title: Functional Analysis of Semi-conserved Transit Peptide Motifs and Mechanistic Implications in Precursor Targeting and Recognition.
abstract: Over 95% of plastid proteins are nuclear-encoded as their precursors containing an N-terminal extension known as the transit peptide (TP). Although highly variable, TPs direct the precursors through a conserved, posttranslational mechanism involving translocons in the outer (TOC) and inner envelope (TOC). The organelle import specificity is mediated by one or more components of the Toc complex. However, the high TP diversity creates a paradox on how the sequences can be specifically recognized. An emerging model of TP design is that they contain multiple loosely conserved motifs that are recognized at different steps in the targeting and transport process. Bioinformatics has demonstrated that many TPs contain semi-conserved physicochemical motifs, termed FGLK. In order to characterize FGLK motifs in TP recognition and import, we have analyzed two well-studied TPs from the precursor of RuBisCO small subunit (SStp) and ferredoxin (Fdtp). Both SStp and Fdtp contain two FGLK motifs. Analysis of large set mutations (∼85) in these two motifs using in vitro, in organello, and in vivo approaches support a model in which the FGLK domains mediate interaction with TOC34 and possibly other TOC components. In vivo import analysis suggests that multiple FGLK motifs are functionally redundant. Furthermore, we discuss how FGLK motifs are required for efficient precursor protein import and how these elements may permit a convergent function of this highly variable class of targeting sequences.
authors.affiliation: Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA.
authors.forename: Kristen
authors.initials: K
authors.lastname: Holbrook
authors.affiliation: Graduate Program in Plant Physiology and Genetics, University of Tennessee, Knoxville, TN 37996, USA.
authors.forename: Chitra
authors.initials: C
authors.lastname: Subramanian
authors.affiliation: Graduate School of Genome Science and Technology, University of Tennessee, Knoxville, TN 37996, USA.
authors.forename: Prakitchai
authors.initials: P
authors.lastname: Chotewutmontri
authors.affiliation: Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA.
authors.forename: L Evan
authors.initials: LE
authors.lastname: Reddick
authors.affiliation: Department of Botany, University of Tennessee, Knoxville, TN 37996, USA.
authors.forename: Sarah
authors.initials: S
authors.lastname: Wright
authors.affiliation: Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA.
authors.forename: Huixia
authors.initials: H
authors.lastname: Zhang
authors.affiliation: Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA.
authors.forename: Lily
authors.initials: L
authors.lastname: Moncrief
authors.affiliation: Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA; Graduate Program in Plant Physiology and Genetics, University of Tennessee, Knoxville, TN 37996, USA; Graduate School of Genome Science and Technology, University of Tennessee, Knoxville, TN 37996, USA. Electronic address: bbruce@utk.edu.
authors.forename: Barry D
authors.initials: BD
authors.lastname: Bruce
mesh_terms: D060365:Chloroplast Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism; D010455:Peptides / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport / Q000235:genetics / Q000502:physiology
chemical_list: D060365:Chloroplast Proteins; D010455:Peptides; D010940:Plant Proteins
keywords: Toc34; cell biology; chloroplast biology; protein translocation; transit peptide