title: Identification of Putative Substrates of SEC2, a Chloroplast Inner Envelope Translocase.
abstract: Most chloroplast proteins are synthesized in the cytosol and imported into chloroplasts. Many imported proteins are further targeted to the thylakoid membrane and lumen by the SEC1, TAT, or SRP/ALB3 translocases. Others are targeted to the inner chloroplast envelope membrane by undescribed translocases. Recently, a second SEC system (SEC2) consisting of SCY2, SECE2, and SECA2 was found in the chloroplast envelope. Null mutants ofSCY2in Arabidopsis (Arabidopsis thaliana) exhibit a severe embryo-lethal phenotype. To investigate the function of the SEC2 system in plants, we used inducible RNA interference to knock downSCY2in Arabidopsis. Seedlings cultured with inducer were chlorotic with aberrant chloroplasts and undeveloped thylakoids, indicating an essential role for SCY2 in chloroplast biogenesis beyond embryo development. InSCY2down-regulated seedlings, several thylakoid membrane proteins, including SCY1, ALB3, and TATC, and inner envelope membrane proteins, including TIC40, TIC110, and FTSH12, were reduced substantially, suggesting that they may be SEC2 substrates. Additional insight was achieved by the in vitro reconstitution of protein integration into chloroplast membranes. The results show that SCY1 and ALB3 target directly to the thylakoid membrane and are likely independent of SEC2. FTSH12 was integrated into the envelope membrane in a coupled import-integration reaction that was impaired by the SECA inhibitor sodium azide. The stromal intermediate of TIC40 integrated into the envelope in a reaction that was largely inhibited when antibodies against epitope-tagged SCY2 or SECE2 were applied. These data demonstrate that the SEC2 translocase likely integrates a subset of inner envelope membrane proteins, such as FTSH12 and TIC40.
authors.affiliation: Horticultural Sciences Department and Plant Molecular and Cellular Biology, University of Florida, Gainesville, Florida 32611 (Y.L., J.R.M., G.A.A., K.C.); and.|Department of Botany, University of Wisconsin, Madison, Wisconsin 53706 (D.E.F.).
authors.forename: Yubing
authors.identifier: 0000-0002-5850-7678
authors.initials: Y
authors.lastname: Li
authors.affiliation: Horticultural Sciences Department and Plant Molecular and Cellular Biology, University of Florida, Gainesville, Florida 32611 (Y.L., J.R.M., G.A.A., K.C.); and.|Department of Botany, University of Wisconsin, Madison, Wisconsin 53706 (D.E.F.).
authors.forename: Jonathan R
authors.identifier: 0000-0001-6187-3022
authors.initials: JR
authors.lastname: Martin
authors.affiliation: Horticultural Sciences Department and Plant Molecular and Cellular Biology, University of Florida, Gainesville, Florida 32611 (Y.L., J.R.M., G.A.A., K.C.); and.|Department of Botany, University of Wisconsin, Madison, Wisconsin 53706 (D.E.F.).
authors.forename: Giovanni A
authors.identifier: 0000-0001-5519-7152
authors.initials: GA
authors.lastname: Aldama
authors.affiliation: Horticultural Sciences Department and Plant Molecular and Cellular Biology, University of Florida, Gainesville, Florida 32611 (Y.L., J.R.M., G.A.A., K.C.); and.|Department of Botany, University of Wisconsin, Madison, Wisconsin 53706 (D.E.F.).
authors.forename: Donna E
authors.identifier: 0000-0003-4266-9813
authors.initials: DE
authors.lastname: Fernandez
authors.affiliation: Horticultural Sciences Department and Plant Molecular and Cellular Biology, University of Florida, Gainesville, Florida 32611 (Y.L., J.R.M., G.A.A., K.C.); and kcline@ufl.edu.|Department of Botany, University of Wisconsin, Madison, Wisconsin 53706 (D.E.F.) kcline@ufl.edu.
authors.forename: Kenneth
authors.identifier: 0000-0001-9625-810X
authors.initials: K
authors.lastname: Cline
mesh_terms: D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D060365:Chloroplast Proteins / Q000235:genetics / Q000378:metabolism*; D020043:Genes, Essential; D015151:Immunoblotting; D009154:Mutation; D030821:Plants, Genetically Modified; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism; D034622:RNA Interference; D000069816:SEC Translocation Channels / Q000235:genetics / Q000378:metabolism*; D036226:Seedlings / Q000235:genetics / Q000378:metabolism; D012639:Seeds / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity; D020524:Thylakoids / Q000201:enzymology* / Q000235:genetics
chemical_list: D029681:Arabidopsis Proteins; D060365:Chloroplast Proteins; D021122:Protein Subunits; D000069816:SEC Translocation Channels