The 24 kDa outer envelope membrane protein from spinach chloroplasts: molecular cloning, in vivo expression and import pathway of a protein with unusual properties

Pepper mild mottle virus coat protein interacts with pepper chloroplast outer envelope membrane protein OMP24 to inhibit antiviral immunity in plants

Pepper mild mottle virus (PMMoV) is a devastating viral pathogen of pepper (Capsicum annuum) but it is unclear whether and how peppers protect against PMMoV infection. The expression of the chloroplast outer membrane protein 24 (OMP24) of C. annuum was upregulated under PMMoV infection and it interacted with PMMoV coat protein (CP). Silencing of OMP24 in either C. annuum or Nicotiana benthamiana facilitated PMMoV infection, whereas overexpression of N. benthamiana OMP24 in transgenic plants inhibited PMMoV infection. Both C. annuum OMP24 (CaOMP24) and N. benthamiana OMP24 (NbOMP24) localized to the chloroplast and have a moderately hydrophobic transmembrane domain that is necessary for their localization. Overexpression of CaOMP24 induced stromules, perinuclear chloroplast clustering, and accumulation of reactive oxygen species (ROS), the typical defense responses of chloroplasts transferring the retrograde signaling to the nucleus to regulate resistance genes. The expression of PR1 and PR2 was also upregulated significantly in plants overexpressing OMP24. Self-interaction of OMP24 was demonstrated and was required for OMP24-mediated plant defense. Interaction with PMMoV CP interfered with the self-interaction of OMP24 and impaired OMP24-induced stromules, perinuclear chloroplast clustering and ROS accumulation. The results demonstrate the defense function of OMP24 in pepper during viral infection and suggest a possible mechanism by which PMMoV CP modulates the plant defense to facilitate viral infection.

An RK/ST C-Terminal Motif is Required for Targeting of OEP7.2 and a Subset of Other Arabidopsis Tail-Anchored Proteins to the Plastid Outer Envelope Membrane

Tail-anchored (TA) proteins are a unique class of integral membrane proteins that possess a single C-terminal transmembrane domain and target post-translationally to the specific organelles at which they function. While significant advances have been made in recent years in elucidating the mechanisms and molecular targeting signals involved in the proper sorting of TA proteins, particularly to the endoplasmic reticulum and mitochondria, relatively little is known about the targeting of TA proteins to the plastid outer envelope. Here we show that several known or predicted plastid TA outer envelope proteins (OEPs) in Arabidopsis possess a C-terminal RK/ST sequence motif that serves as a conserved element of their plastid targeting signal. Evidence for this conclusion comes primarily from experiments with OEP7.2, which is a member of the Arabidopsis 7 kDa OEP family. We confirmed that OEP7.2 is localized to the plastid outer envelope and possesses a TA topology, and its C-terminal sequence (CTS), which includes the RK/ST motif, is essential for proper targeting to plastids. The CTS of OEP7.2 is functionally interchangeable with the CTSs of other TA OEPs that possess similar RK/ST motifs, but not with those that lack the motif. Further, a bioinformatics search based on a consensus sequence led to the identification of several new OEP TA proteins. Collectively, this study provides new insight into the mechanisms of TA protein sorting in plant cells, defines a new targeting signal element for a subset of TA OEPs and expands the number and repertoire of TA proteins at the plastid outer envelope.

The C-terminus of Bienertia sinuspersici Toc159 contains essential elements for its targeting and anchorage to the chloroplast outer membrane

Most nucleus-encoded chloroplast proteins rely on an N-terminal transit peptide (TP) as a post-translational sorting signal for directing them to the organelle. Although Toc159 is known to be a receptor for specific preprotein TPs at the chloroplast surface, the mechanism for its own targeting and integration into the chloroplast outer membrane is not completely understood. In a previous study, we identified a novel TP-like sorting signal at the C-terminus (CT) of a Toc159 homolog from the single-cell C4 species, Bienertia sinuspersici. In the current study, we have extended our understanding of the sorting signal using transient expression of fluorescently-tagged fusion proteins of variable-length, and with truncated and swapped versions of the CT. As was shown in the earlier study, the 56 residues of the CT contain crucial sorting information for reversible interaction of the receptor with the chloroplast envelope. Extension of this region to 100 residues in the current study stabilized the interaction via membrane integration, as demonstrated by more prominent plastid-associated signals and resistance of the fusion protein to alkaline extraction. Despite a high degree of sequence similarity, the plastid localization signals of the equivalent CT regions of Arabidopsis thaliana Toc159 homologs were not as strong as that of the B. sinuspersici counterparts. Together with computational and circular dichroism analyses of the CT domain structures, our data provide insights into the critical elements of the CT for the efficient targeting and anchorage of Toc159 receptors to the dimorphic chloroplasts in the single-cell C4 species.

The Plastid Outer Envelope - A Highly Dynamic Interface between Plastid and Cytoplasm

Plastids are the defining organelles of all photosynthetic eukaryotes. They are the site of photosynthesis and of a large number of other essential metabolic pathways, such as fatty acid and amino acid biosyntheses, sulfur and nitrogen assimilation, and aromatic and terpenoid compound production, to mention only a few examples. The metabolism of plastids is heavily intertwined and connected with that of the surrounding cytosol, thus causing massive traffic of metabolic precursors, intermediates, and products. Two layers of biological membranes that are called the inner (IE) and the outer (OE) plastid envelope membranes bound the plastids of Archaeplastida. While the IE is generally accepted as the osmo-regulatory barrier between cytosol and stroma, the OE was considered to represent an unspecific molecular sieve, permeable for molecules of up to 10 kDa. However, after the discovery of small substrate specific pores in the OE, this view has come under scrutiny. In addition to controlling metabolic fluxes between plastid and cytosol, the OE is also crucial for protein import into the chloroplast. It contains the receptors and translocation channel of the TOC complex that is required for the canonical post-translational import of nuclear-encoded, plastid-targeted proteins. Further, the OE is a metabolically active compartment of the chloroplast, being involved in, e.g., fatty acid metabolism and membrane lipid production. Also, recent findings hint on the OE as a defense platform against several biotic and abiotic stress conditions, such as cold acclimation, freezing tolerance, and phosphate deprivation. Moreover, dynamic non-covalent interactions between the OE and the endomembrane system are thought to play important roles in lipid and non-canonical protein trafficking between plastid and endoplasmic reticulum. While proteomics and bioinformatics has provided us with comprehensive but still incomplete information on proteins localized in the plastid IE, the stroma, and the thylakoids, our knowledge of the protein composition of the plastid OE is far from complete. In this article, we report on the recent progress in discovering novel OE proteins to draw a conclusive picture of the OE. A "parts list" of the plastid OE will be presented, using data generated by proteomics of plastids isolated from various plant sources.

Development-specific association of amyloplasts with microtubules in scale cells of Narcissus tazetta

Narcissus tazetta is one of the major geophyte crops worldwide, but little is known about its cell biology. The narcissus storage organ was studied by monitoring scale cell biology during the growth stage and dormancy, and it was found that amyloplasts gradually increased in size and reached a maximum at dormancy. In parallel, microtubules changed their organisation: during the growth phase (February to March) they were oblique; during April and May, microtubules formed a network with round "holes"; by late June and the beginning of July, when dormancy started, they were organised in parallel arrays. The holes formed in the microtubule array corresponded to amyloplasts. A closer look showed that during a short time window, while the plants were preparing for dormancy, the microtubules surrounded the amyloplasts. In vitro reconfirmation of this phenomenon was obtained when fluorescent bovine brain microtubules enwrapped isolated amyloplasts that had been purified between April and July but not those purified between January and March. Interestingly, protease treatment of amyloplasts did not completely prevent binding of microtubules, which suggests the existence of a protease-resistant factor that docks microtubules to the outer membrane of amyloplasts.

Mutation of the TGD1 chloroplast envelope protein affects phosphatidate metabolism in Arabidopsis

Phosphatidate (PA) is a central metabolite of lipid metabolism and a signaling molecule in many eukaryotes, including plants. Mutations in a permease-like protein, TRIGALACTOSYLDIACYLGLYCEROL1 (TGD1), in Arabidopsis thaliana caused the accumulation of triacylglycerols, oligogalactolipids, and PA. Chloroplast lipids were altered in their fatty acid composition consistent with an impairment of lipid trafficking from the endoplasmic reticulum (ER) to the chloroplast and a disruption of thylakoid lipid biosynthesis from ER-derived precursors. The process mediated by TGD1 appears to be essential as mutation of the protein caused a high incidence of embryo abortion. Isolated tgd1 mutant chloroplasts showed a decreased ability to incorporate PA into galactolipids. The TGD1 protein was localized to the inner chloroplast envelope and appears to be a component of a lipid transporter. As even partial disruption of TGD1 function has drastic consequences on central lipid metabolism, the tgd1 mutant provides a tool to explore regulatory mechanisms governing lipid homeostasis and lipid trafficking in plants.

Chloroplast outer membrane protein targeting and insertion

Proteins in the chloroplast outer envelope membrane are nuclear encoded and post-translationally targeted to the chloroplast. The targeting and membrane insertion of these proteins is not well understood. Although early work suggested otherwise, the best-studied outer membrane proteins (OMPs) use both proteins within the chloroplast and NTPs for insertion. There have been conflicting reports in the field regarding protein targeting and insertion, which have probably arisen because of differences in experimental methodology and different interpretations of reduction (versus abolition) of integration. This review summarizes what is known to date about the mechanism of chloroplast OMP targeting.

Protein transport into secondary plastids and the evolution of primary and secondary plastids

Chloroplasts are key organelles in algae and plants due to their photosynthetic abilities. They are thought to have evolved from prokaryotic cyanobacteria taken up by a eukaryotic host cell in a process termed primary endocytobiosis. In addition, a variety of organisms have evolved by subsequent secondary endocytobioses, in which a heterotrophic host cell engulfed a eukaryotic alga. Both processes dramatically enhanced the complexity of the resulting cells. Since the first version of the endosymbiotic theory was proposed more than 100 years ago, morphological, physiological, biochemical, and molecular data have been collected substantiating the emerging picture about the origin and the relationship of individual organisms with different primary or secondary chloroplast types. Depending on their origin, plastids in different lineages may have two, three, or four envelope membranes. The evolutionary success of endocytobioses depends, among other factors, on the specific exchange of molecules between the host and endosymbiont. This raises questions concerning how targeting of nucleus-encoded proteins into the different plastid types occurs and how these processes may have developed. Most studies of protein translocation into plastids have been performed on primary plastids, but in recent years more complex protein-translocation systems of secondary plastids have been investigated. Analyses of transport systems in different algal lineages with secondary plastids reveal that during evolution existing translocation machineries were recycled or recombined rather than being developed de novo. This review deals with current knowledge about the evolution and function of primary and secondary plastids and the respective protein-targeting systems.

Chloroplast quest: a journey from the cytosol into the chloroplast and beyond

Chloroplasts are characteristic organelles of plants and algae and the site of oxygenic photosynthesis. They are surrounded by a double membrane and possess an internal membrane system, the thylakoids, on which the photosynthetic machinery is located. They originated more than 1.2 billion years ago from an endosymbiotic event between an already photosynthetic ancestor of present day cyanobacteria and a mitochondriate host cell. During the transformation of the internalized cyanobacterium into a cell organelle most of the genetic information of the endosymbiot got lost or was transferred into the nucleus of the host. Chloroplast proteins encoded by nuclear genes are synthesized on cytoplasmic ribosomes and have to be relocated into the organelle. This is achieved by a proteinaceous import machinery in the outer and inner envelope of the chloroplasts. Proteins destined for the thylakoid membrane and the thylakoid lumen are further translocated by several different pathways into or across this membrane. The subject of this review is the quest of nuclear encoded chloroplast proteins into the organelle and to their final suborganellar location.

Without a little help from 'my' friends: direct insertion of proteins into chloroplast membranes?

The chloroplast membranes are highly regulated and biological active regions of the living plant cell, which carry numerous essential proteinaceous components. For example, in the thylakoid membrane the photosynthesis apparatus, one of the most life-relevant biological machineries, is located. How these membrane proteins are targeted to and inserted into their target membranes was one of the questions we aimed to understand in the last few years. Fifteen years ago little to nothing was known about the targeting and translocation of outer envelope proteins (G.W. Schmidt and L.M. Mishkind, Annu. Rev. Biochem. 55 (1986)). Although several protein assisted pathways for translocation of proteins across the membranes have been characterised, only recent results gave insight into how membrane proteins are inserted into the chloroplast membranes. Here we will focus on the mode of insertion of a class of proteins into the outer envelope and the thylakoid membranes, which share a unique feature: they insert apparently directly into the lipid bilayer, i.e. without the help of a proteinaceous translocation pore.

Protein import: the hitchhikers guide into chloroplasts

Plastids originated from an endosymbiotic event between an early eukaryotic host cell and an ancestor of today's cyanobacteria. During the events by which the engulfed endosymbiont was transformed into a permanent organelle, many genes were transferred from the plastidal genome to the nucleus of the host cell. Proteins encoded by these genes are synthesised in the cytosol and subsequently translocated into the plastid. Therefore they contain an N-terminal cleavable transit sequence that is necessary for translocation. The sequence is plastid-specific, thus preventing mistargeting into other organelles. Receptors embedded into the outer envelope of the plastid recognise the transit sequences, and precursor proteins are translocated into the chloroplast by a proteinaceous import machinery located in both the outer and inner envelopes. Inside the stroma the transit sequences are cleaved off and the proteins are further routed to their final locations within the plastid.

Spinach hexokinase I is located in the outer envelope membrane of plastids

The subcellular localization of hexokinase activities in plant cells has been a matter of debate for a long time. We have isolated a hexokinase cDNA fragment from glucose-fed spinach leaves using a differential display reverse transcription-PCR approach. The corresponding cDNA was expressed in Escherichia coli and an antiserum, raised against the recombinant protein, was used in subcellular localization studies. The spinach hexokinase could be localized primarily to the outer envelope membrane of chloroplasts where it is inserted via its N-terminal membrane anchor. We suggest that the chloroplast envelope hexokinase is involved in the energization of glucose export from plastids rather than in the sugar-sensing pathway of the plant cell.

Tic22 is targeted to the intermembrane space of chloroplasts by a novel pathway

Tic22 previously was identified as a component of the general import machinery that functions in the import of nuclear-encoded proteins into the chloroplast. Tic22 is peripherally associated with the outer face of the inner chloroplast envelope membrane, making it the first known resident of the intermembrane space of the envelope. We have investigated the import of Tic22 into isolated chloroplasts to define the requirements for targeting of proteins to the intermembrane space. Tic22 is nuclear-endoded and synthesized as a preprotein with a 50-amino acid N-terminal presequence. The analysis of deletion mutants and chimerical proteins indicates that the precursor of Tic22 (preTic22) presequence is necessary and sufficient for targeting to the intermembrane space. Import of preTic22 was stimulated by ATP and required the presence of protease-sensitive components on the chloroplast surface. PreTic22 import was not competed by an excess of an authentic stromal preprotein, indicating that targeting to the intermembrane space does not involve the general import pathway utilized by stromal preproteins. On the basis of these observations, we conclude that preTic22 is targeted to the intermembrane space of chloroplasts by a novel import pathway that is distinct from known pathways that target proteins to other chloroplast subcompartments.

Insertion of atToc34 into the chloroplastic outer membrane is assisted by at least two proteinaceous components in the import system

Toc34 is a member of the outer membrane translocon complex that mediates the initial stage of protein import into chloroplasts. Toc34, like most outer membrane proteins, is synthesized in the cytosol at its mature size without a cleavable transit peptide. The majority of outer membrane proteins do not require thermolysin-sensitive components on the chloroplastic surface or ATP for their insertion into the outer membrane. However, different results have been obtained concerning the factors required for Toc34 insertion into the outer membrane. Using an Arabidopsis homologue of pea Toc34, atToc34, we show that the insertion of atToc34 was greatly reduced by thermolysin pretreatment of chloroplasts as assayed either by protease digestion or by alkaline extraction. The insertion was also dependent on the presence of ATP or GTP. A mutant of atToc34 with the GTP-binding domain deleted still required ATP for optimal insertion, indicating that ATP was used by other protein components in the import system. The ATP-supported insertion was observed even in thermolysin-pretreated chloroplasts, suggesting that the protein component responsible for ATP-stimulated insertion is a different protein from the thermolysin-sensitive component that assists atToc34 insertion.

Positive charges determine the topology and functionality of the transmembrane domain in the chloroplastic outer envelope protein Toc34

The chloroplastic outer envelope protein Toc34 is inserted into the membrane by a COOH-terminal membrane anchor domain in the orientation Ncyto-Cin. The insertion is independent of ATP and a cleavable transit sequence. The cytosolic domain of Toc34 does not influence the insertion process and can be replaced by a different hydrophilic reporter peptide. Inversion of the COOH-terminal, 45-residue segment, including the membrane anchor domain (Toc34Cinv), resulted in an inverted topology of the protein, i.e., Nin-Ccyto. A mutual exchange of the charged amino acid residues NH2- and COOH-proximal of the hydrophobic alpha-helix indicates that a double-positive charge at the cytosolic side of the transmembrane alpha-helix is the sole determinant for its topology. When the inverted COOH-terminal segment was fused to the chloroplastic precursor of the ribulose-1,5-bisphosphate carboxylase small subunit (pS34Cinv), it engaged the transit sequence-dependent import pathway. The inverted peptide domain of Toc34 functions as a stop transfer signal and is released out of the outer envelope protein translocation machinery into the lipid phase. Simultaneously, the NH2-terminal part of the hybrid precursor remained engaged in the inner envelope protein translocon, which could be reversed by the removal of ATP, demonstrating that only an energy-dependent force but no further ionic interactions kept the precursor in the import machinery.

Insertion of the 34-kDa chloroplast protein import component, IAP34, into the chloroplast outer membrane is dependent on its intrinsic GTP-binding capacity

IAP34 is a 34-kDa component of the outer membrane complex that mediates the initial stages of protein import into chloroplasts (Seedorf, M., Waegemann, K., and Soll, J. (1995) Plant J. 7, 401-411; Kessler, F., Blobel, G., Patel, H. A., and Schnell, D. J. (1994) Science 266, 1035-1039). We have investigated the targeting and insertion of IAP34 at the outer envelope membrane. The analyses of IAP34 deletion mutants and hybrid proteins (consisting of regions of IAP34 fused to the soluble IgG-binding domain of staphylococcal protein A) suggest that the transmembrane domain and C-terminal tail of IAP34 contain information essential but not sufficient for targeting to the outer membrane. Treatment of chloroplasts with exogenous proteases does not affect IAP34 insertion, indicating that targeting does not require surface-exposed receptors at the envelope. GTP or GDP is required for maximal integration of IAP34 into the outer membrane. The GTP/GDP requirement is attributed to the intrinsic GTP binding activity of IAP34 because GTP/GDP binding-deficient mutants are defective in outer membrane insertion. On the basis of these observations, we propose that IAP34 is targeted to the chloroplast by a C-terminal signal and efficiently integrated into the outer membrane by conformation-induced insertion upon GTP/GDP binding.

Import and routing of nucleus-encoded chloroplast proteins

Most chloroplast proteins are nuclear encoded, synthesized as larger precursor proteins in the cytosol, posttranslationally imported into the organelle, and routed to one of six different compartments. Import across the outer and inner envelope membranes into the stroma is the major means for entry of proteins destined for the stroma, the thylakoid membrane, and the thylakoid lumen. Recent investigations have identified several unique protein components of the envelope translocation machinery. These include two GTP-binding proteins that appear to participate in the early events of import and probably regulate precursor recognition and advancement into the translocon. Localization of imported precursor proteins to the thylakoid membrane and thylakoid lumen is accomplished by four distinct mechanisms; two are homologous to bacterial and endoplasmic reticulum protein transport systems, one appears unique, and the last may be a spontaneous mechanism. Thus chloroplast protein targeting is a unique and surprisingly complex process. The presence of GTP-binding proteins in the envelope translocation machinery indicates a different precursor recognition process than is present in mitochondria. Mechanisms for thylakoid protein localization are in part derived from the prokaryotic endosymbiont, but are more unusual and diverse than expected.

A protein import receptor of chloroplasts is inserted into the outer envelope membrane by a novel pathway

The outer envelope protein OEP86 functions as a receptor for precursor proteins in the chloroplastic import machinery. In contrast to most other organellar outer membrane proteins it is synthesized as a precursor polypeptide (preOEP86) in the cytosol and is post-translationally targeted to the organelles. PreOEP86 is targeted to and productively inserted into the chloroplastic outer envelope mediated by a bipartite signal consisting of the presequence and the COOH terminus of the precursor protein. The cleavable presequence alone does not seem to contain sufficient information to target preOEP86 without the COOH terminus or a hybrid protein consisting of the presequence of preOEP86 and the mature form of the small subunit of ribulose bisphosphate carboxylase to intact chloroplasts. The presequence seems to be required to maintain preOEP86 in an integration competent state, whereas interaction of preOEP86 with chloroplasts is accomplished by a short sequence of amino acids in the COOH-terminal portion of the mature protein. The COOH-terminal portion of preOEP86 contains enough information to also direct mature OEP86 into the outer envelope membrane of pea chloroplasts. However, mature OEP86 enters the productive folding pathway much less efficiently than preOEP86. The COOH terminus of preOEP86 not only serves as a membrane anchor but seems to be required for a productive translocation through an interaction with other outer envelope proteins. Although the binding was ATP-dependent, productive folding was not. PreOEP86 seems to follow a unique road into the chloroplastic outer envelope.

Isolation and characterization of a cDNA clone encoding a member of the Com44/Cim44 envelope components of the chloroplast protein import apparatus

Many of the proteins in the chloroplast envelope play an important role in facilitating the biochemical and transport processes of the compartment. For the transport of proteins into the chloroplast, we have recently identified at least three different envelope proteins (Com44/Cim44, Com70, and Cim97) in close physical proximity to a partially translocated chimeric precursor protein (Wu, C., Seibert, F. S., and Ko, K. (1994) J. Biol. Chem. 269, 32264-32271). In this study we report the characterization of a cDNA clone encoding a member of the Com44/Cim44 envelope proteins. The combined data from nucleotide sequencing, and RNA and protein blot analyses indicate the existence of multiple forms of the 44-kDa envelope protein. Depending on the plant species examined, immunologically-related protein bands with molecular masses of 42 to 46 kDa were observed. Organelle subfractionation, protease treatment, and immunomicroscopic studies together provide an indication that the immunologically-related proteins may be present in both the outer and inner envelope membranes. Co-migration of the product synthesized from the cDNA insert with a 44-kDa immunoreactive band of the chloroplast envelope, and the in vitro import results, together suggest that the in vitro synthesized 44-kDa protein is targeted to the envelope membrane without any further processing.

The membrane of leaf peroxisomes contains a porin-like channel

Spinach leaf peroxisomes were purified by Percoll density gradient centrifugation. After several freeze-thaw cycles, the peroxisomal membranes were separated from the matrix enzymes by sucrose density gradient centrifugation. The purity of the peroxisomal membranes was checked by measuring the activities of marker enzymes and by using antibodies. Lipid bilayer membrane experiments with the purified peroxisomal membranes, solubilized with a detergent, demonstrated that the membranes contain a channel-forming component, which may represent the major permeability pathway of these membranes. Control experiments with membranes of other cell organelles showed that the peroxisomal channel was not caused by the contamination of the peroxisomes with mitochondria or chloroplasts. The peroxisomal channel had a comparatively small single channel conductance of 350 pS in 1 M KCl as compared with channels from other cell organelles. The channel is slightly anion selective, which is in accordance with its physiological function. The single channel conductance was found to be only moderately dependent on the salt concentration in the aqueous phase. This may be explained by the presence of positive point net charges in or near the channel or by the presence of a saturable binding site inside the channel. The possible role of the channel in peroxisomal metabolism is discussed.

Import characteristics of a leucoplast pyruvate kinase are influenced by a 19-amino-acid domain within the protein

Two cDNA clones encoding distinct forms of plastid pyruvate kinase (designated Pka and Pkg) have recently been characterized. Pkg is found in both leucoplasts and chloroplasts, whereas Pka is present only in leucoplasts. The precursors of these proteins have different in vitro import characteristics. The Pkg precursor behaves like a typical stromal protein precusor with both types of plastid. In contrast, Pka precursors accumulate on the outer envelope membrane of leucoplasts under the same assay conditions and require a higher level of ATP for import into the organelle. Interestingly, the binding of Pka precursors to chloroplasts cannot be detected at any tested level of ATP even though the precursors are imported into the organelle at higher concentrations of ATP. Various N-terminal deletions and chimeric fusions were used to examine the translocation signaling mechanism of the Pka precursor. The N-terminal 83-amino-acid segment of Pka contains a transit peptide that is capable of directing dihydrofolate reductase and the mature body of Pkg into both types of plastid. Unlike the complete Pka precursor, these fusion proteins behave like typical stromal protein precursors. The behavior of the Pka transit peptide is influenced by a 19-amino-acid domain (-P-S-S-I-E-V-D-A-V-T-E-T-E-L-K-E-N-G-F-) located immediately downstream of the N-terminal 83-residue segment. Deletion of this domain from Pka alters its import properties such that it resembles a typical stromal protein precursor. Re-introduction of the 19-residue domain into the Dhfr fusion protein alters its import characteristics to resemble that of the complete Pka precursor. This 19-amino-acid domain can also influence the function of transit sequences from other precursors when it is placed immediately behind the transit peptide. These results suggest that this 19-amino-acid domain plays an important role in governing the import characteristics of the Pka precursor. We have named this 19-residue segment the "import modifying domain."