title: Alternative transcription initiation and the AUG context configuration control dual-organellar targeting and functional competence of Arabidopsis Lon1 protease.
abstract: Cellular homeostasis relies on components of protein quality control including chaperones and proteases. In bacteria and eukaryotic organelles, Lon proteases play a critical role in removing irreparably damaged proteins and thereby preventing the accumulation of deleterious degradation-resistant aggregates. Gene expression, live-cell imaging, immunobiochemical, and functional complementation approaches provide conclusive evidence for Lon1 dual-targeting to chloroplasts and mitochondria. Dual-organellar deposition of Lon1 isoforms depends on both transcriptional regulation and alternative translation initiation via leaky ribosome scanning from the first AUG sequence context that deviates extensively from the optimum Kozak consensus. Organelle-specific Lon1 targeting results in partial complementation of Arabidopsis lon1-1 mutants, whereas full complementation is solely accomplished by dual-organellar targeting. Both the optimal and non-optimal AUG sequence contexts are functional in yeast and facilitate leaky ribosome scanning complementing the pim1 phenotype when the mitochondrial presequence is used. Bioinformatic search identified a limited number of Arabidopsis genes with Lon1-type dual-targeting sequence organization. Lon4, the paralog of Lon1, has an ambiguous presequence likely evolved from the twin presequences of an ancestral Lon1-like gene, generating a single dual-targeted protein isoform. We postulate that Lon1 and its subfunctional paralog Lon4 evolved complementary subsets of transcriptional and posttranscriptional regulatory components responsive to environmental cues for dual-organellar targeting.
authors.affiliation: Department of Biotechnology, Agricultural University of Athens, Iera Odos 75, Athens 118 55, Greece.
authors.forename: Gerasimos
authors.initials: G
authors.lastname: Daras
authors.affiliation: Department of Biotechnology, Agricultural University of Athens, Iera Odos 75, Athens 118 55, Greece.
authors.forename: Stamatis
authors.initials: S
authors.lastname: Rigas
authors.affiliation: Department of Biotechnology, Agricultural University of Athens, Iera Odos 75, Athens 118 55, Greece.
authors.forename: Dikran
authors.initials: D
authors.lastname: Tsitsekian
authors.affiliation: School of Computer Science, Tel Aviv University, Ramat Aviv 69978, Israel.
authors.forename: Hadas
authors.initials: H
authors.lastname: Zur
authors.affiliation: Department of Biomedical Engineering, Faculty of Engineering, Tel Aviv University, Ramat Aviv 69978, Israel.
authors.forename: Tamir
authors.initials: T
authors.lastname: Tuller
authors.affiliation: Department of Biotechnology, Agricultural University of Athens, Iera Odos 75, Athens 118 55, Greece. Electronic address: phat@aua.gr.
authors.forename: Polydefkis
authors.initials: P
authors.lastname: Hatzopoulos
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism*; D001483:Base Sequence; D018506:Gene Expression Regulation, Plant / Q000235:genetics / Q000502:physiology; D008969:Molecular Sequence Data; D016415:Sequence Alignment; D012697:Serine Endopeptidases; D024363:Transcription Initiation Site / Q000502:physiology*
chemical_list: D029681:Arabidopsis Proteins; C579791:Lon1 protein, Arabidopsis; D012697:Serine Endopeptidases
keywords: Lon; alternative transcription; alternative translation; ambiguous and twin presequences; chloroplasts; mitochondria.; protein dual-targeting