title: Mechanism of Dual Targeting of the Phytochrome Signaling Component HEMERA/pTAC12 to Plastids and the Nucleus.
abstract: HEMERA (HMR) is a nuclear and plastidial dual-targeted protein. While it functions in the nucleus as a transcriptional coactivator in phytochrome signaling to regulate a distinct set of light-responsive, growth-relevant genes, in plastids it is known as pTAC12, which associates with the plastid-encoded RNA polymerase, and is essential for inducing the plastomic photosynthetic genes and initiating chloroplast biogenesis. However, the mechanism of targeting HMR to the nucleus and plastids is still poorly understood. Here, we show that HMR can be directly imported into chloroplasts through a transit peptide residing in the N-terminal 50 amino acids. Upon cleavage of the transit peptide and additional proteolytic processing, mature HMR, which begins from Lys-58, retains its biochemical properties in phytochrome signaling. Unexpectedly, expression of mature HMR failed to rescue not only the plastidial but also the nuclear defects of thehmrmutant. This is because the predicted nuclear localization signals of HMR are nonfunctional, and therefore mature HMR is unable to accumulate in either plastids or the nucleus. Surprisingly, fusing the transit peptide of the small subunit of Rubisco with mature HMR rescues both its plastidial and nuclear localization and functions. These results, combined with the observation that the nuclear form of HMR has the same reduced molecular mass as plastidial HMR, support a retrograde protein translocation mechanism in which HMR is targeted first to plastids, processed to the mature form, and then relocated to the nucleus.
authors.affiliation: Department of Botany and Plant Sciences, Institute for Integrative Genome Biology, University of California, Riverside, California 92521 (Y.Q., C.Y., M.C.).|Department of Biology, Duke University, Durham, North Carolina 27708 (P.A.N., Y.Q., C.Y., P.N.B., M.C.); and.|Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824 (H.I., D.J.S.).
authors.forename: P Andrew
authors.identifier: 0000-0001-5175-2542
authors.initials: PA
authors.lastname: Nevarez
authors.affiliation: Department of Botany and Plant Sciences, Institute for Integrative Genome Biology, University of California, Riverside, California 92521 (Y.Q., C.Y., M.C.).|Department of Biology, Duke University, Durham, North Carolina 27708 (P.A.N., Y.Q., C.Y., P.N.B., M.C.); and.|Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824 (H.I., D.J.S.).
authors.forename: Yongjian
authors.identifier: 0000-0002-1799-6305
authors.initials: Y
authors.lastname: Qiu
authors.affiliation: Department of Botany and Plant Sciences, Institute for Integrative Genome Biology, University of California, Riverside, California 92521 (Y.Q., C.Y., M.C.).|Department of Biology, Duke University, Durham, North Carolina 27708 (P.A.N., Y.Q., C.Y., P.N.B., M.C.); and.|Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824 (H.I., D.J.S.).
authors.forename: Hitoshi
authors.identifier: 0000-0003-4988-337X
authors.initials: H
authors.lastname: Inoue
authors.affiliation: Department of Botany and Plant Sciences, Institute for Integrative Genome Biology, University of California, Riverside, California 92521 (Y.Q., C.Y., M.C.).|Department of Biology, Duke University, Durham, North Carolina 27708 (P.A.N., Y.Q., C.Y., P.N.B., M.C.); and.|Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824 (H.I., D.J.S.).
authors.forename: Chan Yul
authors.identifier: 0000-0001-6159-7443
authors.initials: CY
authors.lastname: Yoo
authors.affiliation: Department of Botany and Plant Sciences, Institute for Integrative Genome Biology, University of California, Riverside, California 92521 (Y.Q., C.Y., M.C.).|Department of Biology, Duke University, Durham, North Carolina 27708 (P.A.N., Y.Q., C.Y., P.N.B., M.C.); and.|Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824 (H.I., D.J.S.).
authors.forename: Philip N
authors.identifier: 0000-0001-5302-758X
authors.initials: PN
authors.lastname: Benfey
authors.affiliation: Department of Botany and Plant Sciences, Institute for Integrative Genome Biology, University of California, Riverside, California 92521 (Y.Q., C.Y., M.C.).|Department of Biology, Duke University, Durham, North Carolina 27708 (P.A.N., Y.Q., C.Y., P.N.B., M.C.); and.|Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824 (H.I., D.J.S.).
authors.forename: Danny J
authors.identifier: 0000-0002-6524-4159
authors.initials: DJ
authors.lastname: Schnell
authors.affiliation: Department of Botany and Plant Sciences, Institute for Integrative Genome Biology, University of California, Riverside, California 92521 (Y.Q., C.Y., M.C.); meng.chen@ucr.edu.|Department of Biology, Duke University, Durham, North Carolina 27708 (P.A.N., Y.Q., C.Y., P.N.B., M.C.); and meng.chen@ucr.edu.|Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824 (H.I., D.J.S.) meng.chen@ucr.edu.
authors.forename: Meng
authors.identifier: 0000-0003-0351-5897
authors.initials: M
authors.lastname: Chen
mesh_terms: D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism; D002467:Cell Nucleus / Q000235:genetics* / Q000378:metabolism; D002736:Chloroplasts / Q000235:genetics / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D015151:Immunoblotting; D018613:Microscopy, Confocal; D009154:Mutation; D010834:Phytochrome / Q000235:genetics; D030821:Plants, Genetically Modified; D018087:Plastids / Q000235:genetics* / Q000378:metabolism; D021381:Protein Transport / Q000235:genetics; D059748:Proteolysis; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012273:Ribulose-Bisphosphate Carboxylase / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; C000604706:HMR protein, Arabidopsis; D014157:Transcription Factors; D010834:Phytochrome; D012273:Ribulose-Bisphosphate Carboxylase