Transit Peptides Play a Major Role in the Preferential Import of Proteins into Leucoplasts and Chloroplasts

Abstract

The in vitro import characteristics of six different precursors of plastid proteins were assessed to determine differences in the protein import pathways of leucoplasts and chloroplasts. Five of these precursor proteins are destined to different subchloroplast sites, and one is a leucoplast stromal precursor protein. The results indicate that some of these precursors can be imported equally into both plastid types and others preferentially into one type of plastid versus the other. The ability of plastids to import different proteins correlates with the in vivo steady state levels of these proteins. Additional differences were also observed in the intraorganellar portion of the translocation pathway for two thylakoidal proteins. The differences in import characteristics were found to be predominantly governed by information in the transit peptides, since attachment of the various transit peptides to different plastid and foreign proteins demonstrated that the import behavior of the proteins is transferable with the transit sequence. These results indicate that the import mechanisms of leucoplasts and chloroplasts are sufficiently different such that the plastids respond differently to the information present in the transit peptides.The NH 2 -terminal transit peptide extension of nuclear-encoded precursors of plastid proteins is considered the primary signal for directing posttranslational import of the precursor (see reviews by Keegstra, 1989;Keegstra et al., 1989). The transit peptide generally possesses sufficient information for the correct targeting of proteins to the plastid compartment and for subsequent intraorganellar sorting. Targeting of stromal-destined precursor proteins appears to be directed by information in a contiguous segment of NH 2 -terminal residues (Smeekens et al., 1986Schmidt and Mishkind, 1986;Van den Broeck et al., 1985), whereas thylakoid lumen proteins, e.g. plastocyanin and Oee1, 1 are directed by bipartite transit peptides (Smeekens et al., 1986Weisbeek et al., 1987;de Boer et al., 1988;Ko and Cashmore, 1989). There are also examples of proteins such as Cab and Ferrodoxin binding subunit of photosystem I that possess suborganellar targeting information within the mature region of the protein rather than within the transit peptide (Cline, 1988;Lamppa, 1988;Van den Broeck et al., 1988;Viitanen et al., 1988;Hand et al., 1989; also see review by Theg and Scott, 1993). In these cases, the NH 2 -terminal transit peptide acts only to import the protein into the stromal compartment.The targeting of proteins into plastids other than chloroplasts and the additional information accompanying this process are even more complex when it is considered that plastids can vary enormously in function. Most of the evidence to date indicates that targeting information for other types of plastids can be transposed from chloroplastic precursor proteins without apparent effect. For instance, the chloroplast Rbcs precursor proteins can be imported into leucoplasts (Boyle et al., 1986; Halpin et al., 1989), ...

L-Amino Acid Oxidases with Specificity for Basic L-Amino Acids in Cyanobacteria

“…This protein is intracellularly located and is mainly attached to the thylakoid membrane. As suggested in the literature (Wan et al, 1996;Kouranov and Schnell, 1996), this implies that the passenger protein behind such a signal peptide also has an influence on the localisation of the protein. Since the l-Aox in Synechococcus elongatus PCC 6301 (and also in Synechococcus elongatus PCC 7942) has been shown to be in part located in the soluble protein fraction of the periplasm, the presence of a putative translocation pathway signal is in agreement with this location.…”

Members of the Toc159 Import Receptor Family Represent Distinct Pathways for Protein Targeting to Plastids

“…The possibility that different preproteins might engage distinct import components was first raised by the observation that preproteins were differentially imported into chloroplasts and leucoplasts (Wan et al, 1996). This led Jarvis et al (1998) to propose that atToc33 and atToc34 might represent distinct targeting pathways for plastid preproteins.…”

Red Bell Pepper Chromoplasts Exhibit in Vitro Import Competency and Membrane Targeting of Passenger Proteins from the Thylakoidal Sec and ΔpH Pathways but Not the Chloroplast Signal Recognition Particle Pathway1

“…These data are predominantly based on the observation that any plastid type can import and localize to the stroma precursors destined for any other plastid type. In vitro studies demonstrated that isolated castor bean leukoplasts and sycamore amyloplasts can import a number of chloroplastic precursors, although there may be precursor-specific variations in efficiency (Strzalka et al, 1987;Halpin et al, 1989;Wan et al, 1996). Conversely, tomato chromoplast-targeted and corn amyloplasttargeted proteins are imported into isolated pea chloroplasts (Klosgen et al, 1989;Lawrence et al, 1993).…”

“…Most nonphotosynthetic plastids are the result of tissue-specific developmental cues. In the one normally nongreen plastid type in which protein targeting to internal membrane compartments has been addressed, castor bean leukoplasts, very little subplastid localization of proteins (thylakoid Sec pathway substrates OE33 and plastocyanin) was detected (Halpin et al, 1989;Wan et al, 1996). Precursors that utilize the other known targeting pathways were not tested.…”