Differential age-dependent import regulation by signal peptides

Area of Science:

• Plant Biology
• Cell Biology
• Molecular Biology

Background:

• Protein transport into organelles is typically constitutive.
• Gene regulation often involves age-dependent mechanisms at transcriptional and translational levels.

Purpose of the Study:

• To investigate if protein transport into chloroplasts is also subject to age-dependent regulation.
• To identify the signals and mechanisms governing age-selective protein import into chloroplasts.

Main Methods:

• Categorization of chloroplast proteins into age-selective import groups (I, II, III).
• Analysis of transit peptide sequences for age-selective signals.
• Mutational analysis by swapping transit peptides between protein groups.
• Identification of specific sequence motifs (e.g., positive charges) in transit peptides.

Main Results:

• Chloroplast proteins are divided into three age-selective import groups: preferential import into young (Group I), age-independent (Group II), and preferential import into old (Group III) chloroplasts.
• The age-selective targeting signal resides within the protein's transit peptide.
• Replacing a Group III transit peptide with a Group I peptide altered import preference.
• A motif of two consecutive positive charges in Group III transit peptides confers preference for older chloroplasts.
• Members of the same gene family can exhibit different age-selective import properties due to transit peptide variations.

Conclusions:

• Organelle-targeting signal peptides are integral components of cellular age-dependent regulatory networks.
• Sequence diversity in organelle-targeting peptides has evolved to mediate age-selective regulation, not due to lack of selection pressure.