Structural features of chloroplast trigger factor determined at 2.6 Å resolution.

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Protein folding is crucial and relies on molecular chaperones.
• Trigger factor (TF) is a ribosome-associated chaperone found in prokaryotes and plant chloroplasts, binding nascent polypeptides.
• Bacterial TF has a well-defined dragon-shaped structure with domains for ribosome binding, peptidyl-prolyl cis-trans isomerization (PPIase), and substrate interaction.

Purpose of the Study:

• To determine the crystal structure of plastidic trigger factor from Chlamydomonas reinhardtii.
• To investigate the molecular mechanism and structural adaptations of eukaryotic TF in plant organelles.
• To compare the structure of eukaryotic TF with its bacterial orthologs.

Main Methods:

• X-ray crystallography was used to determine the structure of Chlamydomonas reinhardtii plastidic trigger factor at 2.6 Å resolution.
• A truncated protein lacking the N-terminal ribosome-binding domain was used due to high intramolecular flexibility.
• Structural comparisons were made between the eukaryotic TF and bacterial TF.

Main Results:

• The eukaryotic TF from C. reinhardtii adopts a dragon-shaped conformation, similar to bacterial TF.
• The C-terminal chaperone domain exhibits altered charge distributions, modified helical arm positioning, and distinct substrate-binding surface characteristics.
• The PPIase domain is structurally conserved but shows weak activity and an unusual orientation relative to the C-terminal domain.

Conclusions:

• Chloroplast TF has diversified from bacterial TF, with structural adaptations in its chaperone domain.
• These adaptations suggest specialized functional roles for eukaryotic TF within chloroplasts.
• The findings provide insights into the evolution and function of ribosome-associated chaperones in different cellular compartments.