摘要
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 Lonl dual-targeting to chloroplasts and mitochondria. Dual-organellar deposition of Lonl isoforms depends on both transcriptional regula- tion and alternative translation initiation via leaky ribosome scanning from the first AUG sequence context that deviates extensively from the optimum Kozak consensus. Organelle-specific Lonl targeting results in partial complementation of Arabidopsis Ion1-1 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 com- plementing the piml phenotype when the mitochondrial presequence is used. Bioinformatic search identified a limited number of Arabidopsis genes with Lonl-type dual-targeting sequence organization. Lon4, the paralog of Lonl, has an ambiguous presequence likely evolved from the twin presequences of an ancestral Lonl-like gene, generating a single dual-targeted protein isoform. We postulate that Lonl and its subfunctional paralog Lon4 evolved complementary sub- sets of transcriptional and posttranscriptional regulatory components responsive to environmental cues for dual-organel- lar targeting.
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 Lonl dual-targeting to chloroplasts and mitochondria. Dual-organellar deposition of Lonl isoforms depends on both transcriptional regula- tion and alternative translation initiation via leaky ribosome scanning from the first AUG sequence context that deviates extensively from the optimum Kozak consensus. Organelle-specific Lonl targeting results in partial complementation of Arabidopsis Ion1-1 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 com- plementing the piml phenotype when the mitochondrial presequence is used. Bioinformatic search identified a limited number of Arabidopsis genes with Lonl-type dual-targeting sequence organization. Lon4, the paralog of Lonl, has an ambiguous presequence likely evolved from the twin presequences of an ancestral Lonl-like gene, generating a single dual-targeted protein isoform. We postulate that Lonl and its subfunctional paralog Lon4 evolved complementary sub- sets of transcriptional and posttranscriptional regulatory components responsive to environmental cues for dual-organel- lar targeting.
