, and MgCl2. We have re-examined the nucleotide requirement for these two reactions using stromal extract and translation products depleted of low molecular weight compounds. LHCP integration activity was up to 10-fold higher when assayed with GTP compared with ATP, CTP, or UTP. A combination of ATP and GTP supported less LHCP integration activity than GTP alone, suggesting that GTP meets the entire nucleotide requirement. Nonhydrolyzable analogs of GTP were inhibitory, consistent with the idea that GTP hydrolysis is required for integration activity. Periodate-oxidized GTP (GTPox) also inhibited the integration reaction when present during the assay. Pretreatment of stroma with GTPox followed by GTPox removal inhibited integration activity, whereas pretreatment of thylakoids had no effect. We interpret this to mean that a GTP-binding protein involved in integration is localized in the stroma. Translocation of OEE33 was more efficient with ATP than with GTP, and the combination of both nucleotides was not additive. Our data implicate the involvement of a GTPase in LHCP integration but not in the translocation of OEE33.     

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.     

A stromal pool of TatA promotes Tat-dependent protein transport across the thylakoid membrane

In chloroplasts and bacteria, the Tat (twin-arginine translocation) system is engaged in transporting folded passenger proteins across the thylakoid and cytoplasmic membranes, respectively. To date, three membrane proteins (TatA, TatB, and TatC) have been identified to be essential for Tat-dependent protein translocation in the plant system, whereas soluble factors seem not to be required. In contrast, in the bacterial system, several cytosolic chaperones were described to be involved in Tat transport processes. Therefore, we have examined whether stromal or peripherally associated membrane proteins also play a role in Tat transport across the thylakoid membrane. Analyzing both authentic precursors as well as the chimeric 16/23 protein, which allows us to study each step of the translocation process individually, we demonstrate that a soluble form of TatA is present in the chloroplast stroma, which significantly improves the efficiency of Tat-dependent protein transport. Furthermore, this soluble TatA is able to reconstitute the Tat transport properties of thylakoid membranes that are transport-incompetent due to extraction with solutions of chaotropic salts.     

Chloroplast Chaperonin-Mediated Targeting of a Thylakoid Membrane Protein

Posttranslational protein targeting requires chaperone assistance to direct insertion-competent proteins to integration pathways. Chloroplasts integrate nearly all thylakoid transmembrane proteins posttranslationally, but mechanisms in the stroma that assist their insertion remain largely undefined. Here, we investigated how the chloroplast chaperonin (Cpn60) facilitated the thylakoid integration of Plastidic type I signal peptidase 1 (Plsp1) using in vitro targeting assays. Cpn60 bound Plsp1 in the stroma. In isolated chloroplasts, the membrane integration of imported Plsp1 correlated with its dissociation from Cpn60. When the Plsp1 residues that interacted with Cpn60 were removed, Plsp1 did not integrate into the membrane. These results suggested Cpn60 was an intermediate in thylakoid targeting of Plsp1. In isolated thylakoids, the integration of Plsp1 decreased when Cpn60 was present in excess of cpSecA1, the stromal motor of the cpSec1 translocon that inserts unfolded Plsp1 into the thylakoid. An excess of cpSecA1 favored integration. Introducing Cpn60’s obligate substrate RbcL displaced Cpn60-bound Plsp1; then, the released Plsp1 exhibited increased accessibility to cpSec1. These in vitro targeting experiments support a model in which Cpn60 captures and then releases insertion-competent Plsp1, whereas cpSecA1 recognizes free Plsp1 for integration. Thylakoid transmembrane proteins in the stroma can interact with Cpn60 to shield themselves from the aqueous environment.     

The identification of a heat-shock protein complex in chloroplasts of barley leaves

In vivo radiolabeling of chloroplast proteins in barley (Hordeum vulgare L. cv Corvette) leaves and their separation by one-dimensional electrophoresis revealed at least seven heat-shock proteins between 24 and 94 kD, of which most have not been previously identified in this C₃ species. Fractionation into stromal and thylakoid membrane components showed that all chloroplast heat-shock proteins were synthesized on cytoplasmic ribosomes, translocated into the chloroplast, and located in the stroma. Examination of stromal preparations by native (nondissociating) polyacrylamide gel electrophoresis revealed the presence of a high-molecular mass heat-shock protein complex in barley. This complex was estimated to be 250 to 265 kD in size. Dissociation by denaturing polyacrylamide gel electrophoresis revealed a single protein component, a 32-kD heat-shock protein. The synthesis of this protein and the formation of the heat-shock protein complex were dependent on functional cytoplasmic ribosomes. Immunological studies showed that the heat-shock protein complex did not contain any proteins homologous to the α-subunit of ribulose bisphosphate carboxylase oxygenase subunit-binding protein. Other features about the complex included the absence of nucleic acid (RNA or DNA) and its nondissociation in the presence of Mg²⁺/ATP. These results suggest that the heat-shock protein complex in barley chloroplasts is a homogeneous octamer of 32-kD subunits.     

Functional and mutational analysis of the light-harvesting chlorophyll a/b protein of thylakoid membranes

The precursor for a Lemna light-harvesting chlorophyll a/b protein (pLHCP) has been synthesized in vitro from a single member of the nuclear LHCP multigene family. We report the sequence of this gene. When incubated with Lemna chloroplasts, the pLHCP is imported and processed into several polypeptides, and the mature form is assembled into the light-harvesting complex of photosystem II (LHC II). The accumulation of the processed LHCP is enhanced by the addition to the chloroplasts of a precursor and a co-factor for chlorophyll biosynthesis. Using a model for the arrangement of the mature polypeptide in the thylakoid membrane as a guide, we have created mutations that lie within the mature coding region. We have studied the processing, the integration into thylakoid membranes, and the assembly into light-harvesting complexes of six of these deletions. Four different mutant LHCPs are found as processed proteins in the thylakoid membrane, but only one appears to have an orientation in the membrane that is similar to that of the wild type. No mutant LHCP appears in LHC II. The other two mutant LHCPs cannot be detected within the chloroplasts. We conclude that stable complex formation is not required for the processing and insertion of altered LHCPs into the thylakoid membrane. We discuss the results in light of our model.     

Analysis of the Import of Carboxyl-Terminal Truncations of the 23-Kilodalton Subunit of the Oxygen-Evolving Complex Suggests That Its Structure Is an Important Determinant for Thylakoid Transport

A series of deletions from the carboxyl terminus of the 23-kD subunit of the photosynthetic oxygen-evolving complex OE23 revealed that these truncations result in various degrees of inhibition of translocation across thylakoid membranes and their subsequent assembly to the oxygen-evolving complex. Import of in vitro translated precursors across the chloroplast envelopes was not inhibited by these truncations. Time-course studies of the import of truncated OE23 precursors into intact chloroplasts revealed that the stromal intermediate was subsequently translocated into the thylakoid lumen, where it was processed to a smaller size and rapidly degraded. In contrast to the full-length OE23 intermediate, the truncated intermediate forms that accumulated in the stroma as a result of de-energization of thylakoid membranes could be found associated with the membrane rather than free in the stroma. Protease digestion experiments revealed that the deletions evidently altered the folded conformation of the protein. These results suggest that the carboxyl-terminal portion of the OE23 precursor is important for the maintenance of an optimal structure for import into thylakoids, implying that the efficient translocation of OE23 requires the protein to be correctly folded. In addition, the rapid degradation of the truncated forms of the processed OE23 within the lumen indicates that a protease (or proteases) active in the lumen can recognize and remove misfolded polypeptides.