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Import, targeting, and processing of a plant polyphenol oxidase
A Sommer1, E Ne'eman, J C Steffens
1Botany Department, Hebrew University, Jerusalem, Israel.
Plant Physiology
|August 1, 1994
Summary
Polyphenol oxidase (PPO) precursor proteins are imported into chloroplasts in a two-step process, first to the stroma and then to the thylakoid lumen. This targeting mechanism is conserved across various plant species and is crucial for PPO function.
Area of Science:
- Plant Molecular Biology
- Chloroplast Protein Targeting
- Enzyme Biochemistry
Background:
- Polyphenol oxidase (PPO) is a key enzyme in plants, involved in various physiological processes.
- Understanding the precise localization and processing of PPO within plant cells is essential for elucidating its function.
- Previous studies suggested complex routing mechanisms for plant PPOs, but experimental evidence was limited.
Purpose of the Study:
- To investigate the import, targeting, and processing of a tomato polyphenol oxidase (PPO) precursor protein (pPPO) in isolated chloroplasts.
- To determine the step-wise pathway and conditions required for pPPO translocation into the thylakoid lumen.
- To examine the conservation of this targeting mechanism across different plant species.
Main Methods:
- In vitro transcription and translation of a tomato PPO gene to produce a [35S]methionine-labeled precursor protein (pPPO).
- Import assays using isolated chloroplasts from tomato, pea, and maize, with analysis of protein processing and localization.
- Investigating the effects of ATP, light, copper ions (Cu2+), and tentoxin on pPPO import and processing.
Main Results:
- The pPPO precursor was imported into chloroplasts in a two-step process: first into the stroma (ATP-dependent) and then into the thylakoid lumen (light-dependent).
- Processing occurred in two stages: a 67-kD precursor was converted to a 62-kD intermediate in the stroma, and then to a 59-kD mature form in the lumen.
- This two-step targeting was conserved in pea and maize chloroplasts, and was not dependent on Cu2+, with low Cu2+ concentrations inhibiting import. Tentoxin specifically blocked import at the envelope membrane.
Conclusions:
- The import and targeting of pPPO into the thylakoid lumen follow a conserved, two-step pathway in plants, consistent with the protein's transit peptide structure.
- This mechanism does not involve unorthodox routing and likely accounts for the observed multiplicity of PPO in vivo.
- The findings provide a clear model for thylakoid lumen protein targeting and PPO biogenesis in plants.