Targeting proteins to diatom plastids involves transport through an endoplasmic reticulum

Area of Science:

• * Marine biology and algal physiology.
• * Molecular biology and cell biology.
• * Photosynthetic pigment-protein complexes.

Background:

• * Diatoms possess unique light-harvesting complexes dominated by xanthophylls and an endoplasmic reticulum (ER) network around their plastids.
• * Light-harvesting complex polypeptides in diatoms are nuclear-encoded and synthesized as precursor proteins in the cytoplasm.
• * Precursor protein gene sequences indicate the presence of a signal peptide at their amino-termini.

Purpose of the Study:

• * To investigate the import pathway of cytoplasmically synthesized light-harvesting complex precursor polypeptides into diatom plastids.
• * To determine if a signal peptide is necessary for traversing the ER before plastid entry.
• * To provide insights into the evolutionary context of plastid protein import.

Main Methods:

• * In vitro import assays using a microsomal membrane system.
• * Analysis of cotranslational import and processing of precursor polypeptides.
• * Deduction of signal peptide features from gene sequences.

Main Results:

• * Precursor polypeptides for the light-harvesting complex were successfully imported and processed in vitro.
• * Cotranslational import and processing occurred in a microsomal membrane system.
• * Evidence suggests the necessity of a signal peptide for ER traversal prior to plastid import.

Conclusions:

• * Cytoplasmically synthesized proteins destined for diatom plastids utilize a signal peptide-mediated pathway.
• * This pathway involves translocation across the endoplasmic reticulum (ER) membrane.
• * Findings contribute to understanding protein import mechanisms and plastid evolution in algae.